[time-nuts] Ground loops in measurements?

[Alan Melia]

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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