Radio Frequency Interference
(RFI) is one of the hardest problems to solve on telephone systems.
You can't see it, it may not always be there, and there is sometimes
no rhyme or reason for what's happening to be happening.
In general, RFI is picked up by a component of
the telephone or system that's "tuned", by coincidence, to
the frequency of RFI. Whether it's a diode, transistor or even a bad
solder joint, something is acting like a radio receiver,
demodulating the RF signal into audio. Once RF is turned into audio,
you can't filter it out (since you'd also have to filter out the
The idea behind an RF filter is to stop the
piece of wire that's acting like an antenna from bringing the radio
signal into the part that's demodulating the signal. Just like on a
radio or TV, if you remove the antenna, the radio signal goes away.
An RF filter simply notches out a portion of the frequency spectrum,
so it can't get through the filter to the part that's demodulating
the RF. The filter allows the rest of the frequency spectrum to flow
through the filter.
Our RF Filters are inductors,
or chokes. They are simply wire wrapped around the proper inductive
(toroid) core material, which blocks a particular frequency range
from getting past the Filter (it sounds
simple, but they are very difficult to engineer). Because
there are no other components, and they're entirely passive, they
don't interfere with electronic or digital telephones when placed on
the line cord, or even with Ethernet when used in-series with a
computer/hub/switch. While it's possible to build an active filter
with more components that would notch out a larger or deeper portion
of the frequency spectrum, it would not be usable on a digital or
electronic phone since it would change the electrical
characteristics of the pair (screwing up the power and data going to
RFI is usually worse when it's closer to the
source of the radio transmission. If you see a big radio tower out
the window when you're putting in a new phone system, I would expect
to have some RF problems once you get the system up and running.
There are no brands of phone systems that are immune to RF, and they
all can get RFI - analog or digital. On the plus side, there's a 99%
chance that you can fix the RF problems by using our Filters,
along with taking some basic RFI elimination steps on the cables
themselves. Even at powerful radio stations that are located right
below a big tower, we've had a better than 99% success rate in
fixing phone system RF problems.
If the customer is changing out a phone
system, and there were filters on the old one, all bets are off. You
have to start over with your RF troubleshooting. The new system may
not have RF problems at all, or they could be worse... or just be in
different places. With RF, even if two phones are 3 feet apart, they
are separate cases of trouble. You need to troubleshoot each one
separately. There's no blanket fix that will always work... "It
fixed that phone, so I'll do the same thing to all of them" doesn't
apply to RFI.
Here are the frequencies that are most
often the source of problems on phone systems:
1. AM Radio (.5 to 1.5 Megahertz)
2. CB & Amateur Radio (3 to 30 Megahertz)
3. Amateur & Plastic Sealing Equipment (26 to 60 Megahertz)
4. FM, Air & Business Band VHF Radio (60 to 150 Megahertz)
5. Neon / Dimmer Noise (60 - 120 Cycles)
6. Welding Noise (500 Hertz to 150 Megahertz)
While you may get interference from higher
frequencies, it doesn't happen very often with telephones. The most
common occurrence at higher frequencies is interference from low
power cell phones or walkie-talkies, right next to the phones or KSU.
Nextel phones put out unbelievable amounts of RF interference
throughout the frequency spectrum (but they'll be gone eventually,
since they were bought out by Sprint).
We don't have filters for the higher
frequencies. The easiest way to fix problems at higher frequencies
is to move the phone / RF device away from each other. Since these
RF devices are usually very low power, you don't have to separate
them very far. I've only heard of a couple of cases where the cell
phone tower next to a building was causing RFI, over the years.
If you hear interference on a wired phone from a
walkie-talkie when it's transmitting next to the phone, don't do that! No phone manufacturer has tested their
phone or system like that, and you're likely to
have problems. I don't know of a fix except to use an external
antenna with good quality coax to get the RF away from the phone.
