53 Circuits and Components for System Evaluations - conocimientos.com.ve: julio 2010

domingo, 25 de julio de 2010

GSM EDGE cellular evolution technology

EDGE is an evolution to the GSM mobile cellular phone system. The name EDGE stands for Enhanced Data for GSM Evolution and it enables data to be sent over a GSM TDMA system at speeds up to 384 kbps. In some instances GSM EDGE evolution systems may also be known as EGPRS, or Enhanced General Packet Radio Service systems. Although strictly speaking a "2.5G" system, the GSM EDGE cellular technology is capable of providing data rates that are a distinct increase on those that could be supported by GPRS.

EDGE evolution is intended to build on the enhancements provided by the addition of GPRS (General Packet Radio Service) where packet switching is applied to a network. It then enables a three-fold increase in the speed at which data can be transferred by adopting a new form of modulation. GSM uses a form of modulation known as Gaussian Minimum Shift Keying (GMSK), but EDGE evolution changes the modulation to 8PSK and thereby enabling a significant increase in data rate to be achieved.

GSM EDGE basics

GSM EDGE cellular technology is an upgrade to the existing GSM / GPRS networks, and can often be implemented as a software upgrade to existing GSM / GPRS networks. This makes it a particularly attractive option proving virtually 3G data rates for a small upgrade to an existing GPRS network.

GSM EDGE evolution can provide data rates of up to 384 kbps, and this means that it offers a significantly higher data rate than GPRS.

There are a number of key elements in the upgrade from GSM or GPRS to EDGE. The GSM EDGE technology requires a number of new elements to be added to the system:

Use of 8PSK modulation: In order to achieve the higher data rates within GSM EDGE, the modulation format can be changed from GMSK to 8PSK. This provides a significant advantage in being able to convey 3 bits per symbol, thereby increasing the maximum data rate. This upgrade requires a change to the base station. Sometimes hardware upgrades may be required, although it is often simply a software change.
Base station: Apart from the upgrade to incorporate the 8PSK modulation capability, other small changes are required to the base station. These are normally relatively small and can often be accomplished by software upgrades.
Upgrade to network architecture: GSM EDGE provides the capability for IP based data transfer. As a result, additional network elements are required. These are the same as those needed for GPRS and later for UMTS. In this way the introduction of EDGE technology is part of the overall migration path from GSM to UMTS.

The two main additional nodes required for the network are the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). The GGSN connects to packet-switched networks such as the Internet and other GPRS networks. The SGSN provides the packet-switched link to mobile stations.
Mobile stations: It is necessary to have a GSM EDGE handset that is EDGE compatible. As it is not possible to upgrade handsets, this means that the user needs to buy a new GSM EDGE handset.

Despite the number of changes that need to be made, the cost of the upgrade to move to GSM EDGE cellular technology is normally relatively small. The elements in the core network are required for GPRS which may already be available on the network, and hence these elements will already be present. The new network entities are also needed for UMTS and therefore they are on the overall upgrade and migration path. Other changes to the base stations are comparatively small and can often be achieved very easily.

GSM EDGE evolution specification overview

It is worth summarizing the key parameters of GSM EDGE cellular technology.

Parameter	Details
Multiple Access Technology	FDMA / TDMA
Duplex Technique	FDD
Channel Spacing	200 kHz
Modulation	GMSK, 8PSK
Slots per channel	8
Frame duration	4.615 ms
Latency	Below 100 ms
Overall symbol rate	270 k symbols / s
Overall modulation bit rate	810 kbps
Radio data rate per time slot	69.2 kbps
Max user data rate per time slot	59.2 kbps (MCS-9)
Max user data rate when using 8 time slots	473.6 kbps **

GSM EDGE specification highlights

Note:
** A maximum user data rate of 384 kbps is often seen quoted as the data rate for GSM EDGE. This data rate corresponds to the International Telecommunications Union (ITU) definition of the data rate limit required for a service to fulfill the International Mobile Telecommunications-2000 (IMT-2000) standard(i.e. 3G) in a pedestrian environment.

Summary

GSM EDGE evolution has been widely deployed across the globe providing an evolutionary path from 2G GSM through to 3G UMTS. It is able to give data throughput rates that enable some Internet surfing as well as sending and receiving emails in addition to a variety of other applications. The use of GSM EDGE cellular technology has therefore been a major enabler in allowing data applications to grow, and to provide a data service where no 3G coverage may be available.

Luiggi Escalante
CI 18.878.611
CRF
Fuente: http://www.radio-electronics.com/info/cellulartelecomms/gsm-edge/basics-tutorial-technology.php

PIN diode variable RF attenuator circuit

Electronically controllable variable RF attenuators are often used in RF design. For example, it is often necessary to be able to control the level of a radio frequency signal using a control voltage. These variable RF attenuators can even be used in programmable RF attenuators. Here the known voltage generated by a computer for example can be applied to the circuit and in this way create a programmable RF attenuator.
Often when designing or using variable or programmable RF attenuators, it is necessary to ensure that the RF attenuator retains a constant impedance over its operating range to ensure the correct operation of the interfacing circuitry. This RF attenuator circuit shown below provides a good match to 50 ohms over its operating range.

RF attenuator circuit description

The PIN diode variable attenuator is used to give attenuation over a range of about 20 dB and can be used in 50 ohm systems. The inductor L1 along with the capacitors C4 and C5 are included to prevent signal leakage from D1 to D2 that would impair the performance of the circuit.
The maximum attenuation is achieved when Vin is at a minimum. At this point current from the supply V+ turns the diodes D1 and D2 on effectively shorting the signal to ground. D3 is then reverse biased. When Vin is increased the diodes D1 and D2 become reverse biased, and D3 becomes forward biased, allowing the signal to pass through the circuit.

PIN diode variable RF attenuator

PIN diode variable RF attenuator circuit Typical values for the variable RF attenuator circuit might be: +V : 5 volts; Vin : 0 - 6 volts; D1 to D3 HP5082-3080 PIN diodes; R1 2k2; R2 : 1k; R3 2k7; L1 is self resonant above the operating frequency, but sufficient to give isolation between the diodes D1 and D2.
These values are only a starting point for an experimental design, and are only provided as such. The circuit may not be suitable in all instances.

Choice of PIN diode

Although in theory any diode could be used in variable RF attenuators, PIN diodes have a number of advantages. In the first place they are more linear than ordinary PN junction diodes. This means that in their action as a radio frequency switch they do not create as many spurious products and additionally as an attenuator they have a more useful curve. Secondly when reverse biased and switched off, the depletion layer is wider than with an ordinary diode and this provides for greater isolation when switching or providing higher levels of attenuation.

Luiggi Escalante
CI 18.878.611
CRF
Fuente: http://www.radio-electronics.com/info/rf-technology-design/attenuators/rf-variable-pin-diode-attenuator.php

Low Phase Noise Frequency Synthesizer Design

Phase noise in PLL frequency synthesizers if of great importance because it determines many factors about the equipment into which it is incorporated. For receivers it determines the reciprocal mixing performance, and in some circumstances the bit error rate. In transmitters the phase noise performance of the frequency synthesizer determines features such as adjacent channel noise and it contributes to the bit error rate for the whole system.

