In Mixers

In microwave engineering, the fundamental problem is coaxing your waves to move through your system without reflecting. This is described by the return loss from a junction, that is how much power is reflected from the junction. A related measure is the voltage standing wave ratio (VSWR), which is measured as the ratio of the maximum to minimum amplitude of the standing voltage wave created by the reflection. When designing a system it is vital for a number of reasons to minimize reflections and thus return loss and VSWR.

So then, VSWR is important to consider when selecting a mixer, right? In our opinion the answer is a qualified no. The main reason that it doesn’t matter is that in our experience the most important metric for a mixer is the conversion loss. If the conversion loss is good there is not enough energy left over to degrade the isolations, multitone products, spurs, and even reflections. Just as an amplifier is sometimes mismatched from the source to improve the gain or noise figure, the mixer conversion loss will sometimes optimize for a mismatched source impedance. In other words, when we design a mixer we will sometimes deliberately mismatch the mixer ports to improve the conversion loss. It isn’t a bad mixer, it’s a really good mixer made by someone that understands the tradeoffs.

Now for the qualifications. First, mixer VSWR matters if your system can’t handle reflections. For example, if you have an antenna and you are worried about rebroadcasting, you would need a circulator, if you have an LNA preamplifier that can’t handle reflections, etc. The second point is that while the goal of a mixer is to optimize conversion loss and not VSWR, that doesn’t mean the VSWR doesn’t tell us anything. Certainly a return loss of 1 dB on the LO port indicates that something has gone wrong.

So in summary, if your mixer doesn’t look like a through line, don’t worry about it as long as the conversion loss is still okay. These are not the droids you are looking for. If, on the other hand, your mixer looks like a short from any port, you have a problem worth investigating.

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