If there is a radio base station (particularly a
high power radio) near the phone with the RFI, make sure you're
using an external antenna that is outside of the
building. If you put 20 or 50 watts of RF into a little stick
antenna near a regular wired phone, you're bound to get some kind of
interference when transmitting. Use coax to run the antenna outside
the building to a point that it's far away from any wired
telephones. Make sure the connectors are good on the coax! Coax is
specially made to prevent leakage from the cable and connectors, so
getting the antenna outside is likely to fix any RFI problems on
If the interference you're hearing is a
buzzing or some other noise, not an obvious radio station, and you
don't know where the interference is coming from, the first thing to
do is turn off everything running on AC.
Turn off the computer, monitor, WI-FI, routers, fans, dimmers,
lights, radios - every single thing that runs on AC. It might be
easier to just turn off the breakers. If the problem goes away, turn
the stuff back on one at a time until the problem returns. If it turns out that some
common piece of equipment like a PC or monitor is causing
interference, replace that item!
It doesn't make sense to try to get
rid of RFI caused by broken or junky equipment. There are a zillion
phones out there that work fine near a zillion other PCs, monitors,
It doesn't make sense to try to filter
interference out of a phone, when it's coming out of a defective or
poorly designed device. There are a zillion telephones sitting next
to these common devices all over the world without interference. For
example, if a computer monitor is causing interference, all you can
do is chase your tail trying to get rid of the interference on the
phone. Replace the monitor! Even swapping
a monitor from another office may fix the problem.
FIRST THINGS FIRST (THE
Before you go and buy a bunch of RF
Eliminators, there are some basic things I would do to try
to get rid of the RFI, without using parts. I guess it's a
toss up whether it's more economical to try our Filters
first, or to go out and spend time doing the basics first, and get
the remaining problems solved with our RF Eliminators. Even if our
RF Eliminators don't work at first, they'll probably fix what's left
over after you do the basics.
The nice thing about our Modular
Filters... is that they're modular!
It only takes a second to plug it in and see if it helps. On some
jobs, you may need the filters at the KSU, or both the KSU and the
phone. They can't all be easy.
Click HERE to view
Eliminator Placement Illustrations.
Putting the filters in the right
place is absolutely necessary! They will
do nothing if they aren't in the right place, to
block the RF from the piece of cable that's acting like an antenna,
which is bringing the RF into the phone or system.
Click HERE to see
our RF Troubleshooting Flow Chart, which
tells you how to determine where you need filters.
Please note that you can call and ask
to put the filters, but we won't know without going through the
Flow Chart ourselves.
If I was on a service call, those are the exact
things I would do (which is how I came up with the Flow
Chart). There are no shortcuts, and there's no magic. You
might luck out and guess correctly where to put the filters, but
you'd be wrong more often than right, and spend a lot of time
chasing your tail (which is true of troubleshooting anything in the
wrong order). The secret of being a good repairman is simply knowing
how to troubleshoot - not necessarily having to know anything about
the gizmo that's broken. Always start at one end or the other. I
personally usually start at the beginning, and add things back until
the problem returns. Starting in the middle will kick you in the ass
more often than not (and take you a lot longer to find the problem).
Here are the basics to try
• #1 - Ground the spare
pairs at the frame!
• Ground the spare pairs in any house cables
you are using - this includes cables running between buildings
(which should always have lightning protection installed).
• Remove unused cables from the frame. If
you're having RF problems, prewires will often cause problems by
acting like an antenna.
• Try longer or shorter base cords or
handset cords. Simply "retuning" the antenna to a
different length will often help.
Before ordering RF
• Find out where the RF is entering the
system... through the CO lines, station cables, or handset cords.
• Find out the frequency of the station you
are hearing! This is very important - what works for one
frequency won't do anything for another using our Filters,
which are frequency band specific. Ask the customer what station it
is, often he'll know only too well. Be careful of broadcast stations
that are simulcasting on AM and FM, it's impossible to tell which
one the interference is coming from. Call the station to check to
see the broadcast schedule where they aren't simulcasting, try one
of our Combo AM/FM Filters, or try an AM, then an
FM Filter, to see which one fixes the problem.
