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Old 06-22-2017, 11:14 AM   #1
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Default Hum on 100-pair cables

I'm trying to figure out the simplest way to minimize noise coming into our facility on a couple 100-pair cables.

The application is we have several remotely controlled radios in an existing installation. The user talks on the remote end, and the radio keys in response to audio on the line. Everything we are doing is in the audio spectrum, nothing digital.

The signals are sent through probably a mile worth of 3 separate cables, with 4 demarcs inbetween. It's an aging mess of copper, to be sure. The lines we use are all dedicated to this purpose - no active electronics along the way, no dial tones, no ring voltages. Just wire wrapped telephone blocks and a punchdown block at either end.

On the lines that do have a lot of noise, if I pull the fuse from the cable head, the noise goes away. So we know that the noise is not coming from inside the building. It's coming in over the 100 pair cables.

On both cables, less than a quarter of the lines are being used. The rest are not connected, and are all sitting open on the demarc. I read in one spot that shorting all the unused pairs can help. Is this true? And should each pair be individually shorted, or just short all of them together? And should they get tied to ground?

The noise is not consistent pair to pair. Some pairs work just fine. Some have a little noise. Some have a lot of noise so as to make the resulting audio unusable.

Any help would be appreciated.

Thank you!
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Old 06-22-2017, 11:29 AM   #2
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Who is responsible for the lines coming into the building? Are they old? The one way I know to reduce noise is to have twists in the pairs. The more twists the better so maybe you need an upgrade. Not sure-- hopefully someone else can help
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Old 06-22-2017, 12:20 PM   #3
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Asymmetric leakage on one wire of a pair can cause noise to be present on that pair.
Another surprisingly common cause for problems is incorrectly assigning a two wire circuit to wires from two different pairs.
This can happen even though the color coding is right at both ends if there are splices along the route.

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Old 06-22-2017, 01:12 PM   #4
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Grounding the unused pairs at the frame sometimes works. I don't think shorting them together without grounding them will be helpful and might be harmful but it's certainly easy enough to test.
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Old 06-22-2017, 03:17 PM   #5
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Yes...ground the unused pairs at every point available. It's easier than testing along each path where they could be exposed to something causing the hum. Use proper grounding paths, not some made up or separate ground.

What modulation scheme is this...for the radios? The EMF may be generated by the gear. Has it always been like this or is it progressively worse? If there is a radio counterpoise, then the grounding scheme should be that the unused pair scheme should bond at a single point at each point along the path, separate from the counterpoise physically, but electrically the same.
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Old 06-22-2017, 03:40 PM   #6
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upgrade from your 19th century technology to 20 or 21st century.
there has to be at least 1000 ways to improve it. not trying to be a smarta$$, just stating the facts.

as always, just my $.02
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Old 06-22-2017, 06:30 PM   #7
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The radios are probably using FSK, (frequency shift keying) and noise can effect it. Pick and choose a few good lines, ground one end only of the unused lines.
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Old 06-22-2017, 06:58 PM   #8
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Also verify your sheath bonding and grounding at your splice cases and demarcs. Water intrusion a posibility?
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Old 06-22-2017, 07:44 PM   #9
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Wet splices and flooded U/G vaults are notorious for induced noise.
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Old 06-23-2017, 03:56 AM   #10
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In answer to some of the replies:

@Dennis

We have access to the demarcs, but not the vaults and splices inbetween. The people who maintain the telephone lines "own" them, but they allow us access to the demarcs to do our own troubleshooting. The lines are old. At least a decade or two.

@splatz

Today I tried shorting all unused pairs, and grounding. Unfortunately, it didn't help. More details below.

@CubaPete

These are just AM radios. This is an ongoing issue that is getting worse as the cables age.

@wildleg

There is a $1Million+ project in the works to get new radios and fiber. But we need to keep the current system working now because the new install probably won't happen for at least 6 months.

@drsparky, jw0445, joebanana

Flooded vaults are an extremely likely scenario. Unfortunately, nothing I can do about that at my level.

Update on today:

Today I tried shorting all the unused pairs together by punching down a single piece of wire to all the unused pins on one of the 100pr cables. This didn't affect the noise. I then connected the wire to a ground, and it made the noise about 15db worse. I then used the cut side of the punchdown tool to cut the wires between separate pairs so that only individual pairs were shorted, and this had no effect.

I measured the voltage across the open pairs, and I'm getting about .3 VAC on a Fluke TrueRMS multimeter.

I also hooked our telephone transmission test set to several ground points in the building and got about 45db of noise. This seems to explain why connecting the open wires to ground made the noise worse on the pair under test.

Right now my plan is to isolate each leg of our spare paths, and do a noise check on each one. I'll then "cherry pick" the quietest pairs. At this point, this seems like my only option that is within my power to implement.

What I really want to do is point out to the operators that our radios are not THX certified.
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Old 06-23-2017, 09:26 AM   #11
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Old 06-23-2017, 09:36 AM   #12
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Quote:
Originally Posted by DanielLW View Post
The application is we have several remotely controlled radios in an existing installation. The user talks on the remote end, and the radio keys in response to audio on the line. Everything we are doing is in the audio spectrum, nothing digital.
So it's a telephone


Quote:
Originally Posted by DanielLW View Post

The noise is not consistent pair to pair. Some pairs work just fine. Some have a little noise. Some have a lot of noise so as to make the resulting audio unusable.

Any help would be appreciated.

Thank you!
I worked a lot with telcos and telephone/data lines to remote locations ... Their outdoor cables go bad. We had pairs go bad all the time ... they come out and change pairs ... problem solved.