Phase noise in a PLL synthesizer

Phase noise is generated at different points around the synthesizer loop and depending upon where it is generated it affects the output in different ways. For example, noise generated by the VCO has a different effect to that generated by the phase detector. This illustrates that it is necessary to look at the noise performance of each circuit block in the loop when designing the synthesizer so that the best noise performance is obtained.

Apart from ensuring that the noise from each part of the circuit is reduced to an absolute minimum, it is the loop filter which has the most effect on the final performance of the circuit because it determines the break frequencies where noise from different parts of the circuit start to affect the output.

To see how this happens take the example of noise from the VCO. Noise from the oscillator is divided by the divider chain and appears at the phase detector. Here it appears as small perturbations in the phase of the signal and emerges at the output of the phase detector. When it comes to the loop filter only those frequencies which are below its cut-off point appear at the control terminal of the VCO to correct or eliminate the noise. From this it can be seen that VCO noise which is within the loop bandwidth is attenuated, but that which is outside the loop bandwidth is left unchanged.

The situation is slightly different for noise generated by the reference. This enters the phase detector and again passes through it to the loop filter where the components below the cut-off frequency are allowed through and appear on the control terminal of the VCO. Here they add noise to the output signal. So it can be seen that noise from the reference is added to the output signal within the loop bandwidth but it is attenuated outside this.

Similar arguments can be applied to all the other circuit blocks within the loop. In practice the only other block which normally has any major effect is the phase detector and its noise affects the loop in exactly the same way as noise from the reference. Also if multi-loop synthesizers are used then the same arguments can be used again.

Effects of multiplication

As noise is generated at different points around the loop it is necessary to discover what effect this has on the output. As a result it is necessary to relate all the effects back to the VCO. Apart from the different elements in the loop affecting the noise at the output in different ways, the effect of the multiplication in the loop also has an effect.

The effect of multiplication is very important. It is found that the level of phase noise from some areas is increased in line with the multiplication factor (i.e. the ratio of the final output frequency to the phase comparison frequency). In fact it is increased by a factor of 20 log10 N where N is the multiplication factor. The VCO is unaffected by this, but any noise from the reference and phase detector undergoes this amount of degradation. Even very good reference signals can be a major source of noise if the multiplication factor is high. For example a loop which has a divider set to 200 will multiply the noise of the reference and phase detector by 46 dB.

From this information it is possible to build up a picture of the performance of the synthesizer. Generally this will look like the outline shown in Fig. 6. From this it can be seen that the noise inside the loop bandwidth is due mainly to components like the phase detector and reference, whilst outside the loop the VCO generates the noise. A slight hump is generally seen at the point where the loop filter cuts off and the loop gain falls to unity.

By predicting the performance of the loop it is possible to optimise the performance or look at areas which can be addressed to improve the performance of the whole synthesizer before the loop is even built. In order to analyse the loop further it is necessary to look at each circuit block in turn.

Voltage controlled oscillator

The noise performance of the oscillator is of particular importance. This is because the noise performance of the synthesizer outside the loop is totally governed by its performance. In addition to this its performance may influence decisions about other areas of the circuit.

The typical noise outline for a VCO is flat at large frequency offsets from the carrier. It is determined largely by factors such as the noise figure of the active device. The performance of this area of the oscillator operation can be optimised by ensuring the circuit is running under the optimum noise performance conditions. Another approach is to increase the power level of the circuit so that the signal to noise ratio improves.

Closer in the noise starts to rise, initially at a rate of 20 dB per decade. The point at which this starts to rise is determined mainly by the Q of the oscillator circuit. A high Q circuit will ensure a good noise performance. Unfortunately VCOs have an inherently low Q because of the Q of the tuning varactors normally employed. Performance can be improved by increasing the Q, but this often results in the coverage of the oscillator being reduced.

Still further in towards the carrier the noise level starts to rise even faster at a rate of 30 dB per decade. This results from flicker or 1/f noise. This can be improved by increasing the level of low frequency feedback in the oscillator circuit. In a standard bipolar circuit a small un-bypassed resistor in the emitter circuit can give significant improvements.

To be able to assess the performance of the whole loop it is necessary to assess the performance of the oscillator once it has been designed and optimised. Whilst there are a number of methods of achieving this the most successful is generally to place the oscillator into a loop having a narrow bandwidth and then measure its performance with a spectrum analyser. By holding the oscillator steady this can be achieved relatively easily. However the results are only valid outside the loop bandwidth. However a test loop is likely to have a much narrower bandwidth than the loop being designed the noise levels in the area of interest will be unaltered.

Reference

The noise performance of the reference follows the same outlines as those for the VCO, but the performance is naturally far better. The reason for this is that the Q of the crystal is many orders of magnitude higher than that of the tuned circuit in the VCO.

Typically it is possible to achieve figures of -110 dBc/Hz at 10 Hz from the carrier and 140 dBc/Hz at 1 kHz from a crystal oven. Figures of this order are quite satisfactory for most applications. If lower levels of reference noise are required these can be obtain, but at a cost. In instances where large multiplication factors are necessary a low noise reference may be the only option. However as a result of the cost they should be avoided wherever possible. Plots of typical levels of phase noise are often available with crystal ovens giving an accurate guide to the level of phase noise generated by the reference.

Frequency divider

Divider noise appears within the loop bandwidth. Fortunately the levels of noise generated within the divider are normally quite low. If an analysis is required then it will be found that noise is generated at different points within the divider each of which will be subject to a different multiplication factor dependent upon where in the divider it is generated and the division ratio employed from that point.

Most divider chains use several dividers and if an approximate analysis is to be performed it may be more convenient to only consider the last device or devices in the chain as these will contribute most to the noise. However the noise is generally difficult to measure and will be seen with that generated by the phase detector.

Phase detector

Like the reference signal the phase detector performance is crucial in determining the noise performance within the loop bandwidth. There are a number of different types of phase detector. The two main categories are analogue and digital.

Mixers are used to give analogue phase detectors. If the output signal to noise ratio is to be as good as possible then it is necessary to ensure that the input signal levels are as high as possible within the operating limits of the mixer. Typically the signal input may be limited to around -10 dBM and the local oscillator input to +10 dBm. In some instances higher level mixers may be used with local oscillator levels of +17 dBm or higher. The mixer should also be chosen to have a low NTR (noise temperature ratio). As the output is DC coupled it is necessary to have a low output load resistance to prevent a backward bias developing. This could offset the operation of the mixer and reduce its noise performance.

It is possible to calculate the theoretical noise performance of the mixer under optimum conditions. An analogue mixer is likely to give a noise level of around -153 dBc/Hz.

There are a variety of digital phase detectors which can be used. In theory these give a better noise performance than the analogue counterpart. At best a simple OR gate type will give figures about 10 dB better than an analogue detector and an edge triggered type (e.g. a dual D type or similar) will give a performance of around 5 dB better than the analogue detector.

These figures are the theoretical optimum and should be treated as guide although they are sufficient for initial noise estimates. In practice other factors may mean that the figures are different. A variety of factors including power supply noise, circuit layout etc. can degrade the performance from the ideal. If very accurate measurements are required then results from the previous use of the circuit, or from a special test loop can provide the required results.

