A Fixed Attenuator can Improve your Harmonic Distortion Test Setup

Typical harmonic distortion test setup:  Signal Generator, Low Pass Filter, DUT (device under test), Spectrum Analyzer.  The low pass filter is used to filter out any high frequency distortion created by the signal generator.  The low pass filter ensures that the fundamental test signal (Fc) is the only RF signal being input into the DUT.  However, this only cleans up the first half of your harmonic distortion test setup.  The other source of distortion that must be accounted for is the harmonic distortion cause by the front end of the spectrum analyzer.  Spectrum analyzers cause a substantial amount of signal distortion.  It must be accounted for before testing the DUT.  Because the distortion caused by the spectrum analyzer is in your testing band you can not use a filter to fix this problem.  Instead, use fixed attenuators.  Take the DUT out of the test setup.  We want to check the harmonic distortion level measured of just our test setup.  If you add a 10dB fixed attenuator on the front end of the spectrum analyzer and the difference between Fc and 2Fc increases, then you just improved your test setup.  Keep adding 10dB fixed attenuators until the difference between Fc and 2Fc stops increasing.  Now your harmonic distortion test setup has been optimized.