I’ve posted about this before, but there is a persistent myth in the industry that conversion loss ripple comes from bad mixer VSWR. In this post we will dispel that myth. First we start with a thought experiment: how could we make mixer VSWR create flatness problems? Obviously if the mixer reflects power from it’s RF or IF inputs, then that power could reflect from whatever else is in the source. This would indeed cause ripple, as with any microwave component. In this case, however, the problem is that the source is not well matched. As we will see this is a problem whether or not the mixer is well matched.
Next, we imagine that if the VSWR is bad then internal reflections could cause conversion loss ripple. If this was the case, then it would show up in any sort of testing, in any environment. It would be obvious in the datasheet. This doesn’t happen, though, because there would be no path length delay from the output, which is the second necessary part of a standing wave. There has to be a reflection, and there has to be a line for the standing wave to exist on.
Where does conversion loss ripple come from then? The short explanation is that the mixer is both a load and a source from all three ports. If you look carefully with a spectrum analyzer, you will see various products coming out of each mixer port at all sorts of frequencies. This is the mixers job. At a minimum it will have both the sum and difference frequencies. What happens when the desired difference frequency is well terminated, but the undesired sum frequency is terminated in a reflective load? This is often the case when a low-pass or band-pass filter is used next to a mixer.
Ferenc Marki performed this experiment this morning. He was upconverting 2-8 GHz to 12-18 GHz using a 20 GHz LO. First he terminated all the ports in 50 ohm loads, and then with an 18 GHz low pass filter. Here are the results:
As you can see the wideband 50 ohm system shows a smooth conversion loss, while the filter-terminated system has significant ripple. The frequency of the ripples can be increased by placing a length of cable between the mixer and the filter:
Here is the spectral output of the upconversion, along with the illustration of the problem from Ferenc:
What he is showing is that the high side product is reflected from the low pass filter, which then reconverts to the I port and becomes a new signal to reconvert to the desired product. Since the phase is rotating across the frequency band, it shows up as a ripple and not just a fixed loss.