microwave integrated circuits (MMICs) sometimes blur the line between converters and mixers as they may contain several amplifier and mixing stages, but they typically don't contain filtering.
Mixers have fundamental parameters that include conversion loss (or gain in the case of MMICs with amplifiers), isolation (of which there are 12 varieties, to be discussed in Chapter 7), compression level, noise figure (which for passive mixers is typically just the conversion loss), input and output match, and most of the other parameters found for amplifiers.
Mixers are sometimes referred to as passive mixers or active mixers. Passive mixers don't contain amplification circuits, are typically constructed of diodes (with a ring or star configuration being most common), and have baluns in some paths to provide improved isolation and reduced higher‐order products. Higher‐order mixing products, also called spurious mixing products or spurs, refer to signals other than the simple sum and difference of the input and LO frequencies. They are often referred to by the order of harmonic related to creating them; for example, a 2:1 spur is created at the frequency of two times the LO plus and minus one times the input. The level of the mixer spurious products, which are sometimes called mixer intermod products (even though the two tones are not applied to the input), change with respect to the RF drive signal at the rate of the order of the RF portion of the spur. For example, a 2:1 spur will increase 1 dB in power for each dB in power the RF signal increases. However, since the LO is creating the non‐linearity in the mixer, as well as the spurious signal, it is difficult to predict how the spurious power will change with respect to LO power. In many cases, driving the LO power higher produces a higher spurious signal, as the relative magnitude of the RF signal to the LO power is reduced; in other cases, the transfer impedance of the non‐linear element becomes more consistent across the RF drive level. The spurious higher‐order products of a mixer are sometimes defined as a spur table, which shows the dBc values of higher‐order products relative to the desired output. Chapter 7 has more details on measuring these behaviors.
Mixers with baluns can suppress some of the higher‐order products, with baluns on the RF port suppressing products that have even‐order LO spurious, and baluns on the LO port suppressing products that have even‐order RF spurious. These are called single‐balance mixers, and it is typical that the LO port is balanced. Double‐balanced mixers have baluns on the RF or IF ports. Refer to Chapter 7 for more details on mixer configurations. Triple‐balanced mixers are usually comprised of a pair of double‐balanced mixers, adding a balun to the IF port. Their main advantage is to divide the RF signal power between the two diode quads, lowering the RF relative to the LO, whereby the spurious signals created will be lower; then the outputs are combined to recover the power. This provides a mixer with the same conversion loss and lower spurious products at the same output power level. The disadvantage is that since the LO drive is also divided, higher LO drive power is needed to achieve the same linearity for each diode quad.
The creation of spurious mixer products is a key aspect of system design, with the goal of eliminating spurious signals from the IF output. Unfortunately, some frequency plans are such that the spurious products must fall into the band of interest. In such cases, system designers move to multiple conversion stages to create a first stage, which produces an output free of spurious signals over the range of input signals of interest, and then has a second conversion stage that produces the frequency of interest at the output. This multiple conversion or “dual‐LO” system is typically called a frequency converter and often contains additional filtering and amplification, as shown in Figure 1.41.
Figure 1.41 Dual‐LO frequency converter.
Because of the multiple components used, the frequency response of converters often has gain ripple and phase ripple, which can distort the information signal. Key figures of merit for converters are gain flatness, group delay flatness, and the related phase flatness, which is also known as deviation from linear phase, and represents residual ripple after fitting the phase data to a straight line. Modern systems employ equalization techniques that can remove some of the flatness effects provided they follow simple curvatures; as such, another specification found on converters is deviation from parabolic phase, which is the residual ripple in the phase data when it is fitted to a second order curve.
Mixers often have quite poor input or output match because of the switching nature of their operation, so their effect on system flatness when assembled into a converter can be quite dramatic. Until recently, it was difficult to predict the effects of output load of a mixer on its input match, but the mathematical tools to model mixers as system elements have been developed (Williams et al. 2005) that describe these relationships. Mixers that produce an output that is the sum of the input and output signals are relatively simple to describe, but mixers that produce an output that is the difference between input and LO have a more complicated behavior in the case where the input frequency is less the LO frequency. These are sometimes called image mixers, and their unusual characteristic is that as the input frequency goes up, the output frequency goes down; this also applies to phase: a negative phase shift of the input signal results in a positive phase shift of the output signal. How these special cases affect the system performance will be described in more precise mathematical terms in Chapter 7.
1.14.4 Frequency Multiplier and Limiters and Dividers
Mixers are not the only way to create new frequencies at the output; frequency multipliers are also used to generate high‐frequency signals, particularly when creating mm‐wave sources. Frequency multipliers produce harmonics by changing a sine‐wave input signal into non‐linear wave. The basic doubler is a half‐wave or full‐wave rectifier, such as a diode bridge. A pair of back‐to‐back diodes turns a sine wave into a square wave, which is rich in odd harmonic content. This is essentially the same as a limiter.
The key figure of merits of a multiplier is the conversion loss from fundamental drive to the desired harmonic. Other important characteristics are fundamental feed‐through and higher‐order harmonics.
Limiters have the key characteristic of maximum output power; that is the power at which they limit. Also important is the onset of limiting and the compression point. Ideal limiters are linear until the onset of limiting, and then they effectively clip the output voltage above that level.
Other multiplier types are step‐recovery diodes, and non‐linear transmission lines that, when driven with a sine wave, effectively “snap” on to produce a sharp edge. Depending upon the design, the on‐time can be short, which produces an output rich in harmonics. Some digital circuits can also be used to create narrow pulses from a sine‐wave input as a pulse generator. Such a pulse will also be rich in harmonics.
One aspect of a multiplier that is not easily discerned is the group delay through it. That is because for some change in input frequency one will see a multiplied change in output frequency. FM that passes through a doubler will have the same rate as the original FM, but twice the deviation. For this reason, doublers or multipliers are seldom used in the signal or communication path of RF or microwave systems but can be used in the base carrier paths and in the LOs of many systems.
1.14.4.1 Frequency Dividers
Frequency dividers provide for a lower value of frequency than the input frequency.
