Circuits and Components for system evaluations and desing - Mixers

RF mixers are used in systems to perform cocoordinated frequency changes. In addi­tion to this, a suitable mixer can be used as a phase detector or as a demodulator. Commercial products are available through the entire spectrum starting from 10 kHz and exceeding 100 GHz. Internal circuit topologies include one-, two-, and four-diode assemblies [13]. Four diodes are normally connected as a double-balanced mixer, two as a balanced device.

A very basic mixer application is illustrated in Figure 4.17. We have some initial RF signal, which we want to transpose to an IF. The mixer needs a second input, the local oscillator (LO) power, to perform this conversion. Critical performance figures of a mixer include its conversion loss, port matching, isolation between various ports, and naturally the frequency range of all three ports. Often the ports do not have equal character!sties. Practical applications must often be designed for a certain LO power level, for example +7 dBm |3J. Sometimes—mainly in phase detection applications as indicated in Figure 4.18—we have to know the dc polarity and dc offset of the IF port.

One of the first unfortunate characteristics of mixers is the real IF spectrum, which tends to contain an overwhelming combination of mutual sums and differ­ences as predicted by the common mixer equation [14]. The user has to provide suit­able filtering at the IF port to select the signal of interest, but this is not enough. We must also take care of the purity of the LO signal and reduce the spectrum coming to the RF input due to similar reasons. An enhanced block diagram is illustrated in

                                       

figure 4.17   A simple RF mixer application. The first VCO produces an RF signal, and Ihe LO pro­duces Ihe mixer LO input; and the mixer generates the respective IF spectrum, which is filtered.

                            

Figure 4.18    If two signals of precisely equal frequency are fed to the double-balanced mixer's RF and LO ports, the IF signal will be a dc voltage proportional to their phase difference, plus a set of higher sum frequencies. Note the sharp lowpass filter, which is appropriate as we are only inter­ested in the dc output.

Figure 4.19. Such an arrangement is quite easy if our system uses only one prede­fined carrier frequency, but far from simple when tunability is needed. Besides, put­ting normal filters in mixer ports involves the risk of perfect mismatch because bandpass filters typically show very poor matching in their stopbands. Multiple reflections may cause oui-of-spec conversion loss variations and yield to degraded intcrmodulation performance. However, filters better in this respect generally pro­vide much less selectivity in the frequency domain.

Assume that we want CO construct a wideband RX. This makes sharp RF filter­ing impossible, and a fixed LO frequency cannot be used. The only way of circum­venting the problem is to have tunable filters, which are often called tracking filters, because their center frequency is assumed to follow that of the system tuning. Wc can alternatively choose a filter bank in front of the mixer and use switches to select the appropriate unit for the specific frequency range at hand. These two con­figurations are highlighted in Figures 4.20 and 4.21. Electronically tuned filters provide speed and may so seem attractive for example in an electronic support

                                  

Figure 4.19   Adding suitable filters at all three mixer ports is quite mandatory in many real sys­tems to get a desired IF output. The RF input is filtered to prevent the arrival ol possible Image fre­quencies, the LO is sharply filtered for best possible purity, and finally we select from the raw IF spectrum only the band of interest (or further processing.

                              

Figure 4.20   Tunable fillers before the RF port can solve the problem ol wide input bandwidth, but selectivity will generally be degraded.

          

Figure 4.21   A filter bank can be a solution if we need wide bandwidth and good selectivity. However, tracking speed is typically much worse than in a tunable filter RX.

measures (ESMs) system, but their drawback is the poor stopband attenuation and sluggish rejection slopes.

Different mixer topologies yield varying conversion losses, but generally 7 to 8 dB is commercially available up to the high microwave frequencies. This figure depends heavily on the LO power level—if lower than suggested, the conversion loss increases drastically. The frequency range of the RF and I.O ports is from about 1 to 2,000 MHz in VHF and UHF mixers and above that often covers one to two octaves in one unit. IF bandwidths come in some relation to the two other ports so that the low-frequency mixers have dc to 1,000 MHz and microwave devices start from 1 GHz and cover to about one-third or one-fourth of the upper RF limit. The higher the frequencies involved, the worse the isolation figures. Wc must often accept 20 to 30 dB as a good result cither for LO-RF or for LO-1F even though manufacturers like to indicate respective figures (40-50 dB) for very low frequencies. This rather unavoidable feature may be a serious limitation on the system level. Consider the case presented in Figure 4.22. We have a mi Hi meter-wave oscillator and want to use that to form a very simple low-IF radar warning RX. The oscillator feeds a mixer

                     

Figure 4.22 An example erf a real situation where the poor LO-RF isolation o( a mixer may pre-vent the operation of a proposed system. Here, the feed-through of the LO signal can be strong enough to reveal the RX and cause hostile ARM activity.

