Hi all,
what is the current scholarly thought about cabling within an enclosure? I mean, for wiring to/from I/O and pots or swtches. I see problems with all shielding options:
- leave shield disconnected at both ends: ineffective shield
- leave shield disconnected at one end, ground the other on signal ground: shield may act as antenna and inject noise to ground
- as above but use enclosure to ground one end: you create an enclosure protrusion, some say this is not good
- connect shield at both ends to ground or enclosure: ground loop.
Incidentally, if using unshielded wiring inside an eclosure, I also see dilemmas:
- twist tightly (as often recommended): create high capacitance, which the usually unprotected outputs of, say, op amps *within* a circuit won't like. Plus, may pick up crosstalk
- don't twist: create a loop antenna
Err. There has got to be a compromise that works. opinions?
MBK
what is the current scholarly thought about cabling within an enclosure? I mean, for wiring to/from I/O and pots or swtches. I see problems with all shielding options:
- leave shield disconnected at both ends: ineffective shield
- leave shield disconnected at one end, ground the other on signal ground: shield may act as antenna and inject noise to ground
- as above but use enclosure to ground one end: you create an enclosure protrusion, some say this is not good
- connect shield at both ends to ground or enclosure: ground loop.
Incidentally, if using unshielded wiring inside an eclosure, I also see dilemmas:
- twist tightly (as often recommended): create high capacitance, which the usually unprotected outputs of, say, op amps *within* a circuit won't like. Plus, may pick up crosstalk
- don't twist: create a loop antenna
Err. There has got to be a compromise that works. opinions?
MBK
not much interest...
Thanks Lucpes for your input. Though Microphone cable is usually too stiff to use it for connecting, say, volume or balance pots in an enclosure, and we still break some rules as of my original post.
This thread didn't generate much interest, I wonder why. It's a situation everyone has in DIY and the recommendations I found are mutually conflicting.
Thanks Lucpes for your input. Though Microphone cable is usually too stiff to use it for connecting, say, volume or balance pots in an enclosure, and we still break some rules as of my original post.
This thread didn't generate much interest, I wonder why. It's a situation everyone has in DIY and the recommendations I found are mutually conflicting.
Actually, I'm very interested in this subject myself. I've seen a wide variety of different methods used in commercial audio equipment and I wonder what works best.
My current thinking is to use isolated RCA jacks and differential amplifiers for input circuits. Output wires would ground the shields at the source end. At switches I'll ground the source end and leave the other side unconnected. No matter what scheme I think of though there's always the chance for ground loops when connecting to external equipment like tape decks.
My current thinking is to use isolated RCA jacks and differential amplifiers for input circuits. Output wires would ground the shields at the source end. At switches I'll ground the source end and leave the other side unconnected. No matter what scheme I think of though there's always the chance for ground loops when connecting to external equipment like tape decks.
Maylar,
I think that's a good idea, to use signal ground, at least you don't get the "outside world" into the enclosure.
As for me, my compromises are evolving... For DIY stuff I now do everything balanced or pseudo-balanced (equal impedance on signal ground output). That makes the shield grounding *much* easier (shield to enclosure and hot and cold to diff input - no shield penetration, no ground loop, no infestation of signal ground). For RCA I use insulated jacks, then all RCA shield/grounds go to the same diff amp as for balanced inputs. Problem here is that the shield should have a zero impedance connection to enclosure at entry point and not penetrate the enclosure... in theory... but then you mix shield currents and signal ground currents. I guess with RCA you can't get it "theoretically" right. I may change the scheme to connect all unbal grounds to enclosure at a single point, then 50R to diff amp input. Then at least the shield does what it's supposed to, and the 50R should be protection enough for the signal ground. All this however leaves the question as to the internal shielding - because here you basically have to connect the shield at receiving end (diff input) or else you get outside shield currents inside your enclosure and it's going to radiate inside.
For switches and pots I now use mini-coaxial, very flexible, but I suspect high capacitance. In some cases the source has an R impedance so that's ok, best would probably be to use 50R or so at the source in case the sourc is a "naked" op amp output, to prevent ringing/oscillation. Outputs, I usually don't bother but try to get the output close to the connector, and twist the wires.
Do you have any idea if shields connected at just one end will really act as significant antennas? This pops up every once in a while up to the point that ppl recommend plastic XLR jacks just so that no portion of the enclosure shield, even the 1/2 inch of an XLR jack, protrudes inside the enclosure. Because if this really matters, then even say, the shield of a cable to a switch, connected at source = signal ground, will amplify the problem rather than help it...
MBK
I think that's a good idea, to use signal ground, at least you don't get the "outside world" into the enclosure.
