1. Attach Specturm analyzer to signal generator. Measure input SNR (be sure to check that the noise floor shown on the analyzer is the true noise floor, and not the noise floor of the analyzer. Hint: when you disconnect the signal generator and load the input of the spec. an. with 50 ohms, the noise floor should drop ~10 dB, to be confident in your noise floor measurement).
2. Connect input signal to mixer, attach Spec An. to output of mixer, measure output SNR.
3. SNR of mixer(dB) = SNR_in(dB) - SNR-out(dB).