gm of an active mixer

Hi all,

What is the most accurate way to simulate the gm of  RF transistors (RF stage) of an active mixer (single balanced or Gilbert cell)?

I tried to simulate it with many ways such as:

1. DC annotation (but of course its incorrect due to the switching operation of the mixer)

2. d(i_ds)/d(v_gs) using HB analysis and then taking the value at zero (since it is a DC characteristic). In this way I chose in the simulator results of HB: Voltage, spectrum, rms, magnitude. 

3. Using the OP, OPT buttons in the calculator and then extracting the gm of the transistor. 

The problem is that each way gives a different value which makes the procedure of designing an active mixer very difficult. In addition, when I simulate the voltage conversion gain of the active mixer and trying to compare it to the formula (2/pi)*gm*RL (either in linear or dB), I get numbers which are way too far from simulations. I understand that I would not get the same results but not different by hundreds percent. 

I see in many publications that people are plotting graphs of mixer's gm vs. different parameters and starting to doubt whether the results are correct.

I would appreciate any help,

Thanks in advance