Grounding an aluminimum enclosure

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <n96s3058svt73teo512r3u62tkd6t3u6g2@
4ax.com>) about 'Grounding an aluminimum enclosure', on Thu, 26 Feb
2004:

But whet's more important

No, it's not necessarily MORE important.

is that, if the pcb is tied to the can at one
point, the result is an RF resonance mode of the pcb circuitry within
the box, sort of the equivalent of the mechanical cantelever resonance
of a beam or plate fixed at one end. The more grounds, the stiffer the
box-to-pcb connection and the higher frequency the resonant modes and
the harder it is to excite them.

It isn't necessarily harder; that depends on the Q and it is in general
unknown and perhaps unknowable.

RF resonances allow external EMI to get into rectifying junctions and
become low-frequency noise and offsets. And RF sources are everywhere
these days.

But with proper enclosure design and connector configuration, RF ingress
is minimised.

 

[Walter Harley]

I'm just wondering - protruding through the same panel of the enclosure we
have mounted a right-angle D-socket. It is PCB-mount but also has bolts
connecting its shell to the outside of the enclosure. If we run a short
track between the grounding pin of the D-socket to the pigtails of the two
PCB-mount RCA sockets, would this be likely to provide a decent solution? As
you say, getting a low resistance connection is quite easy, but low
inductance could be another matter.

Think about that circuit path: RF riding on the shield of the DVD cable goes
to the pigtail, to the short track, and then to the chassis. The pigtail
and ground are inside the enclosure. So, they are capacitively coupled to
anything near them - particularly the "short track," which is on the PCB
probably right next to something you don't want RF on.

Also, the pigtail and track form a little antenna, which will inductively
radiate the RF to anything else inside the enclosure. The antenna
wavelength is short compared to most of the RF you encounter; but, try
holding a cell phone next to the DVD cable... compare the size of the
pigtail+track to the size of the cell phone antenna.

You really want to keep the RF on the outside of the chassis. Connect the
shield to the chassis on the outside, rather than bring it inside with a
pigtail. If you must have a pigtail, keep it as short as possible - don't
run a wire, and a trace, to some other part of the chassis just because
you've got a convenient ground lug there.

Basic rule of thumb with RF: do not assume that a wire creates an
equipotential.

It's like trying to contain a splash of water with a garden hose. A bit of
water might go down the hose, but 99% of it will get on your pants.

 

[John Woodgate]

If we run a
short track between the grounding pin of the D-socket to the pigtails of
the two PCB-mount RCA sockets, would this be likely to provide a decent
solution?

How long are your pigtails? If you need to show EMC immunity up to 2
GHz, even 10 mm may be on the long side.

 

[Jim]

Think about that circuit path: RF riding on the shield of the DVD cable goes
to the pigtail, to the short track, and then to the chassis. The pigtail
and ground are inside the enclosure. So, they are capacitively coupled to
anything near them - particularly the "short track," which is on the PCB
probably right next to something you don't want RF on.

Also, the pigtail and track form a little antenna, which will inductively
radiate the RF to anything else inside the enclosure. The antenna
wavelength is short compared to most of the RF you encounter; but, try
holding a cell phone next to the DVD cable... compare the size of the
pigtail+track to the size of the cell phone antenna.

You really want to keep the RF on the outside of the chassis. Connect the
shield to the chassis on the outside, rather than bring it inside with a
pigtail. If you must have a pigtail, keep it as short as possible - don't
run a wire, and a trace, to some other part of the chassis just because
you've got a convenient ground lug there.

Basic rule of thumb with RF: do not assume that a wire creates an
equipotential.

It's like trying to contain a splash of water with a garden hose. A bit of
water might go down the hose, but 99% of it will get on your pants.

That all makes complete sense, Walter, and is very clear, thank you. Problem
is we are likely to be have to stick with the PCB-mount RCA sockets with
pigtails. So I'm just wondering how on earth we are going to be able to
connect the shield of our RCA socket to the *outside* of the enclosure? The
rear panel is just 4" x1". Our product is due to sell for approx $300, and
looks matter considerably in our market. I can see we are going to have to
compromise somehow.

