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HMC218A vs. HMC218B

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?

Any Clues?

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