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Author Topic: Grounding and RFI  (Read 8676 times)

Stephen Swaffer

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Grounding and RFI
« on: February 15, 2014, 11:09:41 pm »

It has been discussed that an "earth ground" is primarily for lightning protection.  The Egc (equipment grounding conductor) bonded to the neutral at one place exists to prevent potential differential on metal equipment and to trip overcurrent devices in case of  fault.

Since radio transmission depends on the earth for transmission, how important is a good earth ground to preventing RFI in audio systems?
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Steve Swaffer

Mike Sokol

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Re: Grounding and RFI
« Reply #1 on: February 15, 2014, 11:29:11 pm »

It has been discussed that an "earth ground" is primarily for lightning protection.  The Egc (equipment grounding conductor) bonded to the neutral at one place exists to prevent potential differential on metal equipment and to trip overcurrent devices in case of  fault.

Since radio transmission depends on the earth for transmission, how important is a good earth ground to preventing RFI in audio systems?

I do know that any type of corrosion in the earth ground or shield of a sound system will act like a detector and can demodulate AM radio stations into program audio. Also note that AM radio station quarter-wavelengths can be on the order of 125 to 175 ft, which works out to a 100-ft snake plus a 50-ft subsnake plus a 25-ft mic cable. But while both these scenarios will cause RFI, I don't think that either really answer your RFI grounding question.

I'm guessing that RFI mitigation is more about adequate shielding than earth grounding, but I could be full of it. Time to put on the thinking cap. What do the rest of you guys know about this?
« Last Edit: February 16, 2014, 12:27:30 pm by Mike Sokol »
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Frank Koenig

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Re: Grounding and RFI
« Reply #2 on: February 16, 2014, 01:47:17 am »

Since radio transmission depends on the earth for transmission

Radio transmission does not, in general, depend on the earth. Some antennas, however, use the earth as a counterpoise, such as traditional AM broadcast antennas operating in the .55 to 1.6 MHz band, where the intended propagation is by ground wave. They are often located in areas with high ground conductivity, such as salt marshes. Radio transmission to and from spacecraft, aircraft, and wireless mics for that matter, works just fine, no earth required.

Protection from radio frequency interference (RFI) relies mostly on shielding and filters on connections that penetrate the shield.

--Frank
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Kevin Graf

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Re: Grounding and RFI
« Reply #3 on: February 16, 2014, 10:33:27 am »

a] Frank's explanation is most excellent.

b] Jim Brown, being both AES past committee chair on EMI/RFI and a Ham radio operator, often covers connections to dirt in his many papers.

http://www.audiosystemsgroup.com/publish.htm

c] This paper has a section on connections to dirt:
"Testing for Radio-Frequency Common Impedance Coupling (the "Pin 1 Problem") in Microphones and Other Audio Equipment"
The author has shown that a primary cause of VHF and UHF interference to professional condenser microphones is inadequate termination within the microphone of the shield of the microphone's output wiring, a fault commonly known as the pin 1 problem. Tests using only audio frequency test signals generally fail to expose susceptibility to radio frequency (RF) interference. Simple RF tests for pin 1 problems in microphones and other audio equipment are described that correlate well with EMI observed in the field. Preprint Number: 5897 Convention: 115 (September 2003) Author: Jim Brown 
http://www.audiosystemsgroup.com/AESPaperNYPin1-ASGWeb.pdf
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Kevin Graf

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Re: Grounding and RFI
« Reply #4 on: February 16, 2014, 10:47:30 am »

It has been discussed that an "earth ground" is primarily for lightning protection.
Yes the GEC (Grounding Electrode Conductor) system is for lightning protection and power company (POCO) high-voltage failures.

Quote
The EGC (Equipment Grounding Conductor) bonded to the neutral at one place exists to prevent potential differential on metal equipment and to trip overcurrent devices in case of  fault.
[...................]
Most definitely!
However in cases like Festivals and swimming pools the connection to dirt is also part of the  EGC (Equipment Grounding Conductor) system.
 
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Mike Sokol

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Re: Grounding and RFI
« Reply #5 on: February 16, 2014, 12:25:51 pm »

Protection from radio frequency interference (RFI) relies mostly on shielding and filters on connections that penetrate the shield.

--Frank

I've used the Neutrik EMC XLR connector which includes an internal filter to keep RFI out of your mixing console. These are great for keeping AM radio out of old Mackie mixing consoles. BTW: With all due respect to our southern neighbors, this far-off sounding radio noise in a mix is often called "Mexican Radio" by my students. That nickname probably comes from some of the huge radio stations just south of the border that leak into the US.

