We get a lot of questions about phase noise and noise figure, and it is a topic of interest for Ferenc Marki. The question comes up often, what kind of noise does the mixer add during the conversion process?
Our experience is that the actual noise addition from a passive silicon Schottky mixer is negligible. This is why the noise figure is very nearly the conversion loss: the mixer does not add noise, but it does attenuate the signal by an amount equal to the conversion loss. Frequently when customers have a noise problem in their conversion it is coming from the leaking of LO noise into the IF or RF port.
We can demonstrate this by performing an experiment. We take a 10 MHz low phase noise reference oscillator and measure the phase noise. Then we convert the signal to 5.01 GHz with a noisy 5 GHz LO and measure the phase noise of this output. As you can see the phase noise jumps from -154 dBc/Hz to -117 dBc/Hz at a 10 kHz offset. Finally we downconvert the signal with the same LO (with a trick I’ll explain below) and see that the phase noise has dropped back to -154 dBc/Hz, as though it had never been converted at all.
The Trick: Why would the mixer actually remove phase noise from the upconverted signal? Because the LO cancels it’s own phase noise out on the downconversion. As long as the LO is in phase with itself on the up and downconversion, the phase noise will cancel. Here is the same experiment done with a non-coherent LO for the downconversion step. Note that the phase noise actually increases on the downconversion as a result of the addition of the non-coherent LO phase noise.
The other important part of this is in the RF. The RF needs to remain coherent with the LO during it’s travels between the two mixers. If there’s nothing but a cable in between then we’re fine. If there’s something with a non-linear phase delay (a group delay) such as a filter or narrowband amplifier then we will see the phase noise on the output, as the LO doesn’t have a chance to cancel out the phase noise on the input.
So this suggests to us that the mixer doesn’t add a significant amount of phase noise. We’ll keep looking deeper into this problem and happily let you know what we find!