I've recently replaced the obsolete HMC218A in my design to the newer HMC218B. There seems to be a strong degradation in receiver performance in terms of SNR. I suspect that the dominant noise source in my system is phase noise and that somehow LO phase noise is down-converted to the IF. MY LO and RF are around 5.4GHz and IF is near DC (~70KHz). What are the differences between the 2 products in terms of LO-IF isolation/ LO-RF isolation?
I've noticed that in the HMC218A when tuning the LO power - any value above ~7[dBm] results both in signal and noise saturation in the RF. However when using the HMC218B (or even the HMC220) while the signal power saturated, the noise level kept going up when LO power was tuned up.
I'm quite sure this is not a matching issue as the older HMC218A worked perfectly.
Qould this be RF port saturation problem? Or is this LO isolation problem?
Here is a plot for HMC218A LO isolation
HMC218B LO isolation:
The B version LO to IF isolation is better than A version. But the phase noise of LO source can still affect the noise performance significantly.
I would recommend to put a narrowband filter on LO of HMC218B to see if there is any improvements. And drive the LO at nominal power (13dBm). Actually HMC218B has different LO return loss performance compared to HMC218A. So redesign the matching network may be needed.
Return loss comparison: