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
together
- 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)
Volker
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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