QRZ! Ham Radio 4/QRZ Ham Radio Callsign Database - Volume 4.iso/files/arrl/tis/rfiaudio.txt

Internet Message Format | 1994-11-27 | 14KB

From the files of the ARRL Automated Mail Server, ([e-mail]):

Prepared as a membership service by the American Radio Relay League,
Inc., Technical Information Service, 225 Main St., Newington, CT  06111
[phone]. Email: [e-mail] (Internet).

file: \public\info\tis\rfiaudio.txt updated: 06-06-93

Reprinted from: June 1993 QST "Lab Notes"
Copyright 1993 American Radio Relay League, Inc.
All rights reserved.

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-------------------------------------------------------------

Lab Notes

Subject: Curing Interference to Non-Radio Equipment

The ARRL Laboratory staff is continuing to receive phone calls and letters
about interference problems, so this month we will continue our "Lab Notes"
series on EMI/RFI. At first glance, interference to non-radio equipment
sounds like it should be easy to understand and solve, but, as always, the
issues are complex and the solutions are sometimes elusive. In addition to
the "personal diplomacy" involved, you and your neighbor must understand
the regulatory and technical aspects of the problem.

We have called on our EMI/RFI guru again. Ed Hare, KA1CV, our Laboratory
Supervisor and co-editor of our RFI book, who will help unravel some of the
mystery.


Q: Well, it happened again! Just when I got Sam's television and telephone
interference problems under control, my other neighbor, Sally, bought a new
stereo system. Just as I fired up on 20-meters, my telephone rang. Sally
was very upset! My transmitter was interfering with her new purchase. She
says it's all my fault because the stereo system is brand new. I want to
help her, but I think I need to understand more about the subject. Can you
help?

A: I wish I had a nickel for every time I've heard a similar tale. I'll be
glad to offer some advice, but first I want to take you back in time a bit.

We started this EMI/RFI "Lab Notes" series in February and March 1992 QST.
Those two articles discussed the important personal and technical issues
that surround any EMI/RFI problem and told you how to contact a valuable
local resource--your ARRL section Technical Coordinator (TC). If you have
the back issues, read them. If you gave them away to your local library, or
are a new League member (welcome to the hobby!), a reprint is included in
the "EMI/RFI Package,"\1/ available from the ARRL Technical Department
Secretary.


Q: I just sent for the package, but I'm still wondering if the problem
really is my fault after all. Should I put a filter on my transmitter?

A: No. Remember, you are interfering with a non-radio device. The FCC's
Interference Handbook 1990 Edition says "Telephones, stereos, computers,
electronic organs and home intercom devices can receive interference from
nearby radio transmitters. When this happens, the device improperly
functions as a radio receiver. Proper shielding or filtering can
eliminate such interference."

Her stereo is picking up your RF signal. If your transmitter is not
putting out illegal spurious signals, your spurious emissions are 30 to 60
dB weaker than your fundamental. This means that these spurs are 1000 to
1,000,000 times weaker than your fundamental signal. The problem is almost
certainly not caused by harmonics.


Q: I guess the FCC has let me off the hook, right?

A: You are indeed off the hook, at least as far as the technical and
regulatory issues are concerned. But remember, Sally is your neighbor. You
may want to offer a helping hand anyway. Amateur Radio is known for its
public-service contributions. Applying your technical skills, or those of
your TC or local EMI/RFI Committee, to a neighborhood problem is a form of
public service. So let's see how we can help.


Q: Well, the ARRL "EMI/RFI Package" arrived in this morning's mail. You
didn't tell me it had a pamphlet about interference that was written just
for my neighbor! I gave it to Sally and after she read it, she (somewhat
reluctantly) agreed that it might not be my fault. She is ready to work
with me toward a solution. Where do we begin?