When they run out of pairs, they call the crews out to run new cable.
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Old 06-23-2017, 10:56 AM   #13
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Quote:
Originally Posted by drsparky View Post
The radios are probably using FSK, (frequency shift keying) and noise can effect it. Pick and choose a few good lines, ground one end only of the unused lines.
I wasn't sure of the modulation scheme and which grounding method to suggest, so I went with RF multipoint rather than audio single-point source grounding.

Quote:
Originally Posted by DanielLW View Post
In answer to some of the replies:

I measured the voltage across the open pairs, and I'm getting about .3 VAC on a Fluke TrueRMS multimeter.
Volts AC, but what is the primary frequency(ies)? Is it 60, 120, 180, or some other harmonic predominant? Do you use 3-phase power in association with any of the equipment, or near it?

Quote:
Originally Posted by DanielLW View Post
I also hooked our telephone transmission test set to several ground points in the building and got about 45db of noise. This seems to explain why connecting the open wires to ground made the noise worse on the pair under test.
Okay, this statement in an of itself is kind of telling. 45dB in relation to what? If this is an absolute measurement to facility ground, then you have some other issues to resolve first. I would go that direction to figure out where the noise in your grounding system is coming from rather than putting a Band-Aid on the 100-pair cable.

Is the noise predominantly RF hash-like, or is it related to power (inducing hum)?

My facility has very long coaxial and twisted pair runs all using AM schemes (we have others, but they are not as sensitive to noise).

Ground floor noise for my facility is -90dB absolute. I only have about 30dB of separation before I need to worry...that's not much! I test and measure for interference on the order of microvolts, so I can kind of relate to your issue.
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Old 06-24-2017, 06:57 AM   #14
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Do you have the covers on the cans at the frames?
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Old 06-24-2017, 07:10 AM   #15
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Quote:
Originally Posted by DanielLW View Post
In answer to some of the replies:

...

Right now my plan is to isolate each leg of our spare paths, and do a noise check on each one. I'll then "cherry pick" the quietest pairs. At this point, this seems like my only option that is within my power to implement.

...
That's what our telco has always done with the rotten trunk lines around here. When someone complains about noise they find a better pair to put the customer on. Problem is someone else gets the bad pair and then they complain again. Fortunately you have spare pairs to pick from.
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Old 06-24-2017, 01:03 PM   #16
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This first part is probably a waste of time, but do it just to eliminate the possibility. Move your remotes to the radios, disconnect the lines and see if the hum is still there. A long fight with the telco over not passing monitor voltage turned out to be a problem in the remote.

All in all, I'd say you have a ground loop and suggest the following:
  1. Change the "gas tubes" on all the lines; I think it's rare, but they do get leaky.
  2. Check for a good common communications-ground between terminals. Phone companies take great pains with, errr, took great pains with grounds between buildings for good reason.
  3. Never did it on a radio remote, but have installed a couple of fiber modules at the "local" end of an intercom when nothing else would work. You'll need to know the keying method (DC or Tone) to find the right modules.
If there is dark fiber end-to-end, you're home free. Terminate, test for attenuation and you have perfect isolation.
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Old 06-24-2017, 08:32 PM   #17
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fiber optics
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Old 06-24-2017, 09:00 PM   #18
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I know in the "old days " they used to fit a big multi pair filter to deal with the pairs and noise....who knows maybe it's still available .
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Old 06-24-2017, 09:17 PM   #19
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When you grounded the unused pairs, how did you do it? Did you just stick it to a rod in the dirt, or did you actually connect it to the ground that is tied to the neutral of the electrical service?

Turn off circuits one by one in your facility to see if the noise goes away. Could there be a neutral to ground short at some of your amplifying equipment at either/both ends? Try powering the equipment from an isolating transformer or generator just for fun.
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Old 06-24-2017, 11:02 PM   #20
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http://www.3m.com/3MContentRetrievalAPI/BlobServlet?locale=en_US&lmd=1248120420000&assetId =1180615588328&assetType=MMM_Image&blobAttribute=I mageFile


The fact the hum is more prominent on some twisted pairs and not others points to Total Harmonic Distortion, THD.

an excerpt from this article:

Quote:
The signature (frequency) of a harmonic is the key to diagnosis. For example, a 900 Hz harmonic is
typical where the area being served has high impedance ground connections along with numerous buried
power distribution lines. Transformers that are wired in an open Y configuration can also cause this
harmonic. Alternately, harmonic interference at 540 Hz can indicate power system circuit resonance,
a condition that can be exacerbated by oversized shunt capacitor banks or buried power lines. Specific
trouble diagnoses can be similarly inferred from each of many possible harmonic signatures.
Quote:
Noise mitigation always begins with confirmation of proper shield bonding and grounding, and when
possible it is advisable to bond telephone ground rods to power utility ground rods to provide a return path
for induced current. Plant engineers may find it helpful to add more grounding, based on the power utility
setting. If all these steps are taken and the PI is still too high, it will be necessary to contact the power
company to reduce unbalance and THD.
Even though the power company can take steps to reduce THD and balance problems in their system, if
telco bonding and grounding are not up to par, there will still be transient problems that could result in
data transmission corruption, electrical protection issues and possibly safety hazards during power line
switching, phase wire faults or thunderstorms.
The most effective approach to resolving harmonics issues that result from electrical utility deficiencies
is to build a working relationship with a power company engineer with the goal of establishing a clear
understanding of technical issues on both sides.
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