Loop filter

There are a variety of parameters within the area of the loop filter which affect the noise performance of the loop. The break points of the filter and the unity gain point of the loop determined by the filter govern the noise profile.

In terms of contributions to the noise in the loop the major source is likely to occur if an operational amplifier is used. If this is the case a low noise variety must be used otherwise the filter will give a large contribution to the loop phase noise profile. This noise is often viewed as combined with that from the phase detector, appearing to degrade its performance from the ideal.

Plotting Performance

Having investigated the noise components from each element in the loop, it is possible to construct a picture of how the whole loop will perform. Whilst this can performed mathematically, a simple graphical approach quickly reveals an estimate of the performance and shows which are the major elements which contribute to the noise. In this way some re-design can be undertaken before the design is constructed, enabling the best option to be chosen on the drawing board. Naturally it is likely to need some optimisation once it has been built, but this method enables the design to be made as close as possible beforehand.

First it is necessary to obtain the loop response. This is dependent upon a variety of factors including the gain around the loop and the loop filter response. For stability the loop gain must fall at a rate of 20 dB per decade (6 dB per octave) at the unity gain point. Provided this criterion is met a wide variety of filters can be used. Often it is useful to have the loop response rise at a greater rate than this inside the loop bandwidth. By doing this the VCO noise can be attenuated further. Outside the loop bandwidth a greater fall off rate can aid suppress the unwanted reference sidebands further. From a knowledge of the loop filter chosen the break points can be calculated and with a knowledge of the loop gain the total loop response can be plotted.

With the response known the components from the individual blocks in the loop can be added as they will be affected by the loop and seen at the output.

First take the VCO. Outside the loop bandwidth its noise characteristic is unmodified. However once inside this point the action of the loop attenuates the noise, first at a rate of 20 dB per decade, and then at a rate of 40 dB per decade. The overall affect of this is to modify the response of the characteristic as shown in Fig. 10. It is seen that outside the loop bandwidth the noise profile is left unmodified. Far out the noise is flat, but further in the VCO noise rises at the rate of 20 dB per decade. Inside the loop bandwidth the VCO noise will be attenuated first at the rate of 20 dB per decade, which in this case gives a flat noise profile. Then as the loop gain increases at the filter break point, to 40 dB per decade this gives a fall in the VCO noise profile of -20 dB per decade. However further in the profile of the stand alone VCO noise rises to -30dB per decade. The action of the loop gives an overall fall of -10 dB per decade.

The effects of the other significant contributions can be calculated. The reference response can easily be deduced from the manufacturers figures. Once obtained these must have the effect of the loop multiplication factor added. Once this has been calculated the effect of the loop can be added. Inside the loop there is no effect on the noise characteristic, however outside this frequency it will attenuate the reference noise, first at a rate of 20 dB per decade and then after the filter break point at 40 dB per decade.

The other major contributor to the loop noise is the phase detector. The effect of this is treated in the same way as the reference, having the effect of the loop multiplication added and then being attenuated outside the loop bandwidth.

Once all the individual curves have been generated they can be combined onto a single plot to gain a full picture of the performance of the synthesizer. When doing this it should be remembered that it is necessary to produce the RMS sum of the components because the noise sources are not correlated.

Once this has been done then it is possible to optimise the performance by changing factors like the loop bandwidth, multiplication factor and possibly the loop topology to obtain the best performance and ensure that the required specifications are met. In most cases the loop bandwidth is chosen so that a relatively smooth transition is made between the noise contributions inside and outside the loop. This normally corresponds to lowest overall noise situation.

Summary

Although this approach may appear to be slightly "low tech" in today's highly computerised engineering environment it has the advantage that a visual plot of the predicted performance can be easily put together. In this way the problem areas can be quickly identified, and the noise performance of the whole synthesizer optimised before the final design is committed.

Luiggi Escalante
CI 18.878.611
CRF
Fuente: http://www.radio-electronics.com/info/rf-technology-design/pll-synthesizers/low-phase-noise-synthesizer-design.php

DSP - Digital Signal Processing

Today, Digital Signal Processing, DSP, is widely used in radio receivers as well as in many other applications from television, radio transmission, or in fact any applications where signals need to be processed. Today it is not only possible to purchase digital signal processor integrated circuits, but also DSP cards for use in computers. Using these DSP cards it is possible to develop software or just use a PC platform in which to run the DSP card.

DSP has many advantages over analogue processing. It is able to provide far better levels of signal processing than is possible with analogue hardware alone. It is able to perform mathematical operations that enable many of the spurious effects of the analogue components to be overcome. In addition to this, it is possible to easily update a digital signal processor by downloading new software. Once a basic DSP card has been developed, it is possible to use this hardware design to operate in several different environments, performing different functions, purely by downloading different software. It is also able to provide functions that would not be possible using analogue techniques. For example a complicated signal such as Orthogonal Frequency Division Multiplex (OFDM) which is being used for many transmissions today needs DSP to become viable.

Despite this DSP has limitations. It is not able to provide perfect filtering, demodulation and other functions. There are mathematical limitations. In addition to this the processing power of the DSP card may impose some processing limitations. It is also more expensive than many analogue solutions, and thus it may not be cost effective in some applications. Nevertheless it has many advantages to offer, and with the wide availability of cheap DSP hardware and cards, it often provides an attractive solution for many radio applications.

What is DSP?

As the name suggests, digital signal processing is the processing of signals in a digital form. DSP is based upon the fact that it is possible to build up a representation of the signal in a digital form. This is done by sampling the voltage level at regular time intervals and converting the voltage level at that instant into a digital number proportional to the voltage. This process is performed by a circuit called an analogue to digital converter, A to D converter or ADC. In order that the ADC is presented with a steady voltage whilst it is taking its sample, a sample and hold circuit is used to sample the voltage just prior to the conversion. Once complete the sample and hold circuit is ready to update the voltage again ready for the next conversion. In this way a succession of samples is made.

Sampling a waveform for DSP

Once in a digital format the real DSP is able to be undertaken. The digital signal processor performs complicated mathematical routines upon the representation of the signal. However to use the signal it then usually needs to be converted back into an analogue form where it can be amplified and passed into a loudspeaker or headphones. The circuit that performs this function is not surprisingly called a digital to analogue converter, D to A converter or DAC.

Block diagram of a Digital Signal Processor, DSP)

The advantage of DSP, digital signal processing is that once the signals are converted into a digital format they can be manipulated mathematically. This gives the advantage that all the signals can be treated far more exactly, and this enables better filtering, demodulation and general manipulation of the signal. Unfortunately it does not mean that filters can be made with infinitely steep sides because there are mathematical limitations to what can be accomplished.

Luiggi Escalante
CI 18.878.611
CRF
Fuente: http://www.radio-electronics.com/info/receivers/dsp/dsp_basics.php

FM demodulation

Frequency modulation is widely used for radio transmissions for a wide variety of applications from broadcasting to general point to point communications. Frequency modulation, FM offers many advantages, particularly in mobile radio applications where its resistance to fading and interference is a great advantage. It is also widely used for broadcasting on VHF frequencies where it is able to provide a medium for high quality audio transmissions.