As for me, my compromises are evolving... For DIY stuff I now do everything balanced or pseudo-balanced (equal impedance on signal ground output). That makes the shield grounding *much* easier (shield to enclosure and hot and cold to diff input - no shield penetration, no ground loop, no infestation of signal ground). For RCA I use insulated jacks, then all RCA shield/grounds go to the same diff amp as for balanced inputs. Problem here is that the shield should have a zero impedance connection to enclosure at entry point and not penetrate the enclosure... in theory... but then you mix shield currents and signal ground currents. I guess with RCA you can't get it "theoretically" right. I may change the scheme to connect all unbal grounds to enclosure at a single point, then 50R to diff amp input. Then at least the shield does what it's supposed to, and the 50R should be protection enough for the signal ground. All this however leaves the question as to the internal shielding - because here you basically have to connect the shield at receiving end (diff input) or else you get outside shield currents inside your enclosure and it's going to radiate inside.
For switches and pots I now use mini-coaxial, very flexible, but I suspect high capacitance. In some cases the source has an R impedance so that's ok, best would probably be to use 50R or so at the source in case the sourc is a "naked" op amp output, to prevent ringing/oscillation. Outputs, I usually don't bother but try to get the output close to the connector, and twist the wires.
Do you have any idea if shields connected at just one end will really act as significant antennas? This pops up every once in a while up to the point that ppl recommend plastic XLR jacks just so that no portion of the enclosure shield, even the 1/2 inch of an XLR jack, protrudes inside the enclosure. Because if this really matters, then even say, the shield of a cable to a switch, connected at source = signal ground, will amplify the problem rather than help it...
MBK
Seriously doubtful, especially if they're dressed along the chassis. This grounding method is standard in instrumentation, low noise, and other precision applications.
If you want to be a real maniac, use a buffer amp to drive the shield so that the inner conductor and outer conductor track one another. That gets rid of any issues with respect to shielded cable capacitance, and if the buffer has the requisite low-source Z, the shield will still provide shielding. This is wildly more complicated than needed for this application, but so is worrying about a shield injecting stuff into your ground. If you limit the bandwidth of your circuit to any sane value, it's an absolute non-issue.
A Crystal Semiconductor appnote I once read mentioned returning all shield grounds to the star point separately. This was in a different context (DAC output driving coax out of the enclosure) but this will solve the problem of RF pickup from the shield injecting itself into the local ground (which will be isolated from star point by interconnect impedance). Then again, the shield is *inside* a metal box, which is unlikely to have a strong enough RF component to be significant when it's being coupled in by a poorly matched antenna into a very un-cooperative load (the inductance of the wire from the star point to the circuit under considertion).
SY,
RF signals so injected can get rectified and introduce interference (ghost voices from AM radio stations, TV frame buzz, etc)
SY,
RF signals so injected can get rectified and introduce interference (ghost voices from AM radio stations, TV frame buzz, etc)
I think that level of 'precaution' is nonsense.
I'm sure the best solution is to use balanced lines throughout, and only use the shields for RFI protection. As long as no signal passes through them there's no chance of ground currents becomming part of the signal. But that doubles the amount of switch contacts required and makes buffers more complicated.
Single ended signals that use ground as a return are more complicated to evaluate. Especially if signals travel between multiple PC boards. The question I have is how to wire preamp, filter, and output boards together and make sure that only the appropriate return signals travel down interconnect shields (or even separate wires) and not power supply currents.
I need to think about this...
I'm sure the best solution is to use balanced lines throughout, and only use the shields for RFI protection. As long as no signal passes through them there's no chance of ground currents becomming part of the signal. But that doubles the amount of switch contacts required and makes buffers more complicated.
Single ended signals that use ground as a return are more complicated to evaluate. Especially if signals travel between multiple PC boards. The question I have is how to wire preamp, filter, and output boards together and make sure that only the appropriate return signals travel down interconnect shields (or even separate wires) and not power supply currents.
I need to think about this...
Sorry, but a few inches of shield inside a chassis is highly unlikely to pick up RF at anything less than UHF. Take a look inside your oscilloscope, for example.
RF is FAR more likely to originate from stuff outside the box, and is best dealt with using standard methods (e.g., input resistors and caps, ferrite beads, external shields...).
That is a VERY pertinent question. There's some good discussion in Horowitz and Hill, and a very illuminating set of app notes on the Jensen Transformers website. You may not want to implement transformers at every signal junction as they'd like you to, but the basics of grounding problems and solutions are very well treated.
Don't worry, Im just concerned about practical issues that make sense. But what makes most sense (each circuit portion considered as what it is, a complete closed circuit), is also most impractical to achieve, hence the "Common" ground returns that are a compromise.
I also have the problem of connecting multiple proto boards of a single circuit. In my initial projects, I actually ran star ground from all signal returns... ! Worked like a charm, but was very impractical. But I still go as far as possible to separate the returns. One other idea I had was to at least have a separate ground trace reserved for all decoupling caps, and one trace for signal returns.
MBK
I also have the problem of connecting multiple proto boards of a single circuit. In my initial projects, I actually ran star ground from all signal returns... ! Worked like a charm, but was very impractical. But I still go as far as possible to separate the returns. One other idea I had was to at least have a separate ground trace reserved for all decoupling caps, and one trace for signal returns.
