Are ferrite cable shields on coax a good idea?

I've got some very low level signals at up to 150kHz going down some RG174 cable, which is pretty well shielded (85% coverage or more if I recall the cable spec correctly). It struck me I might be able to reduce noise further by adding a ferrite tube to the cable, which I've often done with unshielded wires. However, when asked to explain this to a colleague, I found I didn't know as much as I thought I did, and couldn't convey how ferrites act as common mode chokes to him.

On thinking about how coax works, with the inner and outer current cancelling each others' fields, I wondered if adding a ferrite might actually *disrupt* the flow of current round the coax and make interference worse rather than better - i.e. by increasing impedance to high frequency currents so that the coax shield was poorer at high frequency. And reading further about ferrite cable shields, they seem to be discussed mainly in terms of reducing emissions rather than reducing incoming interference. Another thing that makes me wonder if mixing ferrites and coax is bad is: articles discussing coax as a solution to interference make no reference to ferrite cores. I am now rather confused on the issue. If anyone has any experience in combining ferrites and coax I'd appreciate advice on this matter.

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Nemo
Reply to
Nemo
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The magnetization in the ferrite depends only on the total current threading the loop. The normal transmission mode in coax has current going one way on the centre conductor and an equal and opposite current on the shield. Capacitive pickup leads to a current flowing in the shield only, and inductive pickup occurs more or less equally on both centre conductor and shield. All currents in the coax can be decomposed into even and odd modes:

i_even = 0.5*(i_centre+i_shield) i_odd = 0.5*(i_centre-i_shield).

You can obviously make any centre conductor and shield currents you like from some combination of i_even and i_odd.

If you put a huge common mode choke in the line, it will leave the differential (odd mode) part of the signal unaffected (no net current, so no magnetization, hence no inductance), but it will impede the common-mode (even mode) part.

For capacitive pickup immunity, you have to rely on the shield preventing any of the ac field from getting inside the coax. This is true whether or not you have a choke--if your pickup is producing a differential signal, it'll go right on through the choke, and otherwise you'll never see it. RG-174 is pretty crappy stuff--double shielded, braid-and-foil, or flexible semirigid coax would be much much better. That low shield coverage makes it talk to the whole world, besides the fact that the shielding relies on good contact being made by tinned wires just lying on top of each other--not very confidence-inspiring.

For inductive and radiative pickup (i.e. like an antenna), the CM choke is a big win, because it prevents RF currents from getting into your board in the first place. For ground loops, you need a whole lot of iron, so the usual right answer is a 1-ohm resistor in the ground lead from the connector, with a bypass cap and a differential amp.

Perhaps the source of confusion is the notion that the choke will open-circuit the shield at AC, leaving the coax vulnerable to pickup along its whole length. That isn't so, because for even mode signals it open-circuits the centre conductor as well, and for odd mode signals it doesn't do anything.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Hi Phil, I was with you up to the last paragraph ! Since the currents on the inside of a co-axial cable are separate from any current on the outside. How can a ferrite tube affect current on the inner conductor ?

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Best Regards:
                     Baron.
Reply to
Baron

It's the total current that matters, not how it's arranged in space. Magnetism is weird that way.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

A ferrite dampens the current on anything that goes through it in the same fashion. So if you run the whole coax through there it will not disturb the differential signal (which you want to preserve) but will muffle asymmetrical currents (which you seem to want to get rid of). You said up to 150khz but didn't say how low it goes. Looks like a 77 or J material toroid would be great. The thicker the better, but also the more expensive. You can slide several of those over the cable to improve things some more. If you must muffle stuff in the kHz range as well you'd also need an old transformer that you can scrape out and run the coax through.

Oh, and make sure to secure the ferrite. Ferrite is brittle, if it clangs against something hard it can shatter.

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Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

I could go on for hours on this subject. Here's my take on the matter:

The desired signal over coax is purely odd-mode, i.e., the current flowing over the inner conductor is accompanied by an equal and opposite current flowing along the inside of the screen.

Interfering signals ('noise') are even mode. If the cable screen had been perfect, these would not affect the desired signal. The screen conductor however has a finite, non-zero, resistance, and so, at low frequencies, an even mode signal will leak into the cable.

There are two strategies to reduce this leakage. One seeks to reduce the ingress of interference by lowering the screen resistance: Use double-screened cable or double up the screen by running a copper braid or thick cable in parallel with it. The other seeks to lower even mode currents by reducing the interfering source (if it can be identified) or by increasing the impedance of the loop formed by the cable screen and the

-unknown- return path. (Not by inserting resistance into the screen, of course! That would be the worst you could do!)

Slipping ferrites over the cable falls in that last category. The trouble is that the loop referred to in the previous paragraph usually has an inductance of the order of 10uH or (much) more. Slipping a ferrite over your cable is not going to increase that substantially, and therefore the effect is likely to be minimal.

In an application of these ideas in a particle accelerator here, I managed to gain an additional 20dB of interference rejection on some position pick-ups in the machine. This enabled us to observe beams with a factor of ten lower intensity, which is just what was needed to measure the probe beams for the LHC.

