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1992-02-23
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253 lines
******************************************************************************
Reproduced with permission of Falsoft, Inc. -- PCM Magazine (Oct 91)
"exclusively" for the Delphi Tandy/PC-Compatibles SIG. You MAY NOT DISTRIBUTE
this file elsewhere without permission. All rights reserved by Mike Salisbury
and Falsoft, Inc.
******************************************************************************
MODEM/FAX NOISE MUFFLER
Unless you have been fortunate, there have been times during telecommunications
sessions when Radio Frequency Interference (RFI) has disrupted transmissions.
This manifests itself in strange characters on your terminal screen or CRC
transmission errors (re-sends). Both are less than productive -- especially an
upload/download/fax transmission where you might experience 100 or more block
errors. A file download of 400 Xmodem blocks (51,200 bytes) normally takes
around 9 minutes, at 2400 bps. But, with 100 CRC errors, turns into 11 1/2
minutes.
Commercial communications services, such as Delphi, Genie and CompuServe charge
by the hour. Your monthly bill may be substantially increased when RFI induced
error cause data blocks to be re-sent. This is true of modems and fax machines,
whether the transmission cost concerns your long distance phone bill or Delphi
monthly charges.
The cause and effect of RFI is known -- but, the remedy seems to be a secret.
At best, one searching for a solution has precious few information resources to
guide them.
The scope of this article is intended to present cause, effect and solution to
RF interference. An RFI problem may be caused by something as elementary as a
loose connection or as complicated as an impossible to find noise source
(example: refrigerator, electric light ballast, an air conditioner).
Information presented here will enable you to block 90% of all spurious signals
to modems, facsimile machines and telephones (including cordless).
Causes of RFI
-------------
In this age of convenience, we take for granted the many electronic servants
that populate our surroundings. Take for example, your water and environment
heating systems. An electric hot water heater or furnace contain heating coils
controlled by thermostats. The coil is subjected to large amounts of electric
current, to produce heat. As the current passes through the coil, a magnetic
field is created. From this field are emitted various frequencies -- radio
frequencies.
After a time, the thermostats controlling the temperature (or current passing
through the coils) will pit. This causes the contacts to arc (or spark). Did
you know that the very first Morse Code transmissions used sparking coils to
transmit signals?
You don't have an electric hot water heater or furnace -- you have gas (or
oil). Much more economical. Agreed, both of mine use natural gas. They do have
pump and blower motors, though. That means brushes in the motors and some type
of ignition device -- both create sparks -- both have thermostats.
We are constantly surrounded by magnetic fields -- it is a necessary evil in an
electric/electronic society. Here is a brief list of common RFI generators:
TV sets
Stereo systems
Video cassette recorders
Computers (and peripherals)
Fire, Police, Military, Aircraft, Amateur Radio broadcasts
Power transformers
Appliances
Fluorescent (or neon) lights
Garage door openers
Microwaves ovens
Induction heating systems (furnace & hot water heaters)
Intercoms (including wireless)
Automobile cruise controls and anti-skidding devices
Smoke detectors
Touch-control lamps
It is a good bet that some of the items on the above list have surprised you.
Actually, the listed items barely scratch the surface -- so, you begin to see
the vast number of sources or combination of sources that can produce RFI. If
you ever hear a story about people who receive radio transmissions via their
dental work, don't doubt it for a minute. It is fact, we are surrounded by all
manner of frequencies that can affect us in strange ways.
Modems and Fax Machines
-----------------------
Computer communications peripherals such as modems and facsimile machines use
solid state electronic components. These components are chosen because they are
inexpensive, run cooler, have a lengthily working life and are easy to service.
At the same time, they are very susceptible to stray RFI. Too often, components
that act as protective filter circuitry have been omitted to reduce
manufacturing cost. When the protective circuitry is present, it is often
ineffective, due to poor quality or design. As a general rule, the higher the
transmission speed (baud rate) of the device, the better the filtering. A 9600
baud device should not need external RFI filtering. But, the ever-present
business formula: "cost effective manufacturing = lower consumer prices =
greater sales" often degrades quality.
The transmission carrier for modem and fax machine signals is, of course, the
telephone line. The telephone line bandwidth of 300 to 3000 Hertz was designed
for voice transmission -- not modem/fax carrier frequency and phase
modulation. In truth, the telephone line of today is a poor signal path for
any telecommunication activity; thus, the associated problems.
The Low Pass Filter
-------------------
Understanding the deficiencies of communications devices, how RFI affects these
and the properties of transmission media, an RFI low pass filter can be
designed (Figure #1).