Does it make more sense to carry a bunch of
inexpensive Filters in your case to see if they'll
fix the problem first, without going through the pain of grounding
spare pairs etc.? You'll have to decide. If you don't
carry some of our Filters for troubleshooting, you
won't have that option. It's such a pain to fix RF problems that
you'll probably save a lot of time by having some of these filters
on your shelf, for testing.
ARE THE TYPES OF FILTERS WE HAVE:
• Handset Cord
• 2 Pair Base Cord (RJ-14)
• 1 Pair Base Cord (RJ-11)
• 2 Pair Ethernet (RJ-45 for 10/100 BaseT)
• 4 Pair Ethernet (RJ-45 for Gigabit and/or POE using
all four pairs)
• Hardwire 1 Pair - White/Blue IN,
White/Blue OUT (Comes with 4 Beanies)
• 8 Pin DIP package, in 1 and 2
pair versions (for OEM use in your own equipment)
ARE THE FREQUENCIES WE COVER:
• AM: .5 to 3MHz
• CB: 3 to 30MHz
• AMATEUR / RF SEALING: 26
• FM: 60 to 150MHz
• Straight (Not Coiled) Shielded 6' Handset Cord for Neon Sign Hum
• Very Flexible, Super Strong Braided
Shield (with Ground Wire)
• Will fix Neon Sign hum (and maybe
dimmer hum) if the hum gets louder or softer when you move
around with handset (stretch out the cord)
This is our
RF Elimination Page
where we list all the Filters we sell:
The hardest thing to get rid of is 60 cycle
power type induced noise on an electronic telephone. Neon signs and
dimmer switches are the most frequent causes of 60 cycle hum.
On an actual telephone line, large chokes
(transformers or coils) are eliminate inductively coupled 60 cycle
AC hum. These are sometimes known as "humbuckets" or
"humbuckers"... and they often weigh as much as a full
bucket of water!
It's the phone company's responsibility to
get rid of the hum you can hear on a regular 2500 set at
the Network Interface (NI). If you only hear the hum on a phone
system, or a phone device that plugs into the wall for power (with a
reference to ground), you probably have induced AC on the phone
lines that you need to get rid of.
See our Longitudinal
Imbalance Tech Bulletin for more information.
You will most frequently hear the 60 cycle hum
in the handset of a phone placed near a neon sign. It's usually
picked up by the handset cord, and brought into the phone via the
mod jack. Phones using Dynamic (a receiver cartridge) or Electret (electronic condenser mic) transmitters will pick up this
type of interference (because they have a high power op-amp to
amplify the low output of the mic). Phones using carbon transmitters
hardly ever pick up this kind of hum (they don't need the op-amp).
Most phone systems now use Electret
transmitters, and neon signs / art are "hip" to use in
decorating offices and stores.
Neon at Pharmacies in chain stores is a big
problem. Many are using Neon just above the counter area, which is
causing hum on the phones. One solution called in by Bob Olson of
CommWorld of Portland (OR), is to have a contractor build a sheet
metal shield around the transformer and wiring in the ceiling, and
ground it with a heavy ground. He says this works 100% of the time
No filter currently
exists to eliminate this type of interference. We have a
6' Handset Cord, which should get rid of the noise if it
gets louder or softer as you move around with the handset, but not
everybody is thrilled with the idea of using a straight (not coiled)
TYPE PLASTIC SEALING EQUIPMENT
Companies that manufacture plastic and vinyl
notebooks and similar plastic products often use machines that
produce heat to "weld" the plastic parts together, by RF.
This fairly high frequency (usually around 30MHz, but check on the specific customer's machines), when put in the proximity of the
plastic, causes enough heat to melt the plastic parts together
(without using a hot wire or something similar).
This RFI usually shows up as a tone heard on
the telephones whenever the machine is operated. If there is more
than one machine (and there usually are), you can hear a chorus of
tones all day long on the phones at this type of factory. I haven't
seen any 1A2 key systems that were susceptible to this, but I've
seen it on all types of electronic key systems and PBXs. Keep this
in mind of you're proposing a phone system to a factory that might
use these types of machines!
