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Author Topic: TNC Ground Loop  (Read 923 times)
W0CHH
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« on: April 27, 2007, 09:00:30 PM »

I'm getting substantial noise on transmit on my IC-756Pro when my TNC, a PK-232 with a sound card and USB upgrade is connected to my Astron RS-35M power supply.  If I use a separate wall wart type of power supply, that solves the problem.  All well and good.  However when I plug in the USB cable from the TNC into my laptop, the same thing happens.

The setup is pretty useless like this.  Does anyone have any thoughts on how to solve this dilemma?  Thanks in advance.

Craig W0CHH
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WD8PTB
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« Reply #1 on: April 28, 2007, 04:56:45 AM »

I had a similar problem with a Rascal computer interface It worked fine with a Icom 718. I sold the 718 and I could not get it to work with my 746pro. I ultimatly hooked it up to a 756pro and it worked fine
   Hooking all grounds to the same point should help. Keep in mind that the sheild on your coax is probably a ground also. You might also try disconecting any grounds that are not safety grounds. 73 Don WD8PTB
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N5EG
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« Reply #2 on: April 28, 2007, 06:15:37 AM »

Most HF rigs have very sensitive microphone inputs (millivolts), especially so when the speech processor is turned on (it shouldn't be for data modes). There are many places for ground loops to form, one that is troublesome is the ground connection on an audio source (your TNC) to the HF rig microphone input.

While other ground paths can exist between all your pieces of equipment, you want to avoid those noise currents from flowing alongside your low-level microphone audio.

One of the simplest ways to do this is to use an audio transformer to feed the HF rig microphone input while isolating that ground connection. I don't know what level your audio source puts out - a computer sound card puts out something like 1 or 2  volts pk-pk and this many times will overload your HF rig. In this case an attenuator helps as well.  See the following for a description of the problem and one approach:

 http://www.tapr.org/~n5eg/index_files/page0006.html

  -- Tom, N5EG



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KB4QAA
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« Reply #3 on: April 28, 2007, 08:50:21 AM »

One problem the older PK232MBX had was feedback thru the microphone connection.  As I recall the best solution was to disconnect the mic when doing digital.

Good luck,

Bill
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W0CHH
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« Reply #4 on: April 28, 2007, 09:23:25 AM »

Thanks for the tips.  The 756Pro does not attenuate the mike input when using the aux inputs, so it does need to be disconnected when using a digital mode.

I think it's time to set up a true station ground for all the equipment and antennas. I'll keep you informed of my progress.

Thanks

Craig W0CHH
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W8JI
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« Reply #5 on: April 28, 2007, 12:14:07 PM »

Craig,

Multiple pieces of gear should NEVER be connected through unbalanced lines unless the grounds are floated at one end or the other. Amateurs violate this rule all the time and pay the price with hum and distortion.

You can check to see if your equipment has poor design by ohming the shield to the chassis at every box, and the shield to shield at interfaces. If you read low resistance dc continuity at each end of an audio line to the chassis of each gear, and if that chassis connectes to other equipment like power supplies, you are asking for problems.

The cause of the problem is the power supply and power line currents flowing over the shield from one piece of gear to the other. This causes a voltage drop along the length of the shield, and is exactly like connecting the noise and hum in series with the audio line!! It makes no difference at all if it is induced from end-to-end in the shield or in the center, it will all cause hum and distortion.

We were NEVER allowed to ground the shields to equipment chassis at both ends of a wire in studios. We had to float the shield at one end so there was no dc path between the chassis. If we used balanced lines, that was easy. We simply did not connect the shield at one end.

If it was an UNbalanced line, we had to add an isolation transformer. The shield connected normally at the input but at the source or output we installed an audio isolation transformer. The shield connected ONLY to the secondary winding, and the center to the other side of the secondary. The primary connected to the source. This prevented having a ground loop.

You can buy suitable transformers from Mouser for a couple dollars each.

The proper solution is to isolate the ground paths on audio lines with isolation transformers. This breaks the shield path while the transformer passes audio.

A secondary "no-so-good" patch is to use a heavy ground wire connected to a common point.

In my station, I use dozens of isolation transformers. This prevents hum as I connect the computer, headphones, three or four radios, and some digital interface devices to the audio lines. I don't have a bit of hum. If I omit the interfaces then even with the best grounds I have some hum in things.

73 Tom
 
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W0CHH
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« Reply #6 on: April 28, 2007, 05:57:23 PM »

Thanks Tom.  The PK-232 sound card unit does use isolation transformers for audio signals but I'll check the shields as they may be a problem.  I asked this in another thread so excuse me for re-asking but should DC supply negative connected to common system ground?

Craig W0CHH
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W0CHH
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« Reply #7 on: April 29, 2007, 06:46:26 PM »

Well, I went ahead and grounded my two radios, power supply, tuner and amplifier.  I used 8 gauge stranded wire runs from the equipment to a grounding bar mounted next to my AC service which is in turn connected to service ground.

This made a huge difference.  Before a SSB transmission would be very noisy, almost garbled.  That noise is all gone.

But I still have a residual noise.  It's periodic and occurs about once a second.  It sounds like scratching window screen or sweeping.  My plan now is to try some ferrite beads on my USB cables connecting the laptop to the radios and two TNC's, as well as the USB hub wall wart power supply.  Any additional thoughts are very welcome.

Craig W0CHH
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W8JI
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« Reply #8 on: April 30, 2007, 07:56:11 AM »

Craig,

You should ohm the inputs with the audio cables disconnected with a meter. Proper wiring is one port should have a ground, one port should show NO continuity to ground. The general rule is the INPUT should have the ground, the output should have the isolation.

You also could have RF in the shack. That would mostly be related to an antenna system problem of flaw.

Bonding equipment together can certainly help, but it really is not required for signal quality unless there are other defects in the system. Bonding together and having a ground is primarily a safety measure for power line fault currents.

For dc supplies, I ALWAYS bond the negative rail to my station buss. Otherwise if I have a positive lead short to the chassis someplace it can lift the negative rail negative and damge equipment. Audio lines are the only lines running between different pieces of gear that should NOT have a dc path at each end of a shield.

I'd look closely for ground loops. You also might read about "pin 1 problems" from audio informational sources.  Jim Brown has something about it in some white papers.

73 Tom
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N5EG
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« Reply #9 on: May 01, 2007, 06:56:12 PM »

Craig:

The most likely cause of the once-per-second noise is your computer is 'polling' your radio once each second. Many programs use the RS-232 to talk to the radio to ask it what frequency it is on, and to control it.

The RS-232 signals have sharp rise and fall times, and those can induce a voltage difference between pieces of equipment for various reasons.  It's just like the hum problem, but caused by ground bounce.

Again, the solution is to use that audio isolation transformer to prevent the audio ground (shield) from trying to help out the RS-232 cable ground return. On my FT-1000MP, it completely eradicated the RS-232 noise from my microphone audio.

  -- Tom, N5EG

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