Bad overshoot on HP 3325A function generator - Page 1
Forum section: electronics / projects

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: September 10, 2022, 03:11:08 am
content: Hello all,I have an HP 3325A and have been calibrating it after recapping. One of the performance checks per the manual is to output a 1MHz square wave at 10 V P-P to check the over shoot. It states it should be within 0.5 volts but upon measuring I have a 2 volt overshoot; however whenever I turn on the high voltage output I have no overshoot. Where should I start looking? And yes, I have a 50 ohm load when the HV output is off, so I know it’s not that.Thanks!

author: trobbins
author_rank: Frequent Contributor
timestamp: September 11, 2022, 09:04:57 am
content: Perhaps do a little more background checking - for example is the overshoot the same no matter the base frequency of the squarewave? Do the other waveforms show any abnormal character? Do you have another generator that can confirm your 50 ohm load and scope probe etc are working as expected?Have you identified the main output amp in the service manual? I'm not sure if the HV output option gets its signal directly from the normal output, or prior.

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: September 11, 2022, 06:45:39 pm
content: Quote from: trobbins on September 11, 2022, 09:04:57 amYes the overshoot is the same at various base frequencies, from 100 Hz to 10 MHz.Quote from: trobbins on September 11, 2022, 09:04:57 amNo every other waveform is essentially perfect from super low frequencies up to their upper limit, that is the sine, triangle, and ramp waves are well within spec.Quote from: trobbins on September 11, 2022, 09:04:57 amNo, I do not, couldn't image that it would be that since every other waveform is essentially perfect, even the amplitude and frequency are perfect on every waveform, minus the overshoot on the square wave.Quote from: trobbins on September 11, 2022, 09:04:57 amNo, not yet, but that is probably my next step. The HV output takes the input from the main amp and just boosts it, at least I'm pretty sure but I could be wrong, when looking at the RCA connectors on the inside.I'm almost thinking is may just be a harmonic, because at 10 MHz it is very rounded to which I'm almost certain is caused by harmonics. Still doesn't make sense to me as to why it's such an overshoot no matter the amplitude or frequency. Should I look at the synthesizer circuits since every waveform is synthesized from a sine wave? I mean every other wave except the square wave is fine.Thanks!

author: trobbins
author_rank: Frequent Contributor
timestamp: September 12, 2022, 12:21:14 am
content: Certainly seems like it is related to the final audio amp stage. You may be able to probe directly to that output amp stage, and to disconnect cabling to panel connectors and HV circuit, to isolate the issue further. Is it a damped resonant overshoot with a definable single frequency resonance (if your scope and probe has suitable frequency bandwidth) ? I can check my 3325A for its response if you get stuck - it's just that my bench is full at the moment.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 12, 2022, 01:33:16 am
content: For reference, here's what my 3325A looks like with the same settings (1MHz square wave at 10V P-P into a 50ohm, +/- 0.1ohm load).I can provide further measurements and readings if needed.