Just read this post while currently receiving grief from a scope versus spectrum analyser contest. I'm checking the levels and frequencies coming out of a balanced mixer design being fed by 2 audio frequencies. All was well until I made a mod to the wiring on the protoboard. Bugger!, lost the mixing products and was just left with 2 audio frequencies. Mixer output still looked fine on scope so spent next 15 minutes chasing up the problem. A few mins ago found I'd miswired the audio and was looking at a 'real' mixing of 2 signals, hence no multiplier products on the SA. What's troubling me is that the scope was leading me along me by showing a neat and clean DSBSC signal. Tweeked the frequencies a little and the scope moved to a 'normal' mixed audio-on-audio signal.
Can't leave well alone, so wired the (asynchronous) 50ohm audio sources to10K resistors with their junction to the scope and started looking for audio mixtures that 'look' like DSBSC. To this end I dropped the frequencies down to see the scope spot tracking across the screen (0.5 sec per div =no visual artefacts) and am now looking at a nice DSBSC signal that I know isn't a DSBSC signal. It's 4Hz 2Vpp and 4.4Hz 2Vpp. It all adds up in the wash but aint half a confusing time waster!. john