Most people have tried running shielded cable
only to find that the problem is as bad or worse than with regular
CAT3 cable. If you do run shielded, never leave the shields
ungrounded. Always ground them at the frame. The shield acts like an
antenna, bringing the RF back to the frame (66 blocks), and
spreading the RF throughout the whole system (possible even with the
shields grounded at the frame). Running CAT5 cable would be a better
idea, since it will reject inductive interference due to the high
number of twists per foot. Obviously, you should try to run the
cables as far away from the machines as possible. Running a few
hundred extra feet of cable to stay away from the production area
can really pay. Putting a cable run right on top of one of these
machines is an insane idea.
Make sure you ground the spare pairs on the
cables at the frame. Disconnect any unused cables from the
frame...you can't have any prewires in this kind of environment. THIS
ONE STEP WILL CURE MAYBE 20% OF THE RF PROBLEMS, DOING NOTHING ELSE!
Make sure there is a good ground, and ground the system as per the
manufacturer. Make sure the cover is on the phone system. I tried
putting up a grounded metal plate behind the KSU at one customer,
but it didn't seem to do much good. Make sure that the customer
closes all of the maintenance covers on his machines, and that they
are correctly grounded. Leaving the covers off those machines really
makes things worse!
After you've taken care of the basics, you can try our AMATEUR
/ RF SEALING EQUIPMENT ELIMINATORs. They're available in a
modular version for the Handset that plugs right into the phone -
and the handset cord plugs into it. It's also available in a 1
or 2 Pair Modular Line Cord Filter, which would work on a
two line phone or a system phone that used one or two pairs. You may
need one, or the other. You may also need the Line Cord or Hardwire
version at the KSU end. The filter at the KSU end would prevent the
RF from coming down the station cable, and being spread throughout
the KSU. If you (or the customer) are really unlucky, you may need a
filter at all of the above locations. That's why you want to be
careful bidding a factory with these machines.
On a new system I was bidding on, I'd
make sure there's an understanding that I'd do the RF elimination T&M after the system is
installed. I'd also bring out a system and run some cables along the
floor to see how bad things are, before signing
the contract. It might not be worth dealing with a mess like this.
Digital systems are also susceptible to this
interference, usually picked up by the handset or base cords and fed
into the analog portions of the telephone set.
Putting a second Filter
in-series with the first one of the same frequency range would add
about 10% of the attenuation of the first filter. If the RFI is
really bad, you may need two of our Filters
Welding noise is another hard one to get rid
of, but it can be done! The same basic cable installation rules
apply as with the RF Sealing Equipment. Make sure that any unused
cables or prewires are removed from the frame. If they remain
bundled with the working cables, ground all of the pairs after you
remove them from the frame. Spare cables will act like antennas,
picking up the welding interference and inducing it into the other
cables. Welding noise seems to be picked up by the handset cord in
Don't run cables right over a welding machine,
or even near it! Like RF sealing machine interference, you should
try to run the cables as far away from the welders as possible.
Running a few hundred extra feet of cable to stay away from the
welder(s) can really pay. Putting a cable run right on top of one of
these machines is an insane idea. If the company just installed a
new welder, and the interference just started, you may have to
re-run any cable runs near the machine, or even move the KSU if our Filters
The cure for welding noise has been to use all four of our Handset
or Line Cord filters cascaded together... the AM, CB, CB/RF SEALING
and FM, which will notch out a huge portion of the RF spectrum (.5
to 150 MHz.). You know that you need the HANDSET
versions of the RF Eliminators if you can move
around with the handset, and the noise gets louder or softer.
Welding noise is RF spread throughout the
radio spectrum. We are batting 100% at getting rid of it, but it's
expensive since you have to use 4 or 8 eliminators on each of the
effected phones, and you might need more at the KSU. The Filters
seem to work as close as 8 feet from the welder! This applies to
both analog and digital phone systems.
