It may be worthwhile to use a 6dB attenuator between mixer LO port and low pass filter. That way, the LO port always sees 50 ohms. It requires 6dB more LO signal, but gives 12dB more isolation.?
All the ports of a mixer should be considered as both input and a output port. Even tho you use a gilbert cell mixer, using attenuators to improve the isolation should help.
Remember that shielding is important when doing a project like this, feedthrough capacitors and coax connectors help keeping the unintended signals out of the spectrum analyser.?
What graphing library are you using? it looks great!? This is a interesting project and I will be following along.?
73 de Thomas LA3PNA.?
