Group delay in a receiver
Suppose you had an ideal mixer, operating with fixed LO on the high side.... and you added a "real" IF amplifier. That amplifier has negative phase slope, which is used to calculate receiver group delay. But in this case, as you sweep the RF signal, you are sweeping the IF amplifier phase backwards. Seems like this causes a negative group delay, but I am not ready to believe in time travel. Thoughts on this? Thanks for your help.
The phase response might be tricking the VNA into aliasing your measurement.
If it's a high sided LO, that mean the output frequency is being swept backwards too. So the GD (derivative of phase wrt. freq) still comes out positive.
R&S has a two-tone GD measurement technique. I have never used it, but they say it can even be used to measure satellite links. In this they talk about using a reference mixer fed from the same LO as your mixer, bringing it back up to RF, and I assume that would take care of the frequency inversion as well.
OK, I think I get it. The reason I asked is because of an "Excel mixer" I am playing with. Here is a page on it, it is still a mess
when I "mix" and amplifier's S-parameters to another frequency using high-side LO and look at group delay it is negative. But that is not a physical thing, just a figment of a slightly bogus way to simulate a receiver, right?
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