LO pore. If an external radar is active, its signal goes from the antenna to the mixer, and the respective IF will be detected. However, the leakage of the LO from our RX may be strong enough to alert the enemy of the presence of a warning device and to be used as an ARM guidance signal.

Some means exist to enhance the performance of our simple RX. First, we may redesign the IF part to allow a larger separation between RF and LO frequencies. In the original layout the IF frequency was 80 MHz, which is nice for direct detection or amplification but will cause the millimctcr-wavc frequencies (for example the 35-40 GHz range) to be so close together that any conventional filtering will fail. If, on the other hand, we choose a considerably higher first IF, for example, around 2 GHz, we can insert a bandpass filter before the RF port of the mixer and so reduce the amplitude of the LO frequency at the antenna interface. Another possibility is to put an isolator between the antenna and the mixer. However, this would give per­haps only some 20 dB of reduction and unfortunately would completely destroy the NF due to the 1- to 2-dB insertion loss. Now that we have a higher IF, we might run into processing problems. These can he overcome by adding a second mixer to con­vert the 2-GHz signal into our original 80 MHz, if desired. Adequate filtering must be installed as indicated in Figure 4.23.

A special form of RF mixer is the diode multiplier, into which only one RF sig­nal is fed. They generate normally harmonic multiples of their input and can thus be used as frequency-extension devices (e.g., fur microwave oscillators). Similar limita­tions are valid here, too. The initial power level must be high enough and consider­able filtering is mandatory. Available output levels tend to be fractional particularly, if very high multiplying factors are needed. An example of multiplier usage is shown later in Section 6.2.

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Libro: Circuits  and Components for system  evaluations and desing
Autor: Pekka Eskelinen

Nombre: Josmar Eduardo Depablos Rodriguez
Asignatura: Circuitos de Alta Frecuencia

Circuits and Components for system evaluations and design- Filters

We use filters to select or reject certain signals. A very important task of filters is the reduction of input noise, because the noise bandwidth is rather close to a 3-dB

                                            

Figure 4.3   This is how or why power divider phase character!sties affect antenna array perform­ance. Any phase differences of the divider will be direclly added lo the phasing network values.

bandwidth in most RF equipment. The main characteristics of filters are their fre­quency response {both attenuation and group delay) and impedance matching (4j. Often manufacturers specify the passband attenuation and stopband attenuation separately. Many filters are reflective in nature, which means that there is large mismatch for frequencies within the stopband \S\. The losses in the passband should normally be a lot below 1 dB, but at very high millimeter-wave frequencies this may turn out to be difficult. Some simple filters do not give much more than 20 to 30 dB of attenuation in their stopband, but most are capable of 50 to 60 dB or even more.

Commercially available filters are found in all four main categories: lowpass, highpass, bandpass, and band reject [3]. Normally, the frequency response is fixed. We can also purchase tunable components, which arc based on pure mechanical adjustments. Sometimes the adjusting mechanism is driven by a stepper motor. Faster and durable devices rely on electronics, which can be found as varactor diode designs and as yttrium iron garnet (YIG) blocks. Tuning typically compromises other performance figures. Mechanically tuned filters show generally better attenuation responses but suffer from slow speed and wear in use. YIG and varactor filters can be very fast—several gigahertz per second—but this is achieved only when a couple of decibels of additional passband attenuation and not so steep slopes can be tolerated. The systems designer can obtain additional degrees of free­dom by suitably combining low and high pass designs in order to get tailored pass bands. One such result is illustrated in Figure 4.4. Physical constructions include coaxial ("tubular") filters, microstrip and stripline designs (also using air as the dielectric), and waveguides. Narrowband filters are made in the surface acoustic wave (SAW) scheme or as piezoelectric crystals. Although filters arc normally very low-loss devices also in the stopband, they still have an upper limit of signal levels that thev can handle.

