Frequency measurements with different inputs

[krass]

I just looked again, the sawtooth seems to be a classic harmonic beat frequency of about 2khz (compare the frequency of adjacent troughsand peaks to each other), and the beat is apparent across the full spectrum. Whatever it is I would say its not "real" but definitely something that is being introduced in the test setup. I am intrigued and can't wait to find out what's causing it.....

EDIT: just realised I'm talking rubbish so I've greyed out the rubbish ! This is a frequency domain plot, harmonic beat is time-domain feature. ie I have no clue what's going on. Sorry for the noise folks !