In view of its widespread use receivers need to be able to demodulate these transmissions. There is a wide variety of different techniques and circuits that can be sued including the Foster-Seeley, and ratio detectors using discreet components, and where integrated circuits are used the phase locked loop and quadrature detectors are more widely used.

What is frequency modulation, FM?

As the name suggests frequency modulation, FM uses changes in frequency to carry the sound or other information that is required to be placed onto the carrier. As shown below it can be seen that as the modulating or base band signal voltage varies, so the frequency of the signal changes in line with it. This type of modulation brings several advantages with it:

Interference reduction: When compared to AM, FM offers a marked improvement in interference. In view of the fact that most received noise is amplitude noise, an FM receiver can remove any amplitude sensitivity by driving the IF into limiting.
Removal of many effects of signal strength variations: FM is widely used for mobile applications because the amplitude variations do not cause a change in audio level. As the audio is carried by frequency variations rather than amplitude ones, under good signal strength conditions, this does not manifest itself as a change in audio level.
Transmitter amplifier efficiency: As the modulation is carried by frequency variations, this means that the transmitter power amplifiers can be made non-linear. These amplifiers can be made to be far more efficient than linear ones, thereby saving valuable battery power - a valuable commodity for mobile or portable equipment.

These advantages mean that FM has been widely used for many broadcasting and mobile applications.

Frequency modulating a signal

Wide band and Narrow band FM

When a signal is frequency modulated, the carrier shifts in frequency in line with the modulation. This is called the deviation. In the same way that the modulation level can be varied for an amplitude modulated signal, the same is true for a frequency modulated one, although there is not a maximum or 100% modulation level as in the case of AM.

The level of modulation is governed by a number of factors. The bandwidth that is available is one. It is also found that signals with a large deviation are able to support higher quality transmissions although they naturally occupy a greater bandwidth. As a result of these conflicting requirements different levels of deviation are used according to the application that is used.

Those with low levels of deviation are called narrow band frequency modulation (NBFM) and typically levels of +/- 3 kHz or more are used dependent upon the bandwidth available. Generally NBFM is used for point to point communications. Much higher levels of deviation are used for broadcasting. This is called wide band FM (WBFM) and for broadcasting deviation of +/- 75 kHz is used.

Receiving FM

In order to be able to receive FM a receiver must be sensitive to the frequency variations of the incoming signals. As already mentioned these may be wide or narrow band. However the set is made insensitive to the amplitude variations. This is achieved by having a high gain IF amplifier. Here the signals are amplified to such a degree that the amplifier runs into limiting. In this way any amplitude variations are removed.

In order to be able to convert the frequency variations into voltage variations, the demodulator must be frequency dependent. The ideal response is a perfectly linear voltage to frequency characteristic. Here it can be seen that the centre frequency is in the middle of the response curve and this is where the un-modulated carrier would be located when the receiver is correctly tuned into the signal. In other words there would be no offset DC voltage present.

The ideal response is not achievable because all systems have a finite bandwidth and as a result a response curve known as an "S" curve is obtained. Outside the bandwidth of the system, the response falls, as would be expected. It can be seen that the frequency variations of the signal are converted into voltage variations which can be amplified by an audio amplifier before being passed into headphones, a loudspeaker, or passed into other electronic circuitry for the appropriate processing.

Characteristic "S" curve of an FM demodulator

To enable the best detection to take place the signal should be centred about the middle of the curve. If it moves off too far then the characteristic becomes less linear and higher levels of distortion result. Often the linear region is designed to extend well beyond the bandwidth of a signal so that this does not occur. In this way the optimum linearity is achieved. Typically the bandwidth of a circuit for receiving VHF FM broadcasts may be about 1 MHz whereas the signal is only 200 kHz wide.

FM demodulators

There are a number of circuits that can be used to demodulate FM. Each type has its own advantages and disadvantages, some being used when receivers used discrete components, and others now that ICs are widely used.

Below is a list of some of the main types of FM demodulator or FM detector. In view of the widespread use of FM, even with the competition from digital modes that are widely used today, FM demodulators are needed in many new designs of electronics equipment.

Slope FM detector
Foster-Seeley FM detector
Ratio detector
PLL, Phase locked loop FM demodulator
Quadrature FM demodulator
Coincidence FM demodulator

Luiggi Escalante
CI 18.878.611
CRF
Fuente: http://www.radio-electronics.com/info/receivers/fm_demod/fm_demodulation.php

Synchronous demodulation / detection

Today's radio receivers offer very high levels of performance and boast many facilities. Many radio receivers incorporate memories, phase locked loops, direct digital synthesis, digital signal processing and much more. One facility that can be very useful on the short wave bands is synchronous detection or synchronous demodulation as this can give much improved performance for receiving amplitude modulation (AM) transmissions. Unfortunately little is written about this form of modulation, and often it is a matter of accepting that it must be better than any normal options because it is included as a feature in the receiver specification.
Synchronous detection is used for the detection or demodulation of amplitude modulation (AM). This form of modulation is still widely used for broadcasting on the long, medium and short wave bands despite the fact that there are more efficient forms of modulation that can be used today. The main reason for its use nowadays is that it is very well established, and there are many millions of AM receivers around the world today.
In any receiver a key element is the detector. Its purpose is to remove the modulation from the carrier to give the audio frequency representation of the signal. This can be amplified by the audio amplifier ready to be converted into audible sound by headphones or a loudspeaker. Many receivers still use what is termed an envelope detector using a semiconductor diode for demodulating AM. These detectors have a number of disadvantages. The main one is that they are not particularly linear and distortion levels may be high. Additionally their noise performance is not particularly good at low signal levels.
These detectors also do not perform very well when the signal undergoes selective fading as often occurs on the short wave bands. An AM signal contains two sidebands and the carrier. For the signal to be demodulated correctly the carrier should be present at the required level. It can be seen that the signal covers a definite bandwidth, and the effects of fading may result in the carrier and possibly one of the sidebands being reduced in level. If this occurs then the received signal appears to be over-modulated with the result that distortion occurs in the demodulation process.

The spectrum of an amplitude modulated signal

Diode envelope detector

In virtually every receiver a simple diode envelope detector is used. These circuits have the advantage that they are very simple and give adequate performance in many applications.
The circuit of a typical detector is shown in Figure 2. Here the diode first rectifies the signal to leave only the positive or negative going side of the signal, and then a capacitor removes any of the remaining radio frequency components to leave the demodulated audio signal. Unfortunately diodes are not totally linear and this is the cause of the distortion.

An envelope detector for AM signals

What is synchronous demodulation

Signals can be demodulated using a system known as synchronous detection or demodulation. This is far superior to diode or envelope detection, but requires more circuitry. Here a signal on exactly the same frequency as the carrier is mixed with the incoming signal as shown in Figure 2. This has the effect of converting the frequency of the signal directly down to audio frequencies where the sidebands appear as the required audio signals in the audio frequency band.
The crucial part of the synchronous detector is in the production a local oscillator signal on exactly the same frequency as the carrier. Although it is possible to receive an AM signal without the local oscillator frequency on exactly the same frequency as the carrier this is the same as using the BFO in a receiver to resolve the signal. If the BFO is not exactly on the same frequency as the carrier then the resultant audio is not very good.