Depending on the type of welder (there are
many different welding technologies in use today), you may be able
to eliminate the noise with less than the four frequencies. If four
of them work, start removing / replacing them one at a time to see
if the noise gets worse, and only use the ones you need. We
have a 30 Day Moneyback guarantee on our modular RF Filters. We
can't take the Hardwire Filters back if the wires
have been cut.
AND HAM RADIO
CB radio interference is hands down the
hardest form of RFI to get rid of because you'll hardly ever hear
the problem while you're trying to troubleshoot it.
It's most frequently seen at customers that
are close to highways with a lot of truck traffic. It seems that
truckers have taken to using CB radios with power amplifiers that
take the legal 5 watts of power up to 500 to 3000 watts of power.
Many AM radio stations broadcast at 250 to 1000 watts, so you can
guess how much interference potential these CB radios have as they
roll past your customer's office.
Normally, the interference is only heard for
30 seconds or so, while the truck is driving by. In the worst cases,
a trucker will stop by the side of the road right near your customer
and carry on a long conversation, but that won't happen often.
In major cities, you'll often hear cab drivers
as they pass by (they use the same power amplifiers to talk between
themselves). In Chicago, we hear crazy foreign cab drivers screaming
at each other all the time. In the worst cases, these rigs are
powerful enough to lock up the phone system or cause it to reset.
The first thing to determine is whether the
RFI is coming in before or after the phone system. If all of the
phones are effected, and it's occurring on both the handset and
speakerphones but not the intercom, then it's a pretty good
indication that it's coming in through the phone lines, and you may
be able to get the phone company to work on it for you (doubtful).
If, more likely, there are only a few phones that hear the RFI (most
often closest to the highway), you'll want to find out from the
customer whether it's heard over both the handset and the
speakerphone, or just the handset. If they only hear it on the
handset, that's a pretty good indication that the handset cord is
picking up the RF, and our CB RADIO - RF ELIMINATOR FOR
HANDSETS will take care of the problem.
If the RFI is heard on both the speakerphone
and the handset, and on intercom calls, then the station cable is
probably picking up the radio signal. Ground the spare pairs at the
frame. If the system uses one or two pairs for the phones, use our 1
LINE or 2 LINE CB RADIO RF ELIMINATOR at
the back of the telephone. Sometimes running a new station cable
from a different direction, of a different length, will
"retune" the cable away from the CB frequency. You may
need to put an RF Eliminator in the middle of the
cable (to retune it), or possibly at the frame.
If you have access to a trucker with a power
amplifier, that's the quickest way to fix this problem - because he
can sit out front and talk while you try different approaches to the
problem. Generally, it's a long involved process to cure CB
interference... Because it's almost never doing it while you're
there, so you have to make a change and wait to see if it's fixed,
over and over again.
Ham radio interference, or a CB radio base
station is often easier to track down. Often, you can see a big
antenna nearby that may be the offender (don't jump to conclusions
too fast!). If you can get the cooperation of the guy with the
radio, at least to broadcast while you're trying to fix the problem,
you're going to be ahead of the game.
There doesn't seem to be any brand of phone
system that's immune to CB interference, Analog or Digital. In
general, Digital systems will pick up the RFI at the phone through
the handset cord, since that is analog, and the information coming
down the base cord is digitized (sometimes immune to RFI at this
point). Remember...every digital system is analog going in, and
analog going out, so there's not that much difference from an analog
system. If you don't cover or ground all of the pairs in the station
cable, it's possible the RF will go around the Filter
on the unprotected pairs, and induce back into the voice pair after
the Filter (even more likely at FM frequencies).
You also may need to break up the station cable in the middle and
put in an RF Eliminator, or put it at the frame.
Nothing is very logical about RFI, especially CB!
AND FM BROADCAST STATIONS
This is the most often encountered form of RFI.
Luckily, it's pretty easy to fix. Our filters are running better
than a 99% success level.
