One of the misconceptions we frequently encounter is the idea that a mixer is a passive device. Since it has loss and there is no DC bias, it is considered passive unless a gain element is added. We see the situation differently. Since the diodes in a mixer require a switching signal from the local oscillator to energize them before the device is functional, it is completely active, just with RF energy instead of DC. In fact, when the LO is considered an energizing signal as a DC bias is considered for a transistor in an amplifier, the two situations are almost completely analogous.
In one case the DC bias adds energy and noise to the amplifier, in the other the AC bias adds energy and noise to the mixer.
This has important implications:
– The downconverter/upconverter system is usually limited by the mixer, but it can never be better than the LO. If the LO doesn’t have enough power, it is the same and underbiasing an amplifier. The performance is unpredictable. Similarly whatever noise is on the LO is directly transferred onto the output signal, so it is as important to provide a well filtered LO as it is to provide a well filtered DC bias.
– The mixer VSWR is not an important performance metric. This isn’t a direct implication, but like the transistors in the amplifiers the nonlinear devices inside a mixer have a dynamic impedance that does no not always optimize for a perfect match at all ports. Sometimes the noise figure of an amplifier can be improved by mismatching the input, and sometimes the conversion loss of a mixer can be improved by mismatching one of the ports. If one of our mixers is not well matched, its because we designed it that way.
Hopefully we’ll have an application note out soon on everything you need to know about LO drive for the mixer. Until that time just try to filter it the best you can and drive the mixer within the manufacturer rated range. Also come check out one of our new record performance T3s: the highest linearity, IP3, spurious, and P1db available on the market now at ridiculous bandwidths.