twisted shielded pair cable, using shield as return

Oct 26, 2013 6 Replies
M

I have an application where there is a twisted shielded pair cable with a transformer coupled high frequency communication signal on it. the center taps of the transformers will carry DC for powering the device on the opposite end of the cable. The cable has an outer insulator so the shield is insulated from any other return path.



Unfortunately the ground of the cable will be connected to chassis on both ends.



The load end will have a switching power supply (buck regulator) so the currents on the centertap "phantom mode" will be somewhat noisy.



The differential signal is bidirectional 10Mbit/sec Manchester encoded signal) We do not want this signal to escape the cable either.



A couple of choices.


1) terminate the shield only on one side to make it a Faraday cage and add a separate return wire for the DC power on the CT.

I don't like this one because there will certainly be a loop are between the separate return wire and the twisted pair. Also when the transmitter on the far end sends its signal, there is not return path back to the source for the currents that couple between each conductor and the shield.


2) Terminate the shield on both sides and use the shield as the return for the DC currents as well

Connecting the shield on both ends provides the return for HF current generated between the pair and the shield. Also intimately couples the power supply generated high frequency harmonics of the DC power currents with the pair so the loop area is as small as it can be.


Assuming these are the only two choices, which of the two would radiate the least amount of noise to nearby circuitry.


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B

You can eliminate the high frequency current components on the cable with lumped component filtering at the switching power supply and by using a ferrite on the cable:

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K

That works but adds a wire.

Don't like that at all.

There is a third choice. Couple the power and ground onto the pair, i.e. run a differential DC voltage on the signal pair. I'm using a network that does this, now. The EMI with only UTP looks really good.

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J

That's not unfortunate, that's usually the best way to do it.

2.
John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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R

ABSOLUTELY TRUE! Except, you didn't mention how long the cable is, nor from where to where it goes. For example, if over 1000 ft, not a good idea to tie the two toegther due to AC mains inductance and AC mains ground reference shifting, which can cause so much current in the shield, ...well, don't touch it. Or, if shorter run, but goes from one facility to another, again, not a good idea to tie the two GNDs together.

Short runs, GND to GND is the BEST way to lower noise, even if you have messed up soemwhere else [which is avoidable] and need to put an 'after the fact' ferrite snap on 'bead' on the cable. You'll still be better off GND-GND, Think of it as the continuation of the Faraday cage over BOTH your systems.

Best way to do what you want is to use center tap transformer for data transfer - BOTH ENDS *and* because from experience center tap is more of an 'idea' than a fact and you'll still get a bit of 'wobble' physically place a common mode choke on the pair just as they exit each system and enter the transferring cable shield.

Next, for the DC, filter the noise out! at both ends!! Supply through center tap and use the shield as return. Although not ideal [forces a tight connection between power supply return and the chassis gnd], but with judicious design of the rest of the stuff; you will get away with it.

Even if the remote system is plastic case, you can still make such a power distribution work without failing Medical Class B specs.

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J

The longer the run, the higher the resistance of the shield, so the lower the "ground loop" current for a given driving voltage. It works out.

Aren't cable TV coaxes grounded all over the place? They certainly are at every residential entry.

John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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J

Except for those realistic cases where gnd-to-gnd is 60VAC. BTDT

...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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