MBK
Well, in theory RF techniques should be of little relevance to audio, because audio signals are at much lower frequencies. So, initially I didn't worry about RF, I thought it doesn't apply here.
But from the literature and my own experience, two developments have changed that:
- op amps for audio are very fast now, and perfectly capable to amplify RF picked up somewhere;
- RF pollution has become much stronger: TV, FM, and cell phone transmitters, household appliances etc., not to speak of the RF garbage that comes from your own CD player.
As a consequence, whole books are being written to combat RFI and EMI even *within* circuit boards - not to speak of cable connections to and from. And there is plenty of engineering advice that essentially suggests using almost RF type layout and shielding techniques even for audio, because if you don't, the demodulation of interference signals may indeed intrude into audio levels.
MBK
But from the literature and my own experience, two developments have changed that:
- op amps for audio are very fast now, and perfectly capable to amplify RF picked up somewhere;
- RF pollution has become much stronger: TV, FM, and cell phone transmitters, household appliances etc., not to speak of the RF garbage that comes from your own CD player.
As a consequence, whole books are being written to combat RFI and EMI even *within* circuit boards - not to speak of cable connections to and from. And there is plenty of engineering advice that essentially suggests using almost RF type layout and shielding techniques even for audio, because if you don't, the demodulation of interference signals may indeed intrude into audio levels.
MBK
Last weekend i re-wired a tube amp (old wires were burned in many places).
The old wires were shielded microphone wire with one end of the shield to ground. The noise floor were below audiable.
I re-wired it with unshielded teflon insulated wire (now the owner of the amp wont have to worry about soldering inside it, since the teflon can stand the heat of a soldering iron), and there is still no audiable noise.
My conclusion to that is that shielded wire is for applications that are utterly noise sensitive, or built inside a wooden box.
Magura
The old wires were shielded microphone wire with one end of the shield to ground. The noise floor were below audiable.
I re-wired it with unshielded teflon insulated wire (now the owner of the amp wont have to worry about soldering inside it, since the teflon can stand the heat of a soldering iron), and there is still no audiable noise.
My conclusion to that is that shielded wire is for applications that are utterly noise sensitive, or built inside a wooden box.
Magura
MBK said:
Couldn't you connect one end of the shield directly to ground and the other end to ground via a 100nf ceramic cap?
Wouldn't the shield then act like it was only conected at one end at low frequencies thus preventing ground loops, and for R.F frequencies the shield would act as though it is conected at both ends stopping it from acting like an antenna!
Billy
Magura: RFI problems seem to be very location dependent. While some never have any problems, other people struggle with mysterious effects. Depending on which time of day which emitter is on or off, it may even appear or disappear sporadically in one and the same location.
Billy: good idea. That is recommended pro audio practice anyway for signal ground to enclosure connection. Didn't think of it in the "wires inside enclosure" context.
Traderbam: The shield is part of the signal pathway, but only for unbalanced connections (RCA) where it is unavoidable. The shield is the ground return. The distinction between "signal" and "ground" is arbitrary. It is one and the same electrical circuit.
Incidentally I suppose that balanced connections are used in audio simply because many powered and earthed devices are connected together, so you better use balanced connections to solve grounding problems. It does not seem to be necessary simply because of the shielding issue: A simple shielded wire with signal return in the shield seems to be perfectly fine in applications much more sensitive to RF interference: Oscilloscopes and antenna cables.
MBK
Billy: good idea. That is recommended pro audio practice anyway for signal ground to enclosure connection. Didn't think of it in the "wires inside enclosure" context.
Traderbam: The shield is part of the signal pathway, but only for unbalanced connections (RCA) where it is unavoidable. The shield is the ground return. The distinction between "signal" and "ground" is arbitrary. It is one and the same electrical circuit.
Incidentally I suppose that balanced connections are used in audio simply because many powered and earthed devices are connected together, so you better use balanced connections to solve grounding problems. It does not seem to be necessary simply because of the shielding issue: A simple shielded wire with signal return in the shield seems to be perfectly fine in applications much more sensitive to RF interference: Oscilloscopes and antenna cables.
MBK
SY said:
Exactly what I mentioned in my post:
What I meant was signals coming from outside will carry RF on the shield (WRT the system star ground), and unless the path from the connector to the system ground has low inductance, the RF will get detected by the input amplifier's transistor junction. That's why the shield should go direct to the star point. (I think!)
Don't we all?
traderbam said:
Yes, I guess that is an old and common trick. I have used that
in homemade interconnects, although I used two coax's instead,
one for signal and one for ground and connected the shields to
ground at one end only. I also recently used the same principle
for a headphone extension cord, using a shielded cable with
three conductors, connecting the shield only at the source end.
I suppose one potential problem with this approach is increased
capacitance.
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