Initially, we have to find a filter having the correct frequency range. This is not particularly complicated if we arc dealing with a predefined system {e.g., SSR radar). Then we have to check the interface (i.e., coaxial or waveguide) and the passband loss. This might be surprisingly high in a tunable device even at moderate

                             

Figure 4.4   Suitably selected commercial lowpass and highpass Tillers in cascade can provide a semicustom passband for our system. This shows the response when a highpass filter with a cutoff at 530 MHz is used in series with a lowpass filter with a 630-MHz cutoff. Actually, both filters are sold as 600-MHz devices.

RF frequencies. Many filters have considerable attenuation ripple in the passband (see Figure 4.5), which in FM systems may cause unwanted FM-to-AM conversion. The real struggle in filter selection is often in getting the wanted low passband attenuation in conjunction with sufficient rejection capabilities quite close to the passband. Steep filters are often sought. Modern systems present a further difficulty through the group delay requirement. If a filter has steep slopes in attenuation, we may find severe fluctuations in the delay curve close to the passband edges as indi­cated in Figure 4.6, and some suggested design procedures unfortunately yield to less satisfactory results (6). So-called constant-delay filters are special designs intended to overcome this problem |7|. Unfortunately, they are not broadly available as ready-made units for arbitrary frequencies.

                             

Figure 4.5   Excessive attenuation ripple In the lilter passband (around A) may cause unwanted FM-to-AM conversion in frequency-modulated systems.

                             

Figure 4.6 If a fitter has a reasonably steep amplitude response as desired in many systems, its phase characteristics might be far from linear. Here we show the group delay performance of a seven-stage stripline filter.

One of the cases in which custom designs are easily justified is a tailored system-specific filter. This partly comes from the fact that we—and the whole sys­tem—can benefit from a suitably narrow bandwidth—just tuned to our needs whereby the noise input will be lowest. Military radio systems require specific filters also due to enhanced jamming resistance [8], Of course, exact filter characteristics are kept as classified information due to their importance in electronic countermea-surcs (ECM) and antijamming (AJ) tasks. Examples of design equations and related data can be found in |2]. Recent trials in the author's team with selected commercial simulation software packages have indicated that a fully functional RF filter still requires at least one physical manufacturing iteration cycle for optimum perform­ance [9|. For example, the time needed from the announcement of system specifica­tions to produce the first-in-scries stripline bandpass filter (prototype illustrated in Figure 4.7) for an L-band radar was about 3 months.

                           

Figure 4.7   A high performance stripline bandpass filler (or L-band radar. Completing this all-milled design from the announcement of system specifications took about 3 months.

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Libro:  Circuits and Components for System Evaluations and Design

Autor: Pekka Eskelinen

Nombre: Josmar Eduardo Depablos Rodriguez

Asignatura: Circuitos de Alta Frecuencia

 

Passive Modules

Devices that do not contain semiconductors in their RF paths are classified here as passive. Typical items falling into this category include coaxial or waveguide termi­nations, attenuators, power splitters and combiners, isolators, and filters [1|. Although completely mechanical RF switches do exist and arc frequently used as well, that topic is postponed to Section 4.3, where active modules are discussed in detail.

4.2.1    Terminations

The purpose of an RF termination is to provide a well-known and stable impedance to a Transmission line port, which might belong, for example, to an amplifier or to a power divider. The key performance figures arc the impedance mismatch, the band­width across which that value is maintained, and the power-handling capability. Very high power terminations arc often called dummy loads, because their main usage is in PA and TX testing. Naturally, we must find a termination having the suit­able mating interface (e.g., an appropriate coaxial connector or a waveguide flange). Coaxial terminations are available for 50- and 75-ohm systems, but only excep­tional designs use the latter variant. Most commercial types can withstand CW power up to 100 mW or slightly more and have SWRs better than 1.2 to 26 GHz. Some waveguide terminations are specified below 1.1 but are usable only within the respective guide bandwidth [2]. Microstrip and stripline systems generally do not have ready-made termination modules available but the designer has to use surface-mount device (SMD) resistors having the correct resistance and negligible inducrancc or capacitance. Precision terminations are mainly used as calibration standards for network analyzers and related equipment. For these metrology-grade applications sliding terminations are also manufactured. They enable one to adjust the phase of the residual reflection so that its effect on the overall measurement uncertainty can be estimated.

