EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Chris Wilson on June 09, 2015, 10:10:12 pm
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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 http://www.gatesgarth.com/SA/scope-shots.zip (http://www.gatesgarth.com/SA/scope-shots.zip) 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.
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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.
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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.
Tim