The purpose of a low pass filter is to attenuate frequencies above a designated
range. Most carrier frequencies used by modem and fax are in the middle of the
telephone line bandwidth -- 1650 Hz. This carrier is modulated above and below
the 1650 Hz mid-range frequency to produce the signal information. The highest
useful frequency level for modem/fax transmission is around 2700 Hz. Allowing
for any signal inconsistency that might deviate to a higher range, 3000 Hz is
chosen for the cut-off. No frequency above 3000 Hz should be allowed to pass
through the filter. The frequencies above this range, for modem/fax signal
information purposes, constitute RFI.
Constructing the Muffler
------------------------
The construction of this filter is extremely easy. However, you will need to
solder a few connections. Using a soldering iron is not an art form. The solder
joints need not be perfect. The only restriction to these joints is that you
must not leave sharp points and that the solder joints make good contact with
the leads/wires. If your solder joints are pointed they may act as antennas
that can actually pick up the very signals the filter is intended to reject.
A pencil type soldering iron of any wattage may be used. Do not use a high
wattage soldering "gun." This will get too hot and may cause melting of the
wire insulation or damage to the components.
If you feel that cannot solder, it is possible to "crimp" the connections
together. This is a much less desirable method as the connections may not make
good contact and are susceptible to oxidation.
The following items are needed to build the Muffler:
1> Pencil type soldering iron
2> Wire stripper (a knife will do)
3> Wire cutters
4> Rosin core solder (not acid core)
5> A 100 uH (2 amp) RF choke (Radio Shack # 273-102 - $0.99)
6> A .001 uF (500 volt) capacitor (Radio Shack # 272-126 - $0.49)
7> A 12 inch modular to spade lug line cord (Radio Shack # 273-391 - $1.99)
8> A 42A modular wall jack (Radio Shack # 279-386 - $2.19)
The choke and capacitor listed may be obtained from any source as long as the
values are correct (the ring signal to a telephone is around 92 volts).
Using Figure #2 as a guide perform the following assembly steps:
1> Take the modular wall jack and remove the connector block from within it
(leaving only the 4 spade lug wires, RED-GREEN-BLACK-YELLOW).
2> Cut the BLACK and YELLOW wires close to the rear of the female modular
connector of the block connector (these two wires are not needed). Cut the
spade lugs from the RED and GREEN wires, leaving the entire length of these two
wires minus the spade lugs.
3> Take the modular line cord and cut the BLACK and YELLOW wires close the
outer insulation for the line cord. Cut the spade lugs from the RED and GREEN
wires. Again, leaving the entire length of these two wires minus the spade
lugs.
4> Remove 1/4 inch of insulation from the RED and GREEN wires of the modular
wall jack and modular line cord. Tin all 4 exposed wire leads.
(Tinning: heat the exposed wire ends and apply solder so that the solder melts
into and/or around the wire ends)
5> Take the choke and capacitor and shorten the leads to within 1/2 inch of
their bodies. Tin all 4 leads.
6> Solder one end of the choke to the RED wire of the line cord.
7> Solder the remaining choke lead to the RED wire of the connector block and
one lead of the capacitor.
8> Solder both GREEN wires to the remaining capacitor lead.
9> Arrange the choke and capacitor solder joints inside the housing of the
modular wall jack so that they do not touch (short) each other.
10> Secure the components in the housing using tape of melting candle wax over
them (the wax will not conduct once dry). Make sure that the R.F. coil is not
touching the solder joints.
11> Look on the modular wall jack for a "knock-out" and remove this piece of
plastic to allow the line cord to feed through.
12> Measure and cut a piece of cardboard to glue or tape to the modular wall
jack to enclose the components (any type of non-metalic backer will work as
long as it encloses the components).
Installing the Muffler
----------------------
The male modular connector coming from the filter assembly is plugged into the
modem or facsimile machine. Use the existing phone line cord (used prior to the
filter being placed in-line) to connect the female modular connector of the
filter assembly to the existing wall jack (see Figure #3).
It is important that the filter be as close as possible to the device it
protects. Do not place it at the fixed (existing) wall jack and then run a long
line cord to the modem or fax.
A/C Line Noise
--------------
The Muffler protects only the incoming telephone line from RFI. Although this
is almost always the cause of modem/fax interference, it is possible that noise
is entering through an A/C power cord. This can be eliminated or greatly
reduced by snapping a ferrite choke around the power cord. The properties of
the choke around the power cord are similar to that of an actual RF choke.
Radio Shack sells such a choke as catalog #273-105 ($4.99).
It is difficult to determine which A/C line cord is picking up RFI. If you
decide to use the snap-on choke, use a power strip. Place the choke on the
power strip A/C cord and plug all other A/C line cords into the power strip.
FCC Requirements
----------------
FCC Rules and Regulations, Part 68 pertain to the installation any type of
device to the switched-telephone network. The telephone company may require the
following:
- The telephone number of the line
- FCC registration number of the device
- Ringer Equivalence Number
Mike Salisbury
Delphi Membername "MISAL"
from PCM Magazine - October 1991