DC Supply Noise - RF Cafe Forums

 Post subject: DC Supply Noise

Posted: Tue May 09, 2006 4:23 pm 

Hello,
I am having problems with 60 cycle noise on the DC power supplies in the equipment rack. It does not show up on any of the system performance data but I know it shouldn't be there. At least I think it is coming from the supply(s) since I am using an o-scope to look at the output both at the supply and at the load. The level is about the same in both places (seems to be about 100 mVpp) so I assume it's coming from the supply. The strange thing is that I'm not really confident that the noise is originating in the supply since it seems to be everywhere - even on 50 ohm lines. This one really has me pulling out what little hair I have left. Any guesses as to what's going on here and how I can isolate the problem? Maybe it's a bad ground in my scope but I don't think so. Thanks.

 Post subject: DC Supply noise

Posted: Thu May 11, 2006 12:11 pm 

Hi Ming,

You write "It does not show up on any of the system performance data but I know it shouldn't be there." What do you mean by this? It is not affecting your results or the UUT? The power supply data doesn't spec it? Other?

If the 60 Hz is a sine wave then it is probably facility power leaking thru. This is probably do to poor filtering, shielding or grounding. If it is not a sine wave (ramp, sawtooth, etc) then it is originating in the power supply (I assume supply is a switcher?). I'm sure you have checked the obvious, that the supply has adequate filtering on the output and the facility power is also adequately grounded and filtered.

If there is a poor ground or shield open then the 60 Hz can be radiating so that why it is on the 50 ohm lines. I have experienced this in the past. The external scope is also plugged into the same facility power as the system so the facility power can be coupling into the scope through the power cord (especially the safety ground pin). Try to isolate this scope by using another power source, if possible, or a filter in the power input line. The level is pretty high so you may be able to use a scope probe to "sniff" around to locate the source of the 60 Hz.

Some equipment have internal 60Hz nad 400Hz rejection capability. The DMM and Dig-Oscope I use here do. Check if your does.

You can always try another scope to convince yourself that it is not the problem.

I know my response states the obvious but I can't see any other reason why you would have 60 Hz feed thru. This kind of problem is always hard to isolate and will require a lot of trail & error to locate and eliminate.

Good luck,
RFTEJerry