Jim

 

[Jim]

How long are your pigtails? If you need to show EMC immunity up to 2
GHz, even 10 mm may be on the long side.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

The pigtails of our RCA sockets are 6mm long in fact. As for immunity
requirements, well you were kind enough to address our EMC requirements in
another thread a little while ago - many thanks for all the unpaid work!
http://groups.google.com/[email protected]

Jim

 

[John Larkin]

That all makes complete sense, Walter, and is very clear, thank you. Problem
is we are likely to be have to stick with the PCB-mount RCA sockets with
pigtails. So I'm just wondering how on earth we are going to be able to
connect the shield of our RCA socket to the *outside* of the enclosure? The
rear panel is just 4" x1". Our product is due to sell for approx $300, and
looks matter considerably in our market. I can see we are going to have to
compromise somehow.

Jim

Solder the pcb-mount RCA right to the board, into the ground plane (or
a copper ground island, if it's not a multilayer.) Secure the PCB into
the enclosure using screws into spacers or bosses or whatever, making
a short-distance metallic bond from the pcb ground to the box close to
where the connector is grounded. Do this for all three connectors -
in, out, and power. If you want to float the output connector, a small
cap - 1 nf maybe - from its shell to the local ground is a good idea,
but make sure it can handle a static zap.

This should be fine.

We do stuff like this all the time, with super-wideband low-level
signals, no problem:

http://www.highlandtechnology.com/DSS/T600DS.html

John

 

[John Larkin]

How long are your pigtails? If you need to show EMC immunity up to 2
GHz, even 10 mm may be on the long side.

Surface-mount ferrite beads are magic.

John

 

[Mac]

Hi Mac,

Thank you for your information. The SPDIF digital audio spec says it is 0.5V
pk-pk differential. All outputs are meant to be driven by an isolating pulse
transformer, but in practice this is sometimes not the case.

Ah, I didn't realize we were talking about digital audio. I don't know
much about it, in fact. But now it makes sense that you only have one
channel, instead of two. ;-)

Pulse Engineering make one of the favoured pulse transformers for SPDIF.
They show an application circuit on page 2 (69KB):
http://www.pulseeng.com/pdf/T601.pdf

Transformers are great for EMI/EMC. You can also put in a common mode
choke, which is kind of like a sideways 1:1 transformer. These can
definitely help attenuate high-frequency common-mode signals. A common
mode signal is one which is present on both lines of the differential
pair.

I'm confused by this! The diagram suggests grounding the shield of the
output socket - RCA is only two-pole, so does this mean grounding the
enclosure leaving the lowside output floating (not what people are advising
in this thread). Or does it mean ground the lowside of the output, in which
case there's a connection missing from their diagram isn't there?

I read your post further downstream, where you figured this all out.

Thanks,

Jim

--Mac

 

[Jim]

Solder the pcb-mount RCA right to the board, into the ground plane (or
a copper ground island, if it's not a multilayer.) Secure the PCB into
the enclosure using screws into spacers or bosses or whatever, making
a short-distance metallic bond from the pcb ground to the box close to
where the connector is grounded. Do this for all three connectors -
in, out, and power. If you want to float the output connector, a small
cap - 1 nf maybe - from its shell to the local ground is a good idea,
but make sure it can handle a static zap.

This should be fine.

We do stuff like this all the time, with super-wideband low-level
signals, no problem:

http://www.highlandtechnology.com/DSS/T600DS.html

John

Thank you John for your practical advice - that should all be possible for
us. It's also very good to know that there are products out there working ok
with that method - I was beginning to feel we had a no-win situation.

-Jim

 

[Jim]

On Thu, 26 Feb 2004 12:21:49 +0000, Jim wrote:

Transformers are great for EMI/EMC. You can also put in a common mode
choke, which is kind of like a sideways 1:1 transformer. These can
definitely help attenuate high-frequency common-mode signals. A common
mode signal is one which is present on both lines of the differential
pair.
--Mac

I'll look into the common mode choke, thanks (I realise now though that the
signal is not differential). Anything we can do to minimise EMI/EMC issues,
without affecting the price or aesthetics of the product too much I've very
keen to do.

Jim