Synchronous demodulation Fortunately this is not too difficult to achieve and although there are a number of ways of achieving this the most commonly used method is to pass some of the signal into a high gain limiting amplifier. The gain of the amplifier is such that it limits, and thereby removing all the modulation. This leaves a signal consisting only of the carrier and this can be used as the local oscillator signal in the mixer as shown in Fig. 4. This is most convenient, cheapest and certainly the most elegant method of producing synchronous demodulation.

A synchronous detector using a high gain-limiting amplifier to extract the carrier

Advantages of synchronous detection

A synchronous detector is more expensive to make than an ordinary diode detector when discrete components are used, although with integrated circuits being found in many receivers today there is little or no noticeable cost associated with its use as the circuitry is often included as part of an overall receiver IC.
Synchronous detectors are used because they have several advantages over ordinary diode detectors. Firstly the level of distortion is less. This can be an advantage if a better level of quality is required but for many communications receivers this might not be a problem. Instead the main advantages lie in their ability to improve reception under adverse conditions, especially when selective fading occurs or when signal levels are low.
Under conditions when the carrier level is reduced by selective fading, the receiver is able to re-insert its own signal on the carrier frequency ensuring that the effects of selective fading are removed. As a result the effects of selective fading can be removed to greatly enhance reception.
The other advantage is an improved signal to noise ratio at low signal levels. As the demodulator is what is termed a coherent modulator it only sees the components of noise that are in phase with the local oscillator. Consequently the noise level is reduced and the signal to noise ratio is improved.
Unfortunately synchronous detectors are only used in a limited number of receivers because of their increased complexity. Where they are used a noticeable improvement in receiver performance is seen and when choosing a receiver that will be used for short wave broadcast reception it is worth considering whether a synchronous detector is one of the facilities that is required.

Luiggi Escalante
CI 18.878.611
CRF
Fuente:
http://www.radio-electronics.com/info/receivers/synchdet/sync_det.php

Radio receiver amplitude modulation (AM) demodulation

One of the advantages of amplitude modulation (AM) is that it is cheap and easy to build a demodulator circuit for a radio receiver. The simplicity AM radio receivers AM is one of the reasons why AM has remained in service for broadcasting for so long. One of the key factors of this is the simplicity of the receiver AM demodulator.

A number of methods can be used to demodulate AM, but the simplest is a diode detector. It operates by detecting the envelope of the incoming signal. It achieves this by simply rectifying the signal. Current is allowed to flow through the diode in only one direction, giving either the positive or negative half of the envelope at the output. If the detector is to be used only for detection it does not matter which half of the envelope is used, either will work equally well. Only when the detector is also used to supply the automatic gain control (AGC) circuitry will the polarity of the diode matter.

The AM detector or demodulator includes a capacitor at the output. Its purpose is to remove any radio frequency components of the signal at the output. The value is chosen so that it does not affect the audio base-band signal. There is also a leakage path to enable the capacitor to discharge, but this may be provided by the circuit into which the demodulator is connected.

A simple diode detector or demodulator for AM signals

A simple diode detector (demodulator) for AM signals

This type of detector or demodulator is called a linear envelope detector because the output is proportional to the input envelope. Unfortunately the diodes used can introduce appreciable levels of harmonic distortion unless modulation levels are kept low. As a result these detectors can never provide a signal suitable for high quality applications.

Additionally these detectors ( demodulators ) are susceptible to the effects of selective fading experienced on short wave broadcast transmissions. Here the ionospheric propagation may be such that certain small bands of the signal are removed. Under normal circumstances signals received via the ionosphere reach the receiver via a number of different paths. The overall signal is a combination of the signals received via each path and as a result they will combine with each other, sometimes constructively to increase the overall signal level and sometimes destructively to reduce it. It is found that when the path lengths are considerably different this combination process can mean that small portions of the signal are reduced in strength. An AM signal consists of a carrier with two sidebands.

Spectrum of an amplitude modulated (AM) signal

If the section of the signal that is removed falls in one of the sidebands, it will change the tone of the received signal. However if carrier is removed or even reduced in strength, the signal will appear to be over modulated, and severe distortion will result. This is a comparatively common occurrence on the short waves, and means that diode detectors are not suitable for high quality reception. Synchronous demodulation ( detection ) is far superior.

Luiggi Escalante

CI 18.878.611

CRF

Fuente:
http://www.radio-electronics.com/info/receivers/am_demod/diode_am_demod.php

Light dependent resistor or photo resistor

The light dependent resistor, LDR, is known by many names including the photoresistor, photo resistor, photoconductor, photoconductive cell, or simply the photocell. These devices have been seen in early forms since the nineteenth century when photoconductivity in selenium was discovered by Smith in 1873. Since then many variants of photoconductive devices have been made.

Other light dependent resistors, or photo resistors have been made using materials including cadmium sulphide, lead sulphide, and the more commonly used semiconductor materials including germanium, silicon and gallium arsenide.

The photo resistor, or light dependent resistor, LDR, finds many uses as a low cost photo sensitive element and was used for many years in photographic light meters as well as in other applications such as flame, smoke and burglar detectors, card readers and lighting controls for street lamps.

Basic structure

Although there are many ways in which light dependent resistors, or photo resistors can be manufactured, there are naturally a few more common methods that are seen. Essentially the LDR or photoresisitor consists of a resistive material sensitive to light that is exposed to light. The photo resistive element comprises section of the material with contacts at either end. Although many of the materials used for light dependent resistors are semiconductors, when used as a photo resistor, they are used only as a resistive element and there are no pn junctions. Accordingly the device is purely passive.

A typical structure for a light dependent or photo resistor uses an active semiconductor layer that is deposited on an insulating substrate. The semiconductor is normally lightly doped to enable it to have the required level of conductivity. Contacts are then placed either side of the exposed area.

In many instances the area between the contacts is in the form of a zig zag, or interdigital pattern. This maximises the exposed area and by keeping the distance between the contacts small it enhances the gain.

It is also possible to use a polycrystalline semiconductor that is deposited onto a substrate such as ceramic. This makes for a very low cost light dependent resistor

In order to ensure that the resistance of the light dependent area of the device is the major component of the resistance, all other spurious resistances must be minimised. A major contributor could be the resistance between the contact and the semiconductor. To reduce this component of resistance, The region around the metal contact is heavily doped to increase its conductivity.

Luiggi Escalante

CI 18.878.611

CRF

Fuente:
http://www.radio-electronics.com/info/data/resistor/ldr/light_dependent_resistor.php

RF Mixer

RF mixers are important components for RF design. The correct operation of an RF mixer is essential to any RF design, and part of this process is to ensure that the correct RF mixer specification is generated to choose the correct component for the particular RF design or circuit. In view of the high levels of performance required in some RF circuits and RF designs, it is often appropriate to buy RF mixers as components from specialist suppliers. These RF mixers are able to provide very high levels of performance and normally better than those that could be obtained by using discrete components on a circuit board.

When determining the correct RF mixer for a particular RF design or circuit, there are a few key RF mixer specifications that need to be known. Some are easy to define, but others may need a little more knowledge of the particular circuit of RF design being undertaken.

