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Ground loops

Hello,

I am observing this affect on a few components whereby their frequency response shows the 60Hz power line frequency. This seems to be isolated to just components from a few date codes. However, this does indicate that there is interference with the 60Hz frequency with some parts. I have tried to tie the chassis of the unit under test, the measurement unit and the unit under test to ensure that they are at the same ground potential, but the problem still persists. While the circuit is powered down, the chassis of the unit under test, the unit under test and the measurement unit have no resistance between them. But when I power up the circuit, there exists a negligible resistance of upto 20 ohm between the chassis of the unit under test and the uunit under test and the measurement unit and the unit under test. Does this create the possibility of ground loops? Could the

60Hz power line frequency that I am seeing be related to ground loops?

I have also tried to run the experiment with/without an isolation transformer, but the problem still persists. Any ideas on where the

60Hz power line frequency could be creeping in? If the failing date code is run at a different location, it does not exhibit the 60Hz power line frequency. Therefore, it is something in setup that is causing parts with some date codes to capture the 60Hz power line frequency.

Please help.

Reply to
picoclock
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i would first check to make sure the bad locations have a real earth ground in the receptacle. You didn't say exactly what this component is ? it would shed a lot more light on it. It would appear to me that maybe you have some inductive problems generating EMF, but that is just a guess. I doubt that your meter is actually reading 20 ohms, it most likely is getting energy from it's leads near by.

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Reply to
Jamie

quoted text -

The device is an audio sample rate converter

Reply to
picoclock

what sort of components, transformers, inductors, flux capacitors?

the 60hz, how many dB's worse is it, and what are the specs for 60Hz rejection?

martin

Reply to
martin griffith

Mains frequency injection (and harmonics through MHz) is a real problem down at the mV, uV level. It creeps in everywhere, via all paths and all mechanisms. Textbook 'solutions' such as isolation transformers, filters, star points, baluns, line breakers, shielding etc, provide little help and more usually waste time and effort.

Effective cure is use of battery supplied or isolated power supplies with only the signal commons galvanically connected. Maybe worth adding that signal sources themselves are prone to injecting mains noise along with the wanted reference signals. That 50ohm BNC output socket may be delivering say 100uV of signal on it's centre pin and 10uV mains noise through it's body. (Cleanest signal I've ever seen came from an el cheapo 'Levell TG200' with it's internal switched mode battery adapter . Worst, came from a Wavetek function gen'). john

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Reply to
john jardine

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