QRZ! Ham Radio 1/QRZ Ham Radio Callsign Database - December 1993.iso/arrl/emi_au.dio

Text File | 1993-11-21 | 14KB | 306 lines

>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

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
202-457-4977 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. Whats 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 uF
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 (ARRL-CATALOG) elsewhere in this issue
for ordering information.