RF mixer connections or ports

When specifying RF mixers, reference is often made to the three ports of the mixer. There are two input ports and one output port. These ports are all identified separately as each one has different characteristics. The signal input is often designated "RF" or "RF input". The other input is typically connected to the local oscillator and is normally termed "LO". The output is normally designated "IF" for intermediate frequency.

Circuit symbol for an RF mixer with ports identified

Circuit symbol for an RF mixer with the different ports identified

Key RF mixer specifications

Although many elements of the performance of an RF mixer, there are a number that are of key importance, and they are required for the basic specification of the mixer.

Mixer type
Frequency range
Impedance
Input levels
Conversion loss / gain
Isolation
Noise figure
Spurious outputs

RF mixer type

There is a wide variety of different RF design or circuit configurations for mixers. Typically the types that can be bought as circuit modules or items are of the double balanced diode ring variety. However it is also possible to design single diode mixers, balanced diode mixers, bipolar mixers, FET mixers, etc. As diode mixers do not have any gain, this must be acceptable for the circuit in question.

Frequency range

No single RF mixer will be able to operate at all frequencies. The circuit construction of components will determine the range over which the RF mixer can operate. Typically the double balanced mixers bought as circuit components contain transformers and these are the main frequency limiting element. Despite these frequency limitations, RF mixers are normally able to operate over considerable frequency ranges. However the frequency range must be considered when ordering an RF mixer.

RF mixer impedance

The standard impedance for RF mixers is 50 ohms. It is important to ensure that the source impedance for the inputs, and the load for the output is accurately matched to the required impedance. Often small attenuators may be added into the line to ensure that this is the case. If the ports are not accurately matched to the required impedance, mixer specifications such as the spurious signal and isolation will be impaired.

Input levels

It is important to ensure that the input levels for the RF mixer are met. The RF input will have a maximum input level it can tolerate. Beyond this the mixer may become overloaded and the levels of spurious signals will rise above their limits, along with the isolation falling outside its specification.

The LO input is designed to have a certain input level and this should be maintained reasonably accurately. If it rises too high then higher levels of LO signal will appear at the output. Additionally higher levels of spurious may be experienced. If the signal level is too low, then the conversion loss may increase. Typically a tolerance of + / - 3dB is normally acceptable.

There are a few standard LO input levels. 7 dB is standard for many RF mixing applications, whereas higher LO levels may be required where higher elvels of RF input level may be needed.

Mixer conversion loss

The conversion loss of an RF mixer is a measure of its efficiency. The RF mixer conversion loss is defined as the ratio of the level of one of the output sidebands to the level of the RF input. This ratio is expressed in dB.

As only half of the input power can ever exist in one of the sidebands, this means that the best conversion loss that can ever exist in a diode mixer is 3dB. However other losses are also present in the mixer. These occur as a result of diode insertion loss, spurious signal generation, transformer core loss, etc. As would be expected, the conversion loss of the RF mixer will deteriorate towards the edges of the specified frequency band, mainly as a result of the transformer losses increasing.

The conversion loss is also found to be a function of the carrier drive level, increasing particularly if the LO drive level is not correct. However it is also found that the conversion loss can be improved by providing a short circuit impedance at the output port for the undesired sideband. It is also necessary to provide external decoupling for the IF and RF ports of a single balanced mixer in order to achieve the specified conversion loss.

Isolation

The port to port isolation is one of the parameters of an RF mixer that is of particular importance in most RF mixer applications. RF mixer isolation is defined as the ratio of the signal power available into one port of the mixer to the measured power level of that signal at one of the other mixer ports in a 50 ohm system. This is measured in dB.

Most RF mixer designs aim for the maximum isolation from the LO to the RF ports as the LO signal is normally the highest level. The LO to RF isolation is generally slightly less at the higher frequencies, and this is often important to ensure that the LO signal does not enter the RF drive circuitry and cause problems such as intermodulation. Lastly the RF to IF isolation is the poorest.

The in a diode balanced or double balanced RF mixer, the isolation is a determined by the equality of the diode dynamic characteristics and the accuracy of the transformer's balance. It is also found that a high level of second harmonic in the LO signal will also degrade the isolation performance because the isolation deteriorates at high frequencies as a result of parasitic capacitances.

Noise figure

The noise figure for an RF mixer is important in radio receiver front end circuits. Any noise introduced by the RF mixer at an early stage in the receiver such as the first mixer will degrade the performance of the whole receiver. In this way the noise figure for the RF mixer is important. Above very low frequencies (typically around 10 kHz) the Schottky Barrier diodes used in balanced and double balanced diode mixers contribute negligible levels of noise. This means that the noise figure for the RF mixer is essentially its conversion loss.

Spurious signals

The basic mathematics used to illustrate the way in which an RF mixer works assumes that it is a perfect multiplier. Unfortunately this is not the case, and signals in addition to the sum and difference frequencies are generated. The transfer characteristics for the diodes mean that harmonics for the input signals are generated. If the RF mixer was perfectly balanced, then these components would be cancelled out, but again this is not the case, and as a result, the harmonics of the two input signals also mix together. The lower odd ordered mixer products will be the highest level at the IF port or output of the RF mixer.

The levels of the spurious signals are dependent upon a number of factors. These include the LO and RF input levels as well as parameters such as the load impedance, temperature, frequency, etc. Many of these unwanted products will fall outside the required passband of the mixer, but most manufactured RF mixers will have data for these unwanted products which can be obtained from the manufacturer.

Summary

RF mixers are an important element in most RF designs. Accordingly it is necessary to understand their limitations and know how to specify them to obtain the required performance.

Luiggi Escalante

CI 18.878.611

CRF

Fuente: http://www.radio-electronics.com/info/rf-technology-design/mixers/rf-mixers-specifications.php

RF Coax Connectors

There are a number of commonly available RF connectors that are used with coax cable to provide screened connections. These connectors are used in a number of areas whether to carry RF or radio frequency signals or just to provide a much higher level of screening than would be possible if more ordinary "open" connectors were used.

RF connectors are used in many areas. Naturally their main uses are associated with RF applications. Everything from domestic television, through CB and ham radio to the large number of commercial and industrial applications. However RF or coaxial connectors are also used in areas where screening is one of the major priorities rather than the fundamental RF properties. Coaxial connectors are widely used with a variety of test instruments. For example RF connectors are used on oscilloscopes. These and many other applications all use RF connectors.

RF connector basics

Many connectors, such as the D-type connectors and many other multiway connectors consist of a series of pins with connections in parallel to each other. RF coaxial connectors need to retain the coaxial nature of the cable they are used with. As a result they consist of a central pin for the inner of the coax cable, and then an outer connection around the inner for the outer conductor on the cable. This makes these RF connectors very different to other, more "traditional" connectors.

Coaxial cable has a number of properties, one of which is the characteristic impedance. In order that the maximum power transfer takes place from the source to the load, the characteristic impedances of both should match. Thus the characteristic impedance of a feeder is of great importance. Any mismatch will result in power being reflected back towards the source. It is also important that RF coaxial cable connectors have a characteristic impedance that matches that of the cable. If not, a discontinuity is introduced and losses may result.

There is a variety of connectors that are used for RF applications. Impedance, frequency range, power handling, physical size and a number of other parameters including cost will determine the best type for a given applications.

