I've been running some LTSpice analyses on the mixer and I have a few observations that might be useful. a.. The as-designed mixer seems to have just a little over 6dB conversion loss. It seems like 6.0dB is the best you can get using ideal switches, and the finite on-resistance of the J301's produces about an extra dB. b.. It would seem that best performance can be had when the FET's are selected for highest and best matched Idss. c.. It also seems that the conversion loss is lowest when the source bias is adjusted so that the positive peaks of the LO gate drive signal are just a bit above the source voltage, but not enough to drive the gate-drain diode into hard conduction. d.. The adjustable source bias circuit can possibly be eliminated completely by disconnecting the ground at the center-tap of the LO drive transformer and replacing it with a combination of about 200K resistor in parallel with a 0.1uf cap, both to ground, grounding both FET sources.This produces a self-adjusting gate bias that varies with LO level so that the peaks just barely turn on the gate-drain diodes in the FET. A bonus is that you can measure the DC voltage at the junction of the R & C to determine that the LO drive level is where you want it. e.. Initial analysis indicates that the mixer 1dB compression point is over +10dBm, so IP3 will probably be somewhere around +25dBm. I will be looking at this more closely. I made these analyses by simulating with an LO freq of 10MHz, an RF freq of 9.9MHz, and an RC-filtered IF output freq of 100 kHz. As Farhan mentions, the use of FET's instead of diodes for mixer switches would seem to have a bonus of better mixer termination immunity. In a diode mixer, the on-state voltage drop across the diodes is a non-linear function of the signal current. This non-linearity is where distortion produces are born. The FET's on resistance is pretty linear. This should help keep it predictable when driving wildly reactive loads like a crystal filter. Until I get around to building and testing myself, I'd appreciate anyone else's observations. Joe W3JDR