Are ferrite cable shields on coax a good idea?

[Ross Herbert]

On Mon, 26 Jan 2009 20:22:41 -0800, Archimedes' Lever

:On Mon, 26 Jan 2009 18:49:35 -0800 (PST), MooseFET <[email protected]>
:wrote:
:
:>> On Sun, 25 Jan 2009 12:18:33 -0800 (PST), MooseFET <[email protected]>
wrote:
:>>
:>> :On Jan 25, 2:47 am, "Frithiof
:>>
:>> :> "Nemo" <[email protected]> skrev i
:>> meddelelsen:>> :>
:>> :> > 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.
:>
:>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.
:
:
: When you get up into GHz ranges, they are referred to as 'spurs'.
:
: One reason why there are not very many switchers used in these bands.
:Linear supplies make a friggin' rack module pretty heavy.
:
: We need a switcher that has a super-smooth output that has no HF
:switcher noise included with it. So what? Super caps, computer grade
:cap banks?
:
: How do we reduce switcher noise so that they do not pose a problem with
:GHz range RF bands?


In my experience the average switchers pump out wideband noise consisting of
many harmonics and they only vary in the degree and level of noise produced. As
such, it is very difficult to suppress pickup on low level input lines in
sensitive equipment. Filtering individual input lines will usually help but the
best solution is to stop it from being produced in the first place, but that
requires special attention in the design of the switcher. It is for this reason
that military and medical grade switchers cost more than the run-of-the-mill
switchers.

 

[Archimedes' Lever]

On Mon, 26 Jan 2009 20:22:41 -0800, Archimedes' Lever

:On Mon, 26 Jan 2009 18:49:35 -0800 (PST), MooseFET <[email protected]>
:wrote:
:
:>> On Sun, 25 Jan 2009 12:18:33 -0800 (PST), MooseFET <[email protected]>
wrote:
:>>
:>> :On Jan 25, 2:47 am, "Frithiof
:>>
:>> :> "Nemo" <[email protected]> skrev i
:>> meddelelsen:>> :>
:>> :> > 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.
:>
:>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.
:
:
: When you get up into GHz ranges, they are referred to as 'spurs'.
:
: One reason why there are not very many switchers used in these bands.
:Linear supplies make a friggin' rack module pretty heavy.
:
: We need a switcher that has a super-smooth output that has no HF
:switcher noise included with it. So what? Super caps, computer grade
:cap banks?
:
: How do we reduce switcher noise so that they do not pose a problem with
:GHz range RF bands?


In my experience the average switchers pump out wideband noise consisting of
many harmonics and they only vary in the degree and level of noise produced. As
such, it is very difficult to suppress pickup on low level input lines in
sensitive equipment. Filtering individual input lines will usually help but the
best solution is to stop it from being produced in the first place, but that
requires special attention in the design of the switcher. It is for this reason
that military and medical grade switchers cost more than the run-of-the-mill
switchers.

The military switchers AND medical switchers we made had no such
considerations. They had reduced ripple specs, but all our supplies
were superior in that area, so we already complied there. Our supply had
to have specific rails that stay up in a failure, long enough for the
system to perform a core dump. Some several tens of milliseconds on a
specific for that purpose +5V rail.

 

[MooseFET]

:> On Sun, 25 Jan 2009 12:18:33 -0800 (PST), MooseFET <[email protected]>
wrote:
:>

:>
:> :> "Nemo" <[email protected]> skrev i
:> meddelelsen:> :>
:> :> > 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.
:
: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.

I dare say that a ferrite core around a coax cable might achieve some degree of
"bulk" suppression of noise pick-up

It got me enough to be worth the effort. Stopping the RF current from
flowing in the shield and eating the RF power in the process worked
well. Non-lossy parts tend to reflect the RF power and send it
looking for another way to cause trouble.

but this would not be optimal.

Not creating the RF in teh first place would be the right answer.

The preferred
method of suppressing pickup in sensitive equipment is to do so on each of the
individual input lines at the input.

Yes and also to keep it off the power lines etc.

This can be achieved using filtered input
connectors

Filtered connectors aren't enough, often.

(if the extra expense warrants it) or simply passing the input lines
through individual ferrite beads as close to the input point as possible.

The ferrite beads method usually works out to cost more in
production. It takes manual labor.

 

[Joerg]

Read 'almost certainly will shatter'.

How about iron powder cores ? Not enough permeability I suppose.

I rarely use iron powder much these days. It's more expensive than
ferrite and ferrite usually does the job.