Spectrum Analyzer Residual Spurious Response

When the RF input is terminated, the spectrum analyzer may still show discrete responses while the LO's are sweeping. This occurs because at certain LO frequencies, the harmonics of the first and second LO mix together forming a product that is equal to an IF frequency. If this "spurious" IF frequency gets into the IF path, the detector will see a signal and therefor the display will show a blip at the RF frequency where m*LO1-n*LO2 = +/-IF. IF can be any of the IF frequencies in the instrument.

Other possible residuals:

m*LO1-n*LO3
m*LO2-n*LO3
m*LOX-fixed frequency oscillation (due to cavity resonances, clock frequencies, etc..)

Anything leaking into the front end RF path before the first mixer