Generally, terminations do not cause too much trouble in systems design, if devices of adequate quality and bandwidth are purchased. On some occasions

voltage transients may destroy a termination even though we do not exceed the maximum power. The main reason for damage is improper handling. This is par­ticularly true of small coaxial 1.8-mm, 2.4-mm, 3.5-mm, SMA. or K-type connector devices, which do not withstand excessive force or misalignment during mating. Coaxial modules come as male and female alternatives for the best impedance matching.

4.2.2   Attenuators

The main goal of an RF attenuator is—as the name implies—attenuation. However, we must rake into account, the frequency range, the unavoidable mismatch at both of the ports, the phase or group delay response, and the power-handling capability. Commercially manufactured devices arc often processed in a series of 1-2-3-5-10 dB and from there on, in steps of 10 dB up to about 100 or 120 dB. Both fixed and step attenuators are available, and their control can be either mechanical or fully electronic. Some applications require a continuously variable attenuator, the range of which is typically from 0 to about 50 or 60 dB [3], The actual lossy elements can be simple high-quality tcsistors, pin diodes, or lossy fin-like designs in waveguides. Fast pin-diode attenuators act like amplitude modulators, if needed, and one of their additional parameters is switching time, which can be further divided into set­tling time and active time. Where mechanical step attenuators require 10 to 20 ms to change state, all-electronic counterparts operate within 10 \is or less and pin switches even in the nanosecond class. All step and variable attenuators have the same kind of settling uncertainty and residual attenuation, which is present even if we select 0 dB. Mechanical devices do quite well with a typical residual term around compensate for the attenuation by adding an amplifier of similar gain. Actually, this scheme often gives somewhat better isolation values due to the fact that the ampli­fier, too, has some reverse isolation characteristics, and, if necessary, we can meas­ure the performance and adjust the parameters accordingly. Special high isolation amplifiers are available as well. If wc have to improve matching as well, we can try to divide the attenuation into two parts, one before and the other after our amplifier.

4.2.3   Power Dividers and Combiners

in terms of theory, power dividers also act as combiners, but this is not necessarily the ease in real life. The name describes the wanted function very well. Wc may need to give the same signal to several different processing elements or wc perhaps want to feed several signals to the same antenna. This is just a perfect place for a good RF combiner or divider. Different constructions are available having a sum port and something up to 32 or 64 individual channels to be summed up or divided into. The division itself will cause a respective decrease in the power level; for example, a two-way divider has output signals at-3 dB, but additional losses are unavoidable—typi­cally 1 to 2 dB per division depending on the frequency ranpe and vendor. Resistive power dividers have further attenuation due to their operating principle but often give wider bandwidth and better matching.

When selecting a power-dividing or power-combining clement for a system, we naturally have to look at the number of ports needed and take care of the frequency range. The power-handling capability is limited, too. Summing devices have to with­stand much more than the single input signal |3J. Phase-coherent systems behave in this respect differently to noncoherent designs because voltages may add up in-phase.

In many cases, the phase imbalance between the ports is important. This figure depends on the frequency and may be one of the limiting factors (e.g., in adaptive antenna arrays); see Figure 4.3. Although there might be no reason to ask for isola­tion between ports when considering the initial input signal, the overall system per­formance surely benefits from it. Wilkinson dividers found in the coaxial and microstrip worlds give easily more than 20 dB; waveguide structures, on the other hand, do not necessarily yield very much. Isolation and matching of individual ports also depends on the impedance conditions of the remaining inputs or outputs.

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Libro:  Circuits and Components for System Evaluations and Design

Autor: Pekka Eskelinen

Nombre: Josmar Eduardo Depablos Rodriguez

Asignatura: Circuitos de Alta Frecuencia

Circuits and Components for System Evaluations and Design - intro

A real RF system or a piece of RF equipment is made of real components or circuits. This chapter tries to introduce some of the fundamental building blocks, which the designer can use, for example, to construct a demonstrator or even sometimes the final system as well. Both passive and active circuits will be highlighted with the exception of antennas and related hardware, which have a chapter of their own later. In this context, a demonstrator is a special piece of equipment or an entire sys­tem that does not necessarily have all the external characteristics of the prototype to come and that might also lack some of the software features. The demonstrator's main purpose is to be a test bed for evaluating the key problems and their solutions. Its main benefits when compared to a real industrial prototype are cost and time savings and the possibility to focus at the essential issues.

