HP 5359A - VCO drift issue?

[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.