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Chapter 7: Mixer Design
Mixers are 3-port active or passive devices. They are designed to yield both a sum and a difference frequency at a single output port when two distinct input frequencies are inserted into the other two ports. This process is called frequency conversion (or heterodyning), and is found in most communications gear. It is used so that we may increase or decrease a signal's frequency. The two signals inserted into the two input ports will normally be a continuous wave, produced within the radio by a local oscillator, and the incoming (for a receiver) or outgoing (for a transmitter) signal. If we want to produce an output frequency that is lower than the input signal frequency, then it is called down-conversion; if we want to produce an output signal that is at a higher frequency than the input signal, it is referred to as up-conversion. Indeed, most AM, SSB, and digital transmitters require mixers to convert up to a higher frequency for transmission into space, while superheterodyne receivers require a mixer to convert a received signal to a much lower frequency. This lower received frequency is then called the intermediate frequency (IF). Receivers must use this lower frequency signal, as it is much easier to efficiently amplify and filter with the IF stages tuned and optimized for a single, low band of frequencies, and the receiver's gain and selectivity are thus increased. The frequency conversion process within the nonlinear mixer produces the intermediate frequency by the heterodyning, or beating, of the input signal with the receiver's own internal LO. This mixer circuit can consist of a diode, BJT, or FET that is overdriven or biased to runwithin the nonlinear part of its operation. However, the beating of the two input signals yields not only the signal, the local oscillator, and the sum and difference frequencies of these two signals, but also many spurious frequencies at the mixer's output port. Nonetheless, most of these undesired frequencies will be filtered out within the receiver's IF, resulting in the new desired frequency (consisting of the carrier and any sidebands) now at the difference frequency. This new lower frequency will then be amplified and further filtered as it passes through the fixed tuned IF strip.
There are three basic classifications for both active and passive mixers. Briefly:
Unbalanced mixers have an IF output consisting of fs, fLO, fs - fLO, fs + fLO, and other spurious outputs. They will also exhibit little isolation between each of the mixer's three ports, resulting in undesired signal interactions and feedthrough to another port.
Single-balanced mixers will at least strongly attenuate either the original input signal or the LO-but not both-while sending less of the above mixing products on to its output than the unbalanced type.
Double-balanced mixers (DBMS) supply superior IF-RF-LO interport isolation, while outputting only the sum and difference frequencies of the input signal and the local oscillator, and significantly attenuating threequarters of the possible mixer spurs. This makes the job of filtering and selecting a frequency plan a much easier task.
7.1 Passive Mixers
7.1.1 Introduction Passive mixers permit a much higher RF input signal level than active mixers before severe distortion products within the output IF becomes unacceptable. These distortion products are in the form of intermodulation distortion (IMD), along with compression distortion. The IMD may fall in-band, or cause other signals to fall in-band, possibly swamping out or creating interference to the baseband signal. This causes additional noise, which will degrade system performance and BER.
Passive mixers also possess a lower noise figure than active mixers, which is very important for any stage within the front end of a low-noise receiver. However, passive mixers will have an insertion loss of around 7 dB, instead of the insertion gain that active mixers produce.
The passive mixer conversion losses are caused by the mixing diode's internal resistance, port impedance mismatches, mixer product generation, and the inevitable 3 dB that is wasted in the undesired sum or difference frequency. (This sum or difference frequency is removed by filtering, thus cutting a mixer's final output power in half).
Figure 7.1 is a common double-balanced mixer, which utilizes a diode ring to achieve frequency conversion of the input signal. The mixer's diodes are being constantly switched on and off within the ring by the LO, while the RF signal is alternately sent through the diodes, this mixes the two signals in a nonlinear manner, producing the IF output frequency. DBMS can function up to 8 GHz (and beyond) by using hot-carrier (Schottky) diodes, which possess low noise and high conversion efficiency.
DBMS made of discrete lumped components and placed on the wireless devices' PC board are seldom utilized today. Instead, double-balanced mixers are available in a module, with the diodes and transformers already balanced within a single, low-cost dual in-line package.
Lower-end passive mixers are available that employ either a single diode (Fig. 7.2a) or double diodes (Fig. 7.2b). These mixers are not double-balanced and, in contrast to an active mixer, must be supplied with a high-amplitude LO signal (but not as high as a DBM's). Nonetheless, they are cheap and require few components.
7.1.2 Types of passive mixers
There are several types of passive mixer designs available, depending on cost and performance levels required. Some of these passive diode mixers have already been introduced above, but will be further investigated in this section.
Figure 7.3 shows a one-diode, single-ended mixer. This type of device is found only in low-cost circuits, with the isolation between ports being supplied by bandpass and low-pass filters that are separated in frequency. The mixer circuit can also take advantage of the low level of LO power needed to drive the single-diode mixing element compared to the higher drive levels required of a DBM. The single-ended mixer shown, however, has a rather narrow bandwidth, poor port-to-port isolation, a low intercept point, and inferior intermodulation distortion suppression. If we would like to increase the specifications and overall quality of this device, we will need to increase the number of diodes. This will allow a higher LO drive input level, which automatically forces an increase in the mixer's 1-dB compression point because the P1dB is always about 10 dB below the LO for all diode mixers. Thus, the higher the LO drive that can be inserted into a mixer, the higher the P1dB possible. As we are then demanding a more powerful LO, this will unfortunately cost more and radiate a higher level of EMI...
