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