[time-nuts] Ground loops in measurements?

Volker Esper ailer2 at t-online.de
Mon May 20 10:15:00 EDT 2013

Moin Attila,

yes ground loops can cause serious measurement problems. And solving 
those could fill a hole book. Here's what I do in practical:
1.) avoid the loop
2.) if you can't, try harder to avoid it
- depending on the problem: break up the dc loop by using capacitors 
(most often you only need to kill 50/60 Hz so you can possibly insert a 
C in the shield)
- if you need dc current or extremly low frequencies flowing in the 
shield, use inductors in the shield to get rid of 50/60 Hz
- if you have to transfer low frequency rectangular pulses, you have to 
decide or even to try, what will be the better choice
- but that induces new problems if you have to be synchronous to within 
some ns...
- on the lab bench - if you can't avoid loops - make the area of the 
loop as small as you can to reduce the inducing field -> keep shields 
- use a well grounded!! metal plate (use iron, if you can) under your 
experiment and lay the coax cables flat down on it
- as far as you can connect all case grounds at one point only
- if you are experimenting with low frequency on your bench you can try 
to not connect the shield on one side of the cable - be aware, that the 
current now takes another way, so that is practicable in only few 
situations (and if you fumble around it will change measurement conditions)
- use floating power supplies - but remember, they can be coupled to 
earth or the power line over the stray capacitance of the transformer 
(rather a problem for higher frequencies than 50/60 Hz)


Am 20.05.2013 14:08, schrieb Attila Kinali:
> Moin,
> A couple of weeks ago, there was a short discussion on "bad" connectors
> and cables and the coupled in noise of those. Summarized it said that
> measurements in the time-nuts scale are very sensitive to even the lowest
> noise levels and coupled in signals.
> But, all the measurements we do are done using some sort of coax which
> have their shield connected to the case of the devices. As the invovled
> devices in a measurement are also grounded over their power supply
> this will lead to ground loops and thus a 50/60Hz noise. Also, because
> loops are good magnetic antennas, a lot of other noise floating around
> in the ether is coupled in (eg a nearby radio station).
> How do you handle this kind of problems?
> 			Attila Kinali

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