Standard or Custom Design?

Sometimes already ar the starting phase of a project we face the question of whether the entire system or at least some parts of it should he tailored to the specific task. Could we base the realization on ordinary commercial-off-the-shelf (COTS) tech­nology? Circuit designers might want to rush to the workshop to pick some bread­board and wire and immediately switch on the soldering iron. Alternatively, if those who are more theory-oriented could click MATLAB or MathCAD or Maple to the screen or perhaps go straight to some electromagnetic simulation package. The real solution for systems engineers is to simplify.

Custom designs, either homemade or ordered from a vendor, tend to have far more risks than benefits. If performance criteria can he met with existing hardware and software, that is the path to take. Often, we can even adjust the overall design so that system specifications will be met, even though the original configuration indi­cated severe constraints. For example, if the initial plan calls for previously unavail­able TX output power in an end-to-end system, wc can sometimes compensate for this by taking more out of antenna gain or by selecting a little hit better RX NF—doing so, of course, assuming that such LNAs are readily available.

Custom designs cannot be avoided, though. Particularly novel military and sci­entific systems are forced to use such designs to be able to comply with their mission requirements. Typical—and often neglected—difficulties appearing with such mod­ules include the following:

·         Severe delays in the schedule and often within the critical path;

·         Budget collapses due to labor and hardware costs;

·         Unexpected technical side effects (e.g., power supply, temperature, and avail­ability of semiconductors);

·         Maintainability problems (only the specialist who designed the module knows it thoroughly);

·         Documentation challenges.

Practical experience indicates that even if the financial and timing estimates are made with the best available professionalism, final conclusions after completing a project show a two- to threefold increase in the use of funding and other resources. Seldom can the designed special component or module be applied in, for example, industrial production without considerable refinements and sometimes even redesign.

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Libro:  Circuits and Components for System Evaluations and Design

Autor: Pekka Eskelinen

Nombre: Josmar Eduardo Depablos Rodriguez

Asignatura: Circuitos de Alta Frecuencia

Circuits and Components for system evaluations and design - Amplifiers

  

The main purpose of RF amplifiers in our system is to enhance the signal level. Ini­tially, two distinct cases existed, where we might have been interested in the capabilities of the module to dig up the weak signal from noise or to give our TX output rhe full power needed. Recently, however, monolithic microwave ICs (MMICs) have appeared with sufficient performance for both tasks—of course, with reasonable limits. The most important parameters | IS) describing an RF ampli­fier arc listed as follows:

• Frequency range;
• Amplitude transfer function (gain as a function of frequency);
• Phase transfer function {also for stability analysis);
• Input and output matching;
• NF (mainly of preamplifiers);
• Maximum output power (often at 1-dB compression, mainly of power stages);
• IP3 or third-order intercept point (distortion behavior);
• The dc power consumption and voltage (often also of preamplifiers due to the

         cellular devices);

• Gain control range (when applicable);
• Cooling (when applicable).

The majority (the quantity in use) of today's amplifiers are based on semicon­ductors bur the highest powers still come from tubes. Particularly TWTs are indis­pensable in radar systems and certain satellite system TXs. These tubes are based first on an electron beam that is accelerated with a high dc voltage between the elec­trodes and additionally on a multiple resonator structure in which the interaction between the original input signal and the beam takes place. Exotic lower frequency applications can also make use of classic tetrodes and sometimes also klystron amplifiers turn out to be feasible. Most higher microwave and lower millimeter-wave amplifiers rely on GaAs MESFETs [16J, and above about 60 GHz also IMPATT and Gunn-diode re flection-type designs become practical.

Often the first question in system or equipment design related to an amplifier is "how much gain." Commercial devices start from about 10 dB, and the more expensive TWTs, for example, give above 50 dB. Many MMIC building blocks make a nice compromise around 20 to 30 dB with a variation of about 2 to 3 dB across their entire useful frequency range [17]. Figure 4.24 illustrates one less typi­cal measured result. The higher microwave range is naturally more complicated, and we easily end with a cascade of four to five modules just for the same 30-dB net gain due to additional losses in connectors and transitions to and from the microstrip MMIC board. NFs have practically achieved the man-made noise limit so that a further reduction seldom makes sense. Of course, the millimeter-wave devices still have some progress to show. Typical commercially available figures range from 0.3 to 3 dB, depending on frequency. AGC blocks tend to have inher­ently poor noise performance.