RF connector types

There is a large host of different types of RF connector. Some are in widespread use whereas others are les widely used. Some of those in widespread use and likely to be encountered in the standard electronics laboratory or by the hobbyist are:

BNC The BNC coax connector is widely used in professional circles being used on most oscilloscopes and many other laboratory instruments. The BNC connector is also widely used as an RF connector, being used on RF test equipment, transmitters, receivers and almost any RF equipment. The BNC connector has a bayonet fixing to prevent accidental disconnection while being easy to disconnect when necessary.

Electrically the BNC coax cable connector is designed to present a constant impedance and it is most common in its 50 ohm version, although 75 ohm ones can be obtained. It is recommended for operation at frequencies up to 4 GHz and it can be used up to 10 GHz provided the special top quality versions specified to that frequency are used.
N-type The N-type connector is a high performance RF coaxial connector used in many RF applications. The N-type RF connector is larger than the BNC connector and it has a threaded coupling interface to ensure that it mates correctly. It is available in either 50 ohm or 75 ohm versions. These two versions have subtle mechanical differences that do not allow the two types to mate. The connector is able to withstand relatively high powers when compared to the BNC connector. The standard versions are specified for operation up to 11 GHz, although precision versions are available for operation to 18GHz.
UHF connector (SO239 / PL259): The UHF connector is also sometimes known as the Amphenol coaxial connector. The plug may be referred to as a PL259 coaxial connector, and the socket as an SO239 connector as these were their original military part numbers. The connectors have a threaded coupling, and this prevents them from being removed accidentally. It also enables them to be tightened sufficiently to enable a good low resistance connection to be made between the two halves.

The drawback of the UHF or Amphenol connector is that it has a non-constant impedance. This limits their use to frequencies of up to 300 MHz, but despite this these UHF connectors provide a low cost connector suitable for many applications, provided that the frequencies do not rise. Also very low cost versions are available for applications such as CB operation, and these are not suitable for operation much above 30 MHz. In view of their non-constant impedance, these connectors are now rarely used for many professional applications, being generally limited to CB, amateur radio and some video and public address systems.

The BNC, N-type and UHF (S)239 / PL259) connectors are possibly the most widely used types of connector in many circles. Although many other types of RF connector exist, they tend to be found more in specialised areas of RF technology or in higher cost applications.

Other RF connector types

There are very many other types, but these are not as likely to be encountered in more everyday usage, although some are still used in very large quantities. These include:

SMA
SMB
SMC
MCX
MMCX
C-type
TNC

Summary

RF connectors are widely used in many areas of electronics, covering frequencies from DC up to many GHz. They are an essential part of many electronics equipment, enabling screened connections to be made tot e quality required. The performance required of these RF connectors means that costs are not always cheap. While sometimes it is possible to buy cheap RF connectors, these may be of inferior quality, and they may prove to be a false economy.

Luiggi Escalante

CI 18.878.611

CRF

Fuente:
http://www.electronics-radio.com/articles/electronic_components/rf-connectors/coax-cable-connectors.php

UHF Connector

The UHF connector, also sometimes known as the Amphenol coaxial connector was designed in the 1930s by a designer in the Amphenol company for use as an RF connector in the radio industry. The UHF connector was initially intended for use as a video connector for radar equipment, but it later became used in a variety of RF applications.The plug may be referred to as a PL259 coaxial connector, and the socket as an SO239 connector. These are their original military part numbers

... was originally intended for use as a video connector, but was later used as an RF connector....

UHF connector description

These RF coaxial connectors have a threaded coupling, and this prevents them from being removed accidentally. It also enables them to be tightened sufficiently to enable a good low resistance connection to be made between the two halves. The thread for the UHF connector is a 5/8 inch 24tpi UNEF standard. It is also useful to note that the center conductor jack on the SO-239 will also accept a banana plug. This can be useful for some test applications where access is required.

PL259 RF connectors come in two sizes: for thick and thin coaxial cable. Typically the larger sized version of the PL259 would be used with RG-8/U or RG-9/U while the smaller versions are more suited to cables including RG-58/U. The basic connector remains the same, but the cable entry region is modified for the relevant type of coax cable. Thin coaxial cables are often used for short runs or 'patch' leads but not for long runs as the thinner cables have a higher loss than thicker ones. When a thin coax cable variety is needed, PL259 plugs are commonly used with a "reducer" to fit the large cable entry hole in the plug to the thin cable. It is also interesting to note that the term "PL259" originally referred to one specific mechanical design, although now it is a more generic term referring to a UHF male connector (plug).

UHF connector and reducer

UHF connector limitations

Although UHF connectors, PL259 and SO239 variants are often used in semi-professional applications they are not widely used in all areas. The drawback of the UHF or Amphenol connector is that it has a non-constant impedance across the length of the cable. This limits their use as an RF connector to frequencies of up to an absolute maximum 300 MHz.

Although the absolute maximum frequency for UHF connectors is 300 MHz, care should be taken when using them. Low cost, and hence low quality versions are often sold, and these use inferior and cheaper materials. These connectors may be sold with CB equipment that is manufactured to a price and their performance will often be suitable to frequencies of 30 MHz maximum.

Soldering UHF connectors

Soldering PL259 UHF connectors is not always easy. It is necessary to know exactly what to do to ensure a reliable connection is made. If the connector is not made properly then there is the possibility of an open or short circuit appearing.

The tools required are quite straightforward and available in most home, college or company workshops:

A tool to strip the cable - normally a sharp knife is the preferred item (but be very careful when using it!) in the absence of a special tool for this purpose.
Soldering iron, and solder.
A pair of pliers to hold the item being worked upon.
A vice or other item to hold the work is a distinct advantage.
A pair of wire cutters

NB: A word of warning - the connectors become very hot when being solder and retain their heat for some time, so be careful when handling the connector after it has been soldered.

The assembly of the PL259 can be undertaken in a few easy stages:

Strip back the outer sheath of cable to be attached to the PL259 by about 35 mm (1.5 inches). Take care when doing this not to cut into the outer conductive braid, damaging the individual copper strands.
Leave around 13 mm (0.5 inch) of the copper braid or shielding in place and then remove about 13 mm (0.5 inch)of the plastic core.
Tin the exposed central copper core of the coax cable to ensure that there is a thin but even covering of solder on the copper. This must be done relatively quickly otherwise the dielectric spacing between the outer and inner conductors of the coax will melt.
Once the cable has cooled slide the inner part of the PL259 plug over the cable with a screwing action until the copper core appears at the end of the centre pin. The trimmed shield will have become trapped between the core and the inside of the PL259. The outer sheath or covering or covering of the coax cable will ensure a snug fit and any protruding shielding can then be removed using the sharp knife.
Take the soldering iron and heat the centre pin of the PL259 and cable core. Add solder to fill the void in between the core and the plug.
Once the connector is cool, trim off any protruding core and screw back on the outer cover of the PL259.

Nuevas Tecnologias para el Acceso a INternet

INTERNET POR CABLE

Algunas empresas que ofrecen servicios de televisión por cable en México, han introducido al mercado un innovador sistema que a través de un dispositivo denominado Cablemodem permite conectar tu computadora a Internet, con una velocidad hasta 10 veces superior a la de un sistema telefónico tradicional.

