Hei PA4TIM
Hmm- the 40MHz signal would not show the 100kHz pulses that I'm experiencing.
My current theory (if there is somerthing wrong with my setup) is that there is noise on the PSU lines, caused by the digital section in my AM503B and possibly a dryish tantalum cap. I don't know the age of my 503B (anyone have a link to tektronix SN decoding?). Of course this is just speculation, but I did see some 500Hz modulated 16MHz being radiated at a low low level from the 503B front panel, when I checked with my H-field probe - a similar spacing to the noise peaks near 100kHz.
As to what is creating the 100kHz - well the manual does mention that the micro controller controls the front panel at a multiplexed 200kHz rate - again, perhaps some cross-contamination to the probe amplifier sections there?
I know all this is speculation, but I really can't imagine that tektronix have left so much noise in the system at 100kHz, which much above the 20kHz Hall crossover (Manual Theory of operation, sec. 3-6)