The impedance matching of amplifier blocks has evolved considerably and thus quite easy-to-use modules have appeared requiring just one or two external compo­nents. Wideband units have SWR values generally below 2 at their inputs, but 4 or even 5 occasionally appears as a respective output parameter. The best NF is nor­mally not obtained simultaneously with optimum matching. Figure 4.25 shows the actual measured input return loss of our sample amplifier. The maximum output power of normal laboratory-grade blocks at the 1-dB compression point lays some­where between +10 dBm and +40 dBm, again depending on frequency. Up to 40 GHz we can rather easily get about 200 mW, but from there on semiconductors tend to exhaust. Commercial VHF/UHF transistor amplifiers arc available up to 20 to 40 kW of CW power, but these devices are actually rather complicated paral­lel amplifier matrixes used mainly for broadcasting and radar work.

Good spectral characteristics, which are mainly indicated as low spurious lev­els, are obtained at the expense of dc power consumption. Most transistor ampli­fier blocks work below 30% efficiency, but unfortunately very many cannot even achieve the 5% limit. The good thing in semiconductor blocks is that wc normally rely on dc voltages less than or equal to 24V. Tubes are known to require huge anode voltages, up to and above 50 kV, which makes system prototyping interest­ing and sometimes also colorful. Actually TWT power supplies tend to be as

complicated as the tubes themselves and, according to recent experience, have more faults.

A typical example of a set of amplifiers in a system is illustrated in Figure 4.26. There is one millimeter-wave LNA, and after the mixer we have a number of IF amplifiers, which have been arranged according to the best overall noi.se per­formance. This means that both gain and NF are taken into account. The total amplification in the chain is about 80 dB when we add the conversion loss in the mixer.

Cooling may be necessary both in LNAs and in final power stages. Cryogenic front ends often use liquid nitrogen. Extreme needs are fulfilled by helium, whieh

provides an operating temperature of about 20K. If no other means exist to satisfy the over ail noise floor requirement, this is the way to go, but operating complexity and costs tend to be considerable. Remember, that not all active modules can with­stand such low temperatures either—the whole design must often be reconsidered. Even some conventional materials may suffer and become brittle. PAs from the 100-W class upward typically cannot rely solely on convection cooling through fins. Forced-air cooling is the most common choice, but its efficiency is limited. Systems involving liquids, mainly water, arc again complicated and expensive and cause reli­ability problems. Two main variants are employed. Systems in which normal tap water runs isolated from the blocks to be cooled is easier to maintain but the cooling capability is limited. If we use electrically purified water, which is produced through

ion-exchanging, for example, we can push the water directly in to the electrodes of the TX tube, but already the frequent change of the liquid may be too much of a bur­den. The highest cooling performance is obtained if we let the warer vaporize in the tube and later circulare rhis water through heat exchangers.

Many amplifier problems are related either to neglected cooling of seemingly low-power units or to poor connection arrangements, which cause oscillations and spurious emissions. First of all, even amplifiers operating at, say +10-dBm power levels, need proper cooling due to their extremely low efficiency. We often have to dissipate 1 to 2W of heat from a small module. Take care to prevent any uninten­tional RF coupling into an amplifier block through its power supply lines. Of course, any direct coupling between the input and the output may be disastrous. Sometimes the shielding as supplied from the manufacturer is inadequate and allows a coupling through the electromagnetic field. Oscillation problems tend to be more severe, if we have to cascade modules for higher gain. Often manufacturers indicate whether a certain device is not recommended to be use in a series connec­tion (cascaded). A RF amplifier can oscillate totally outside its nominal frequency range, and this characteristic may not come out until complaints from other users of the spectrum start to arrive.

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Libro:  Circuits and Components for System Evaluations and Design

Autor: Pekka Eskelinen

Nombre: Josmar Eduardo Depablos Rodriguez

Asignatura: Circuitos de Alta Frecuencia