Dear all,

I am simulating a direct-conversion Rx. It comprises an LNA followed by a passive mixer followed by a "current-input" baseband filter (By current input I mean input stage is transimpedance amp) as shown in Fig A.

Instead of using PSS/PAC/PNOISE to simulate everything, I tried to create an sp-model of the baseband filter using an SP analysis. As shown in Fig B, I run an SP saving all S-parameters along with noise of the filter in the 2-port.

Then I used the 2-port to load the mixer and run PSS/PAC/PNOISE as shown in Fig C. For some reason, I do not get the same results (I mean the difference are some dBs in gain and NF). My fundamental is 2.5GHz and the 2-port contains data up to 20GHz so I guess this is enough for 7th harmonic.

Any ideas of what could I get wrong is much appreciated.

Regards
Marios