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HP 5359A - VCO drift issue?
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Raindog:
I have a HP 5359A Time Synthesizer that I'm not sure if I am having an issue with or not. I can set a pulse width, period and when observed on an oscilloscope the pulse train seems fine. But when measured on a frequency counter the pulse train frequency drifts +/- 200-300 Hz, pressing the "CAL" button sometimes fixes it but begins to drift after a few seconds. Changing the pulse to a symmetrical or near symmetrical shape seems to fix the issue. The service manual suggests performing the realignment procedure for the multiplier and VCO boards. However I have done this procedure only to see little to no improvement, even swapped out the VCO board for another and also saw no improvement. Any ideas?

Best Regards,

Craig Petersen. 
trukresom:
I also have a HP5359A here and it shows the same behavior as you describe.

When reading the article in the HP-Journal (August 1978, page 12) carefully,
there are 2 circuits providing precise timing, one used for a delay between sync
output to delayed output and the other for the pulse width.
I did'nt found any specification of the precision of the output frequency.

But Your question was also subject to a discussion on the time-nuts forum:

https://www.febo.com/pipermail/time-nuts/2008-June/031885.html
https://www.febo.com/pipermail/time-nuts/2008-June/031889.html
https://www.febo.com/pipermail/time-nuts/2008-June/031890.html
https://www.febo.com/pipermail/time-nuts/2008-June/031893.html





Raindog:
Thanks for the links to timenuts, I thought I was going crazy or there was something wrong with the unit. After posting this I did a deeper dive on the timing of the pulses with my HP 5371A. What I found is that the pulse width and spacing are within 70-100pS, 120pS worst case (over a 1000 pulse sample) regardless of the variances I am observing in pulse train frequency. After reading the "theory of operation" in the service manual I became suspicious that the unit was only intended to generate precise pulse widths, delays but not continuous pulse trains, this also explains why pulse train frequency accuracy improves with a wider pulse width. Reading the responses at timenuts verified that there was nothing wrong with this instrument, very interesting reading.

Best Regards,

Craig Petersen.
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