Author Topic: Are these PSU spikes enough to cause trouble?  (Read 839 times)

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Offline Chris Wilson

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Are these PSU spikes enough to cause trouble?
« on: June 09, 2015, 10:10:12 pm »
I am battling with a highly complex HP spectrum analyser that shows noise spikes and jitter from time to time on it display. Thanks to the generous help of others I am slowly trying to track the cause down. I have measured the power supply lines after the regulator and on the -15V rail I see the spikes shown at   The last two photos are of the scope looking at the +15V test pin. Whenever i see noise on the SA screen I also see it on the -15V test pin. I never see spikes on + or -5V, or +20V or +15V supplies. The PSU in the SA is a linear, the machine an HP8568B.

My test methodology is probably poor, with a fairly long ground lead, but the fact only -15V shows these spikes is surely significant? The probe was set to X1. The scope an old Tek 7854

Thanks for looking.
Best regards,

                 Chris Wilson.

Offline slateraptor

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Re: Are these PSU spikes enough to cause trouble?
« Reply #1 on: June 10, 2015, 06:06:42 am »
It sounds and looks and smells like noise coupling, but I suspect only you can tell us if |?V/V| = 0.27% is trouble or not.

Online T3sl4co1l

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Re: Are these PSU spikes enough to cause trouble?
« Reply #2 on: June 10, 2015, 09:24:15 am »
Can you focus the scope trace?  :-\

Also zoom on the spikes, they look ~sort of~ consistent, not that there should be any reason for such a thing to be produced randomly from a linear supply regardless.  A DSO (or single sweep + long exposure in dimly lit room) would be helpful here.

Possible culprits: electrical breakdown (capacitor? diode/transistor avalanche?), coupling (AC line noise? nearby transients?), frayed wires (intermittent shorting or breakdown), crummy resistors..?!

Aside from aging capacitors (electrolytics are always prime suspect), almost anything else is a stretch.

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