AM radio towers normally stand on the ground,
and are pretty tall. How tall the tower is depends on the wavelength
of the AM frequency. A full wave AM antenna, like for 1000khz, would
be 984 feet. Because it's usually impractical to have an antenna
that tall, the tower is probably a 3/4 wave or 1/2 wave (half the
size). Because AM towers need to be tall, you won't see them on the
top of tall buildings They're were usually out in the boonies, since
it takes a lot of land for guy wires, and for multiple antennas for
directional arrays - which beam the signal in certain directions.
Since the boonies is getting to be pretty built-up, more AM RFI
problems are seen in these areas.
FM towers are usually on the top of tall
buildings, mountains, or other high structures. Since a full wave FM
antenna for 100MHz is only 10 feet, and how far you can pick up the
station depends on the height of the antenna, FM antennas are often
on top of the highest buildings, which are often in a downtown area.
Although you'll have FM interference problems anywhere the radio
station can find something tall to put their antenna on, many FM RFI
problems are clustered in downtown areas (with the tall buildings).
A radio antenna to receive a particular
frequency would work best when its a full wave, but since that's
impractical (especially for AM), an antenna would be a fraction of
the full wave. The closer the length of an antenna (in this case a
station cable or handset cord) is to an even
fraction of a full wave, the better it's "tuned" to
receive a particular frequency. A 1/4 wave of 1000 AM is 246 feet,
which could be the length of a station cable run. A 1/2 wave of FM
100 is 5 feet, which could be the length of a handset cord
Headsets seem to have a real affinity for picking up AM or FM radio
stations. You can be sure that you need one of our HANDSET
CORD RF ELIMINATORs if you hear the RFI on the headset, but
not the handset. This is the most common scenario. Every brand of
headset seems to be equally susceptible to RFI. Just plug our RF
Eliminator into the handset jack on the phone, and plug the
headset adapter box into it. We are at a 100% success rate with this
If the phone or console has two jacks for
handsets (like for training), be sure nothing is plugged into the
second jack as it can also pick up the RFI - even if you don't hear
it on that handset or headset. If you need
to have both jacks used, you may need an RF Eliminator
for each handset / headset.
Our RF Eliminators have
modular jacks, and cover both the transmit and receive pair. We do
not have a "327 plug" (2 prong) version available for
operator's headsets, but we do have "327 to modular"
adapters that would let you use our RF Filters.
A pretty high percentage of RFI problems come in through the handset
cord - about 50% according to our sales. There are two ways that you
can tell if RFI is coming through the handset cord. First, move
around with the handset, or bunch up the handset cord so that it's
very short (3 or 4 inches), or stretch it out so it's long. If the
RFI gets louder or softer, our HANDSET CORD RF ELIMINATOR
should help. Make sure you know whether it's AM or FM RFI, or use
our COMBO Filter. Second, if the telephone has a
monitor or speakerphone, listen for the RFI through the speaker. If
you don't hear any, it must be coming in through the handset cord
(although one customer did tell me that the handset filter DIDN'T
work, but putting one on the base cord did
work... go figure!). Occasionally, making the handset cord longer or
shorter will solve your problem by "retuning" the antenna.
NEC issued a service bulletin years ago for
it's key and PBX phones that use Dynamic transmitters. You can take
four .01 or .02mf disc capacitors (48V is fine), and wire one
inside the phone before the modular handset jack, across the two
wires going to the transmitter, and one across the two wires going
to the receiver. Then put one between one of the receiver leads and
one of the transmitter leads, and put the last one between the
remaining transmitter and receiver lead. Basically, you have made a
little square between the four handset leads. This definitely
doesn't work with an electret transmitter, and may or may not work
with a carbon transmitter.
Occasionally you may hear hum on the handset
that is not from a neon sign. This is often heard in downtown area
where there is lots of RF (TV?). Our RF Eliminators may
or may not do anything for this hum, but I'd try the FM first, which
would cover the VHF TV band. I was able to just about get rid of it
at one customer site by substituting 15 foot handset cords for the 6
Paging (PA) Systems
RFI problems most often occur on self
amplified speakers. You've got a long cable run going to the speaker
that works great as an antenna. Putting a Hardwire Filter in-front
of the speaker with the RFI usually works, but you may need to put
one in front of each speaker, or put one in-series, somewhere in the
middle of the cable going to the speakers, to prevent the cable run
from acting like an antenna.