See http://www.neutrik.com/en/xlr/xlr-cable-connectors/emc-series/ for the link.

« Last Edit: February 16, 2014, 12:32:07 pm by Mike Sokol »
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John Roberts {JR}

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Re: Grounding and RFI
« Reply #6 on: February 16, 2014, 12:45:26 pm »

I've used the Neutrik EMC XLR connector which includes an internal filter to keep RFI out of your mixing console. These are great for keeping AM radio out of old Mackie mixing consoles. BTW: With all due respect to our southern neighbors, this far-off sounding radio noise in a mix is often called "Mexican Radio" by my students. That nickname probably comes from some of the huge radio stations just south of the border that leak into the US.

See http://www.neutrik.com/en/xlr/xlr-cable-connectors/emc-series/ for the link.

Yes it is interesting when a product with an arguable design flaw (input stage rectification) becomes so popular that a connector company designs and sells a band-aid for it.

Appreciate all the good answers already posted.

In the context of RF is may be worth mentioning that a low impedance ground path for DC or LF, may not be a low impedance at RF frequency. RF detection inside products is mostly a relative thing, the RF on the hot vs the RF on the ground. Truly common mode RF should be ignored.

JR
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Stephen Swaffer

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Re: Grounding and RFI
« Reply #7 on: February 16, 2014, 03:42:22 pm »

Part of the reason I asked this is that I have fought RFI on our church choir mics for years. 

For a while it was quite noticeable-we had a Mackie 32-8 mixer.  Was going to build low pass filters as recomended by Mackie (not sure why they just didn't incorporate them themselves?) but then the Mackie became to unreliable so we replaced it with an A & H GL3200.  I expected RFI to go away, and it did improve a LOT, but still have trouble on one choir mic-but maybe Mike answered that.  If I recall correctly, the choir mic snake is 100 feet long, plus the mic in question has a 25 mic cord and the Shure MX212s come with a 30 foot cord-or 155 feet.

JR mentions a DC/LF ground may not necessarily be a good RF ground.  An RF ground needs to be as short as possible, but running a direct, shorter ground to the input box of the snake would create a ground loop-and violate the principle of a "star" topology for grounding?
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John Roberts {JR}

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Re: Grounding and RFI
« Reply #8 on: February 16, 2014, 04:52:24 pm »

Part of the reason I asked this is that I have fought RFI on our church choir mics for years. 

For a while it was quite noticeable-we had a Mackie 32-8 mixer.  Was going to build low pass filters as recomended by Mackie (not sure why they just didn't incorporate them themselves?)
Mainly because they did not realize that the design was marginal until they got a bunch of them out into the field.
Quote
but then the Mackie became to unreliable so we replaced it with an A & H GL3200.  I expected RFI to go away, and it did improve a LOT, but still have trouble on one choir mic-but maybe Mike answered that.  If I recall correctly, the choir mic snake is 100 feet long, plus the mic in question has a 25 mic cord and the Shure MX212s come with a 30 foot cord-or 155 feet.

JR mentions a DC/LF ground may not necessarily be a good RF ground.  An RF ground needs to be as short as possible, but running a direct, shorter ground to the input box of the snake would create a ground loop-and violate the principle of a "star" topology for grounding?

 ;D ;D  I love it when I hear design theory regurgitated like that.

It's a little more complicated than that and not sure we need to get into circuit design.

My point is that for RF frequencies an apparent ground may not be very low Z. They make antennas out of wire so RF voltage varies over the length of the antenna wire.

Keeping RF out of audio is mostly about design inside the product input.

Grounds are about human safety. For properly designed professional gear interfaces not a signal quality issue. 

JR
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Stephen Swaffer

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Re: Grounding and RFI
« Reply #9 on: February 16, 2014, 11:52:14 pm »

So where would you look to deal with the problem?  Admittedly the RF is weak, (but if I want to listen to the local news I don't need a radio)- grounds/shielding all appear to be fine.  Known issues with the A & H design?  The Shure MX212?  Maybe just going with the aforementioned connector would be a good starting point?

Not trying to get into circuit design, just trying to understand what I can do to deal with the problem without redesigning the input circuit on the mixer-and still follow good install/field practices.
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Steve Swaffer

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Re: Grounding and RFI
« Reply #9 on: February 16, 2014, 11:52:14 pm »


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