A: Let me start with a stern warning! Do not attempt to repair your
neighbor's equipment. The problem can often be fixed with the proper
application of internal filtering and shielding, but this is something that
is best done by the manufacturer. Think of the possible consequences. If
you dismantle Sally's new stereo system, you may be setting yourself up for
some real problems. You'll probably void her warranty. This means that
anything that goes wrong with that system, ever, will be your fault as far
as she is concerned!

If you work on her system, you may also be breaking the law. Most states
have laws that prohibit you from working on your neighbor's electronic
equipment (even for free) unless you hold a valid state service license.
Your ham license, although a valued document, is no substitute.


Q: Should I tell Sally that there is nothing I can do?

A: No. There is a lot you can do! You can help her contact the manufacturer
of the stereo system. The Electronic Industries Association, 2001
Pennsylvania Ave. NW, Washington, DC  20006, tel [phone] maintains a
database of key contact people at each manufacturer.

You may discover that the manufacturer already has some easy fixes
available.  Some manufacturers even offer to do the work under warranty, or
supply free filters.

Once you and Sally have located a manufacturer's representative to repair
the stereo, you can help that representative understand the technical
issues involved. The manufacturers are usually willing to assume their
responsibilities fairly, but they do not always understand the fundamental
causes  of the problem, nor all the solutions. The ARRL has published an
RFI book, Radio Frequency Interference -- How to Find It and Fix It.\3/ Buy
a copy and read the chapters on fundamentals and curing audio equipment
interference.

You can also make your station available for testing when the electronic
service person arrives. Sometimes several things need to be tried before the
magic combination is found. If you're not there to put the radio signal on
the air, the service person will have no way of knowing whether the cures
actually worked.


Q: The manufacturer has agreed to send a service person to investigate the
problem. If I'm going to help, I guess I'd better sharpen my skills. I've
have read the "EMI/RFI Package" and learned a bit about troubleshooting.
What's the best way to start?

A: Start by simplifying the problem.  Disconnect all inputs to the stereo
system, one by one, and see if the interference suddenly goes away. If you
disconnect the FM tuner, for example, and the interference stops, you've
found a culprit.  Now you know where to apply the cure.  (Of course, this
doesn't apply to a unit that is all self-contained.)


Q: We disconnected everything, including the long audio cable from Sally's
stereo TV. The interference is still there.

A: That's unfortunate. Those long cables are prime suspects. They make make a
nice long-wire antennas and they pick up a lot of RF.  Let's recall an old
electromagnetic compatibility (EMC) engineer's adage: Components don't pick
up RF energy--wires do! Take a look at the wires that are connected to her
amplifier.


Q: Well, the only wires I see are the speaker wires. She has the speakers
located about 15 feet from the stereo.  Hmmm... the wires are just about
the size of my 20-meter dipole. Is that why she gets interference when I
operate on 20 meters?

A: Could be! Speaker wires are often 8- or 16-feet long. When you put two of
them together, you make an efficient ham-band receive antenna--not
something you want to connect to your neighbor's stereo. Try bundling the
speaker wires to reduce their effectiveness as an antenna. This procedure
has been known to eliminate the interference all by itself.


Q: Well, we moved the speakers and tried bundling the wire. It helped
somewhat, but not enough. "CQ DX" is still interspersed with her favorite
songs. What's next?

A: You're on the right track. Try this: Disconnect the speaker wires
altogether and plug in a set of headphones. What happens?


Q: The interference is gone! Sally is understandably reluctant to use her
headphones whenever she wants to listen to her stereo, though. What now?

A: Well, you've just learned that the RF is being picked up on the speaker
wires and being conducted into the amplifier.  (The output transistors are
possibly rectifying the RF into audio, and the amplifier's internal
negative feedback circuitry is conducting it back to the high-gain stages
of the amplifier.) Let's start with a common-mode choke.


Q: I think Sally had a different kind of choke in mind--one that involves my
neck! What is a common-mode choke?

