[time-nuts] Ground loops in measurements?

Bob Camp lists at rtty.us
Mon May 20 17:16:02 EDT 2013


Hi

The braid becomes less effective at lower frequencies due to skin effect.
Once that happens you have a magnetic loop that is not as effective at
rejecting signals as a twisted pair. 

Looking at it another way, the threat signal is induced on the shield
(braid) and it penetrates to couple to the center conductor. 

There are a couple of other ways to look at it, they all ultimately come
back to the same point. There are a few other odd things that happen as well
as you drop frequency, none of them really very helpful for moving signals. 

---------------

Now - is it actually a problem or not? As always, that depends. Most of the
nasty stuff in a 1 pps is in the edges. That energy (with a fast edge) is
nicely captured by the coax. The low frequency stuff is not captured as
well, but it also doesn't couple all that well either. Making a transformer
work to 1 Hz isn't very easy. So, coax is fine for getting the signal to
it's destination. It's only when you look at the ground loops and threat
signals that there could be an issue.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Alan Melia
Sent: Monday, May 20, 2013 2:50 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ground loops in measurements?

Hi Bob whats the problem at low freqs ?? I thought leakage was a function of

the size of the "holes"v the wavelength....or are we into braid skin effect 
below 100kHz?? so as not to drag this OT a reference will suffice in answer.

Best Wishes
Alan G3NYK
----- Original Message ----- 
From: "Bob Camp" <lists at rtty.us>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
Sent: Monday, May 20, 2013 6:00 PM
Subject: Re: [time-nuts] Ground loops in measurements?


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