I tested this in Analog Artist, IC6.1. When I tried to plot S11 I only managed to plot different versions of S11 (mag, phase, dB, real, Smith ...). I do not think you can plot the input impedance of the DUT directly from the sp analysis. I guess my question is how have you plotted the impedance?

Of course you can do some math of the S11 data to get that, but in that case it seems easier to do an AC analysis and measure input voltage and current and take it from there.

Hi Pancho,

Thanks for pointing it out. My simulation has only one port (there is no Z12 term) so ZP should give the input impedance. Do you agree?

Hi Rfidea,

Thanks for your analysis. I think i know what the problem is - the impedance really depends on the circuit that you are looking at. In my case, I was looking at a schottky diode based charge pump. Changing the port impedance changes the amplitude of the RF signal at the chip front-end and since the front-end is highly nonlinear, it shows a different impedance.

Yeah, I agree. Even if the non-linear effect is there it should only change the large-signal behavoural. Since SP is a small-signal analysis we should not see the effect there. You are right.

I was thinking of one other explaination. VP has an input with a charge pump structure. If there is a feedback at the input defining the input impedance and the port impedance is part of the feedback network, we should get this effect.

Hi Pancho,

I just checked again and the measured load impedance does change with the port impedance.

I used a capacitor coupled diode connected nmos as my test circuit (in general some sort of a rectifying circuit is needed to see this effect). First i measure the input impedance usig 50 ohm source and the measured impedance is 8-j500.

Then i use port impedances of 8 ohms and 8+j500 to measure the impedance of the test circuit and the load impedance changes each time. Not surprising as there is almost a 40x increase in the amplitude of the voltage at the test circuit between the two port impedances so some change in impedance is to be expected.