Esta nueva tecnología te permite conectar tu computadora con Internet a una super velocidad de 256Kbps (es la más común, pero también hay de 128 Kbps y 512 Kbps). Cabe señalar que la velocidad de conexión obtenida por medio de una línea telefónica estándar es alrededor de 50kbps.
Esto se logra gracias a que tanto la señal que recibes como la que envías viajan a través de una red híbrida de fibra óptica y cable coaxial (HFC), a una velocidad y ancho de banda mucho mayor que la soportada en una línea telefónica común.
Para interconectar la red híbrida a la computadora se utiliza un Cablemódem, el cual se conecta a una tarjeta de red que deberás tener instalada en tu computadora.
El Cablemódem se encarga de regular la velocidad de transmisión y recepción de datos. Al encender tu computadora automáticamente estarás en línea, tendrás acceso directo en cualquier instante que lo requieras las 24 horas del día, de manera similar a la señal de tu televisor.

Hay que aclarar que dependiendo de la infraestructura instalada por el proveedor de servicios, este tipo de conexión se puede ofrecer en alguna de las siguientes modalidades:

Modalidad de retorno telefónico.- Consistente en que el usuario recibirá la señal de Internet a través del cable coaxial, pero si desea enviar algún dato tendrá que hacerlo por medio de una línea telefónica, es decir deberá utilizar una conexión convencional. Por lo tanto, es necesario activar dos conexiones para contar con acceso completo a Internet.

Modalidad de doble vía.- Esta es la modalidad ideal, consistente en que toda la información que se envía y recibe, viaja a través del cable coaxial.

Contratar los servicios de una compañía que brinde el servicio en tu localidad.
Un Cablemódem que te proporciona la empresa con la que contrates el servicio.
Una computadora PC, Mac o Laptop con una velocidad superior a los 100Mhz.
Una tarjeta de red ETERNET 10/100 baseT.
Un navegador de Internet instalado en tu computadora, como por ejemplo Internet Explorer, Netscape, Opera o el de tu elección.
En el caso de que el servicio sea a través de la modalidad de retorno telefónico, necesitarás además de una línea telefónica.

Para la modalidad de doble vía, se cuenta con una alta velocidad de transmisión y recepción de datos (256Kbps), y para la modalidad de retorno telefónico únicamente alta velocidad de recepción.
Por ejemplo, un módem telefónico tardaría hasta 40 minutos en bajar un archivo de video de 10 Mb, con este servicio puedes hacerlo en solamente 5 minutos.
Permite recibir a través de Internet: gráficos de alta calidad, audio con calidad de CD y vídeo en tiempo real.
Señal de excelente calidad.
Se otorga una cuenta de correo electrónico, con una capacidad de almacenamiento de alrededor de 10 Mb.
Para usos domésticos y empresariales se cuenta con la posibilidad de conectar varios equipos de cómputo a un mismo Cablemódem (normalmente existe un costo adicional por equipo adicional).
Se garantiza un alto nivel de seguridad.
Ahorro del costo telefónico.
Se puede hacer uso de la televisión al mismo tiempo que se está conectando al Internet, ya que lo único que hay que hacer es instalar un splitter para hacer una derivación hacia el Cablemódem.

Debido al alto costo económico que implica la instalación de la infraestructura requerida para brindar este servicio, se presentan las siguientes desventajas:

No se cuenta con cobertura a nivel nacional. Únicamente en las ciudades más pobladas o con mayor demanda. Además no todas las zonas con covertura ofrecen la modalidad de doble vía.

El costo de este servicio para los usuarios es de casi el doble del pagado para el acceso vía telefónica, pero es muy razonable si consideramos la ventaja ofrecida en velocidad.

Finalmente podemos decir que el acceso a Internet por cable tendrá en un futuro muy cercano mayores oportunidades de negocio, ya que para las nuevas generaciones el acceso a Internet a grandes velocidades resultará muy atractivo, por no decir indispensable.
Además las posibilidades que brindará la red de televisión por cable, abarcarán en forma integrada señales de televisión, telefonía e Internet.

Luiggi Escalante

CI 18.878.611

CRF

Fuente: http://www.cuentame.inegi.gob.mx/museo/estreno/cablemodem.htm

Nuevas Tecnologias para el acceso a Internet

Internet Por Cable

Algunas empresas que ofrecen servicios de televisión por cable en México, han introducido al mercado un innovador sistema que a través de un dispositivo denominado Cablemodem permite conectar tu computadora a Internet, con una velocidad hasta 10 veces superior a la de un sistema telefónico tradicional.

Hay que aclarar que dependiendo de la infraestructura instalada por el proveedor de servicios, este tipo de conexión se puede ofrecer en alguna de las siguientes modalidades:

Modalidad de retorno telefónico.- Consistente en que el usuario recibirá la señal de Internet a través del cable coaxial, pero si desea enviar algún dato tendrá que hacerlo por medio de una línea telefónica, es decir deberá utilizar una conexión convencional. Por lo tanto, es necesario activar dos conexiones para contar con acceso completo a Internet.

Modalidad de doble vía.- Esta es la modalidad ideal, consistente en que toda la información que se envía y recibe, viaja a través del cable coaxial.

Contratar los servicios de una compañía que brinde el servicio en tu localidad.
Un Cablemódem que te proporciona la empresa con la que contrates el servicio.
Una computadora PC, Mac o Laptop con una velocidad superior a los 100Mhz.
Una tarjeta de red ETERNET 10/100 baseT.
Un navegador de Internet instalado en tu computadora, como por ejemplo Internet Explorer, Netscape, Opera o el de tu elección.
En el caso de que el servicio sea a través de la modalidad de retorno telefónico, necesitarás además de una línea telefónica.

Para la modalidad de doble vía, se cuenta con una alta velocidad de transmisión y recepción de datos (256Kbps), y para la modalidad de retorno telefónico únicamente alta velocidad de recepción.
Por ejemplo, un módem telefónico tardaría hasta 40 minutos en bajar un archivo de video de 10 Mb, con este servicio puedes hacerlo en solamente 5 minutos.
Permite recibir a través de Internet: gráficos de alta calidad, audio con calidad de CD y vídeo en tiempo real.
Señal de excelente calidad.
Se otorga una cuenta de correo electrónico, con una capacidad de almacenamiento de alrededor de 10 Mb.
Para usos domésticos y empresariales se cuenta con la posibilidad de conectar varios equipos de cómputo a un mismo Cablemódem (normalmente existe un costo adicional por equipo adicional).
Se garantiza un alto nivel de seguridad.
Ahorro del costo telefónico.
Se puede hacer uso de la televisión al mismo tiempo que se está conectando al Internet, ya que lo único que hay que hacer es instalar un splitter para hacer una derivación hacia el Cablemódem.

Debido al alto costo económico que implica la instalación de la infraestructura requerida para brindar este servicio, se presentan las siguientes desventajas:

No se cuenta con cobertura a nivel nacional. Únicamente en las ciudades más pobladas o con mayor demanda. Además no todas las zonas con covertura ofrecen la modalidad de doble vía.

El costo de este servicio para los usuarios es de casi el doble del pagado para el acceso vía telefónica, pero es muy razonable si consideramos la ventaja ofrecida en velocidad.