My issue...and I am not sure it's real (is it within specs? I couldn't find it in the manual). It seems I have amplitude modulation even when the VCA input is shorted (frequency is lower than the dialed frequency, maybe some harmonics).
Methodology:
-Using the TDS320' averaging function, read the Vp-p of the waveform
-Using the TDS320' enveloppe function in order to use the cursors to measure the accumulated amplitude differences; Vvar
-Calculating % = Vvar / (Vp-p/2) or 20x log (%)
Here are examples.
At 5x1K, I have 14.81Vp-p and 0.6V of variation, about +/-2.025% or -21.8dB of amplitude noise
At 5x1M, I have 14.04Vp-p and 0.14V of variation, about +/-0.98% or -34dB of amplitude noise.
At 5x10M, I have 10.68Vp-p and 0.28V of variation, about 5.2% or -25.6dB of amplitude noise.
There are no specs in the service manual about this exact information. It seems to me that the way they generated the switching of current direction is highly dependent on the stability of these matched diodes in the bridge, couple that with the choice of the charging capacitors used, proposes these performances.
Any clue?