[time-nuts] Ground loops in measurements?

Bob Camp lists at rtty.us
Mon May 20 13:00:33 EDT 2013


Coax is interesting stuff. The shielding is only good down to some lower
frequency limit. For anything practical that's going to be > 100 KHz.

At the frequencies you *should* use coax at, transformer coupling is the
easy way to break the ground loop. In this era of cell phones all over the
place, a transformer plus some sort of common mode choke is the standard

For things like 1 pps, you should be using some sort of balanced
transmission. Twisted pair, or better, shielded twisted pair. You can either
run into a balanced receiver IC and dc couple or into a transformer and do
something a bit fancier. With the IC you have a maximum voltage offset that
can be tolerated. With the transformer you have the cost / delay / possible
error in picking up the edges. 

If your environment is noisy enough, you may have to transport your pps on
some sort of carrier. RF and optical both have their fans. 

None of that is going to be easy. The alternative is to do what you would do
in a screen room. Single point ground, everything tied tightly together. Put
reasonable filtering on everything in and out. Tie the filters to the common
ground point. This also is not easy, but possibly not as hard as redesigning
the ins and outs of every box in sight. I have seen this approach used on
some *very* large systems. 

A some what extreme approach (that I have seen used). Forget about all the
shielding and stuff. Buy a farm, put up a small metal shack in the middle of
a large field. Bring a hand cart with batteries. Run everything on a big
copper covered table. 

Lots of ways to go. 


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Attila Kinali
Sent: Monday, May 20, 2013 8:08 AM
To: time-nuts at febo.com
Subject: [time-nuts] Ground loops in measurements?


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

The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
		-- unknown
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