A: For a detailed explanation, I will refer you to the RFI book. However, I
will gladly tell you how to build one (two actually, you will need one for
each speaker output). Wrap ten to fifteen turns of speaker wire onto an
FT-140-43 ferrite core. (Use and FT-  240-43 if the speaker wires are large,
and use -73 material for interference from 80- or 160-meter signals.) The
common-mode chokes should be installed right at the amplifier. (If the
system uses amplified speakers, you should install one at each speaker,
too.) Another possibility is to try a few commercially available filters.
Here are two companies that sell filters specifically  designed for audio
systems:

Industrial Communications Engineers, POB 18495, Indianapolis IN,
46218-0495, tel [phone].  Model 461 series.

TCE Labs, RR9 Box 243D, New Braunfels TX 78133, tel 800-KILL-TVI.
Model SP-1.

Both of these companies have fine reputations, and sell a broad line of
other interference-reduction products.  Contact them for more information.


Q: I read an old QST article that recommended placing a 0.01 microfarad
capacitor across the speaker terminals.  Wouldn't that be a lot easier?

A: Don't do it! That was good advice when amplifiers used tubes. Many
modern solid-state amplifiers don't work well into capacitive loads. In
fact, they can break into a full-power (sometimes ultrasonic) oscillation.
This can result in the near-instantaneous destruction of the output module
or transistors. And if you think you had a diplomacy problem before you
blow up her stereo, think what will happen after you let out all the smoke!


Q: Thanks for the warning! Where do I get the right ferrite chokes?

A: The ARRL "RFI Package" contains a list of EMI/RFI materials suppliers.
You can also refer to the advertisements in QST -- there are a few
advertisers offering ferrite materials and chokes.


Q: I installed the speaker-lead chokes, and they worked like champs!
However, as soon as I hook up the long cable to the TV set, the
interference returned. Should I put chokes on that cable, too?

A: Yes. Many interference problems have multiple causes. This is why we
simplify the problem into manageable-sized pieces.  Try a ferrite
common-mode choke at one or both ends of long cable.


Q: The chokes helped somewhat. What else can I try?

A: Now we are getting into the more complicated cures. You many need to use
an L/C filter in the input lines as well.  The ARRL RFI book contains
information about how to build one.


Q: We're going to order the filter. In the meantime, Sally has decided to
forgo her stereo TV connection. I guess everything is under control. Am I
going to have similar problems if one of my neighbors installs an intercom
or alarm system?

A: You might. Many of these devices are not designed with EMC in mind. But
you have learned many things from your stereo experiences that you can
apply to nearly any non-radio interference problem.  You can make use of
those 0.01 $microF capacitors, putting them across (or from both terminals
to ground) of simple alarm systems, or across input connections to
intercoms. You can also install some common-mode chokes on long wire runs.
Some people have reported that a single choke for both wires in a twisted
pair works best; others have reported that a separate ferrite choke for
each wire cured the interference.  In any case, contact the manufacturer
through the Electronic Industries Association. This not only assures that

you get the best possible source of help, but it also ensures that the
manufacturer knows when people are having problems with their systems.  You
can also contact me here at ARRL Headquarters to get advice on how to fix
an interference problem that has stumped you and your ARRL section
Technical Coordinator. Good luck!


Notes

\1/ The ARRL Technical Department "RFI Package" can be obtained by sending
a large (9X12) self-addressed stamped envelope to the ARRL Technical
Department Secretary. Include a specific request for the "RFI Package." The
package is rather large so three units of postage is appreciated.

\2/ A copy of the FCC Interference Handbook can be obtained by sending a
large (9X12) self-addressed stamp envelope to the ARRL Regulatory
Information Branch. Include a specific request for the FCC Interference
Handbook. Three units of postage is appreciated.

\3/Radio Frequency Interference--How to Find It and Fix It is available
from your local dealer or directly from ARRL HQ. See the ARRL Publications
Catalog elsewhere in this issue for ordering information.