Jeroen Belleman

Reply to
Jeroen Belleman

Ferrite can help with RF, but not with ground loops. Also, whether pickup is purely even mode or not depends on how the two conductors are loaded.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

How about RF ground loops?

Reply to
MooseFET

I have this gadget in the lab to measure cable leakage. Basically it's a brass pipe with a piece of cable mounted on-axis inside it, so that the space between the cable screen and the pipe wall also is an approximately 50 Ohm transmission line. So we have a coax inside a coax. All four ends are looking into 50 Ohms. The idea is to inject a signal between the cable screen and the pipe wall, and then look how much of it gets picked up on the inner line.

For very low frequencies (

Reply to
Jeroen Belleman

Aha! The infamous ground loop. Whatever you choose to call it, the thing that matters is current flowing over the cable screen. Normally the screen is grounded at both ends. There may be something that forces a potential difference, not necessarily DC, between the two ends. That will cause a current to flow in the screen and since the screen isn't perfect, some of it will find its way into your signal.

The nature of the remedy will depend on the coupling mechanism and the frequency of the agressor signal. The agressor source and the way it couples are often difficult to identify, if only because the 'components' of the equivalent circuit are not on the schematic diagram and of unknown magnitude. Once you get a model of the way the interference gets into your box, the fix is usually obvious.

Jeroen Belleman

Reply to
Jeroen Belleman

If you're working near a transmitter, then yep, you have to worry about those too. Most of the time in my world, ground loops are 60 Hz affairs--a 1-m loop of cable has about 3 uH of inductance, so you wind up with millivolts of signal and circulating currents of hundreds of milliamps. The reactance is of the same order as contact resistances, which is why jiggling the cable often makes a temporary difference.

I'm most commonly trying to keep the spurs down at -170 dBm or someplace like that, right next to a synthesizer or acousto-optic cell driver putting out a watt or so. I have a disgracefully leaky Agilent sweeper (brand new) that produces -58 dBm in my measurement system at 6 GHz, with the sweeper completely disconnected. Pathetic.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Cute idea.

RG223 is double-shielded, though, isn't it? At high frequencies the holes in the shield dominate, particularly if there are any kinks in the cable. Also you were using brand new coax, I expect--once the braid has a chance to oxidize a bit, life gets much worse (again, at high frequency--the strands ought to be continuous throughout the braid).

I'd certainly be interested.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Thank you everyone, loads and loads of useful info there.

To answer your concerns, I don't think ground loops aren't a problem, but I'll look into that. I usually secure ferrites to cables with some heat shrink. I suspect the lowest frequency of interest is about 5kHz - I'll need to check the system specs there, but certainly not DC.

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Nemo
Reply to
Nemo

For 5kHz you'd have to loop the cable several times through the ferrite. Or just use an old audio transformer core or small line transformer core.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

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Unless you loop both of the cable's ends back to exactly the same place, you can't really say that both ends are grounded without qualifications. Consider a cable that is run between two points that are 1/2 a wavelength apart at some frequency. The shield in the cable is never perfect and there are always external sources of radio waves in the environment.

If the COAX in question is on the order of 10,000 feet and is running from down in the ocean and onto a ship, "RF" starts to mean some fairly low frequencies.

Reply to
MooseFET

"Nemo" skrev i meddelelsen news: snipped-for-privacy@furfur.demon.co.uk...

For frequencies that low you may be better off with an "iron" core or maybe a iron powder toroid.

Reply to
Frithiof Jensen

snipped-for-privacy@furfur.demon.co.uk...

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Remember that the core is intended to remove things that are unwanted. The frequencies of the unwanted currents is what really matters.

Reply to
MooseFET

Agreed. That was my point. 'Ground' isn't universal. Between two different points called 'ground' there is always some impedance and depending on conditions, there may be a sizable voltage.

Jeroen Belleman

Reply to
Jeroen Belleman

:On Jan 25, 2:47 am, "Frithiof Jensen" : wrote: :> "Nemo" skrev i meddelelsennews: snipped-for-privacy@furfur.demon.co.uk... :>

:> > Thank you everyone, loads and loads of useful info there. :>

:> > To answer your concerns, I don't think ground loops aren't a problem, but :> > I'll look into that. I usually secure ferrites to cables with some heat :> > shrink. I suspect the lowest frequency of interest is about 5kHz - I'll :> > need to check the system specs there, but certainly not DC. :> > -- :> > Nemo :>

:> For frequencies that low you may be better off with an "iron" core or maybe :> a iron powder toroid. : :Remember that the core is intended to remove things that are :unwanted. The frequencies of the unwanted currents is what really :matters.

Yes, and the ferrite on the coax is intended to prevent the shield of the cable radiating EMI/RFI from the equipment it is connected to where it can be picked up by other sensitive equipment. The ferrite is not there to prevent the equipment it is connected to from picking up stray noise.

Reply to
Ross Herbert

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I have used them to prevent the equipment they are on from picking up noise. If you have a large signal source nearby and a coax that is some significant part of a wavelength long, you can end up with large RF currents flowing in the shield and coupling into the core of the coax.

Reply to
MooseFET

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