Chassis -- shielding and ground

Very, if you don't let RF sneak in over the power and signal leads.

Might make things better, might make things worse.

John

Steel might be better at reducing magnetic interference. I just put a mu metal shield around a sensitive camera. High gain stages will obviously be affected more. Internal transformers are a problem.

greg

Using my magnetic/electrostatic/vibration/light detector around a system, we found 24 Hz comming in from the arc lamp. Can't be sure if its the lamp or power supply right now. There was considerable magnetic fields at 60 Hz around the camera, as well as extranious 120 hz from lamps around the corner. There is also 200 hz light noise by the lamp being modulated by the fan.

greg

Common steel boxes, like Hoffmanns, are not much better magnetic shields than cardboard.

John

It's an equipotential so it can stop electric fields dead in their tracks. If it is dicast aluminium it is transparent to magnetic fields.

It's worthless if you don't. If it is floating then it will go up an down in sympathy with any local electric field, and thus is will create a similar electric field inside itself. If you connect it to a fixed potential i.e. ground it will stay at that potential and external electric fields will not appear inside it.

Cheers

Ian

Well the aluminum lets through magnetic fields when the thickness of the box is less than the skin depth at that frequency. So the aluminum box may be no good at 60 Hz, But it will block magnetic fields at say 20 kHz.

There are no electrostatic fields inside the box whether it is grounded or left floating. But as John L. said sometimes I find that connecting the case to earth ground helps and sometimes it makes things worse. It's always a bit of a mystery why this is... Sometimes I find it depends on what other 'crap' is connected to the earth ground.

George H.

Quite. Not so much B as E, though.

It depends on your grounding scheme. For unbalanced, unless you need it for code, don't.

-- Les Cargill

For unbalanced audio, this isn't necessarily a problem. You're borrowing ground from the shield of the cables to and from. If you *do* ground it, you may get ground potential on the shields.

There are, of course, exceptions. Guitar stompboxes are usally zinc, not Al, and they make fine little farady cages - you get EMI/RFI from the jacks, not through the case.

-- Les Cargill

It all depends. Man with two grounds never know where ground truly is...

-- Les Cargill

Between ears ;-) ...Jim Thompson

Crazy.

John

I've use the spray-on nickle coating. Certainly effective for RFI.

What kind of RF environment ?

Is this some kind of AM (1 MHz) or FM (100 MHz) broadcast station or perhaps close to some radar installation on a ship (9 GHz) ?

A diecast box is very effective, when the lid is connected to the rest of the box with screws less than 1/4 wavelength apart (8 mm@9 GHz).

However, the real problem is connecting signals in and out of the box and the power arrangement.

In a high RF environment, any wires (signal and power) going through the wall should use proper RF bypass capacitors, however, the problem is how to solder the capacitor ground electrode to aluminum. A copper or steel enclosure might be easier in this sense.

After this, all wires should have some series inductance (preferably on some lossy ferrite core to avoid resonances) and small parallel capacitance (preferably low inductance surface mount) to keep any RF out of the signal and power lines.

Grounding is a very controversial issue with unbalanced audio systems due to the problems with 50/60/150 Hz hum. I would design an audio product assuming that signal and chassis grounds are different and provide an easy way for the end user to connect the signal and chassis ground together (adjacent terminals etc.).

Paul

Let me clarify a bit -- (I asked the original question). Also, I'm talking about a "typical" home environment, not a radio station or industrial one.

There are 2 scenarios I'm interested in --

(1) The PCB and the unbalanced audio would be "floating" inside the chassis. (with chassis only at earth ground, not the signal shield) (2) The ground side of the unbalanced audio would be part of the chassis. (like just about everything is designed) -- (usually without any chassis earth ground)

Wouldn't #1 be the ideal way to go IF the surrounding chassis were at earth ground potential?

Of course, the hard part would be getting the audio in and out of the enclosure but there are jacks that can be insulated when you mount them.

In that case, why bother with a metal enclosure ?

Just use a plastic box with a two sided PCB, in which one side is a common ground plane. The EMC filtering should be grounded to this ground plane, but the real question is, should this ground plane be connected to the audio signal ground and if so, how ?

Here in Finland, up to the 1990's, the real EMC issue with guitar and PA systems was keeping the multiple megawatt MW/SW transmitters just on the other side of the border out of audio equipment. After that the problem was keeping the GSM mobile phone (900/1800 MHz) out of the audio circuitry.

In both cases, the essential thing is keeping the RF out of the PCB, so effective "border control" (ferrites, chip capacitors) is essential both on signal input and output lines as well as power lines.

Due to the 50/60/150 Hz hum issues, I would definitively keep the audio signal ground separate from the chassis ground. If these grounds are connected, then this should be done with a single wire to the audio ground. The EMC filter capacitors (less than 1 nF) can be grounded to the enclosure.

When using 6.35 mm jacks in the enclosure will effectively allow RF to penetrate the box wall and reradiate inside the box, thus, it is essential that the PCB has proper RF filtering on all ports or the RF is killed by soldering a bypass capacitor directly on the 6.35 mm jack.

Paul

I guess what I'm trying to get at is the importance of an earth grounded shield (chassis) in any type of circuit -- whether it's sensitive to incoming noise or emits noise itself.

Assuming you're doing everything else as you should or can, filtering etc.

And forget about unbalanced for a minute -- I would always keep the unbalanced shield off earth ground in audio but balanced, yes because it's not part of the signal.

I've always thought it was good practice.

Like I say, if the circuit itself is floating (independent) than isn't surrounding it with an earth ground package a PLUS?

The reason I originally asked, was I was always amazed at the amount of consumer unbalanced audio equipment that never used a 3 prong outlet -- yet some higher end, especially power amps did. (probably because they needed a higher gauge wire, so what the hell?, right?)

Anyway what I was getting at is just this --

is the cabinet alone a good enough shield OR Is the cabinet with an earth ground a BETTER shield.

Went back and read the answers carefully.....

For electrostatic fields -- yes?

For electromagnetic -- no? (or not necessarily)?

For electromagnetic, aluminum is a poor choice compared to steel (plus zinc and some others?)

For EM, depends on frequency. My induction heater lives inside a box of 0.093" aluminum. Nothing is getting through that (although it would be hard to measure, since I've got the work coil blasting nearby, anyway ;-) ). It runs at 20-100kHz. Aluminum of that size isn't even too bad at 60Hz, IIRC, though mild steel will do better.

Zinc is almost as conductive as aluminum, and a lot heavier, so if you need something mechanically and electrically dead, a diecast zinc sacophagus will do an excellent job.

Static magnetic fields aren't usually a problem, but where they are (e.g., CRT scope?), mu metal shields are excellent.

As for grounding, the ground line is for protection only. It often carries noise, which is why it's such an awful problem to hook up grounded equipment. If you have something sensitive, you're better off grounding the case to the circuit, leaving it isolated from the line.

Tim

But lets say you had a rack of sensitive equipment and you could use a separate copper rod as an earth ground and NOT the AC breaker box earth ground? Even with it's own insulated heavy gauge wire to the ground rod.

And there were no ground loops.

Would you then use the copper rod earth ground?

In that case earth ground is the way to go, isn't it? (don't recording/radio studios do this?).

You would actually leave off the ground wire going to the junction box?

(Or would you have an inferior "protection circuit" then?)