If you hear RFI on a paging system with a
central page amplifier, you'll usually need the filter at the input
to the amp. Swapping tip and ring on the input to the amplifier may
also help. I've never seen anything on the output of a 70V page amp
pick up RF (because there are no components there to demodulate and
amplify the RFI).
Our Ethernet Filters
work on 10 / 100-BaseT
Ethernet CAT5 cables that are picking up AM, FM, CB, and
Plastic Sealing Machine radio
interference and causing network problems. If you have an Ethernet
network with problems and you're near a radio tower, RF would be a
good thing to check for!
Wireless Internet providers (WISPs) are fixing problems
with their access points on FM towers using our 4 Pair Ethernet
Filters (may need one at each end of the cable run).
We have 2 Pair and 4 Pair
Ethernet (RJ-45) Filters for networks. You may need it at the
patch panel, at the workstation, or both ends.
Our 2 Pair Filters only let the two active pairs for Ethernet
through (in which case you'd want to use one at each end of the
cable to prevent the RF from being induced into the two filtered
pairs from the unfiltered pairs along the run).
If you're using POE or Gigabit (1000 BaseT) Ethernet, you
need our 4 Pair Ethernet Filters.
LINES, MODEMS, PAY PHONES, STATION CABLES
If you hear RF on the CO lines at the NI
(Network Interface) with a Butt-set, you have to get rid of it there
before you go any further. Once RF is changed to audio,
there's nothing you can do to filter it after that point.
If you hear RF at the NI you may have to put a filter at the
pedestal, if putting it at the NI doesn't help (because the cable feeding
the pedestal is the antenna). In many cases, you won't hear the RF
in your Butt-Set, but you'll hear it on the phone system. That's
because your Butt-set doesn't have the particular components that
are demodulating the RF and turning it into audio (but the phone
Modems, fax machines, pay phones and other
single line devices may be more prone to RFI than other equipment
(like headsets are more susceptible than handsets). This is due to
the internal design of the device. RFI can cause false triggering of
answering machines, corruption of fax and modem data and strange
problems on electronic telephones (these symptoms can also be caused
by high or low loop current on the telephone line - see our
Current Tech Bulletin).
It's possible that as a result of poor design
or component failure, a telephone device can be the
cause of interference. Although it's a rare
occurrence, this device may continue to operate - even with a
malfunction that causes interference to other devices on the line.
Putting an RF Filter either at the KSU or the
electronic telephone (they both contain microprocessors), you may
get rid of noise that's being picked up on a radio or some other
device that's near the station cable (which is now acting like a broadcast
antenna). If you had interference on an AM or FM radio, you'd choose
the Filter at the frequency you're having the
interference at, on the radio (AM, or FM).
RFI can come down the station cable into
the station card on a single station port, and then get broadcast
back out all of the station ports on the same card or the whole
system. For this reason, it's important to "simplify" your
diagnosis by putting a phone at the KSU (or NI) and stripping the
system down as far as possible. Replace the cards, plug in the
station cables or punch down the cables one by one until the trouble
reappears (this is standard troubleshooting procedures for any
system problem). I know it's a pain stripping the system down to the
basics, but I did say that troubleshooting RF isn't easy.
If your troubleshooting brings you down to a
single station card or phone, try swapping them with another one to
see if the trouble follows.
For maximum interference rejection, make sure
your system ground is of the highest possible quality. Grounds other
than known-to-be conductive cold water pipe (it's hard to find water
pipes that aren't PVC these days, so a cold water pipe isn't
considered a good ground), building frame ground or driven ground
rods should be considered suspicious. Good grounding goes a long way
toward shielding your telephone wiring from intruding RF. An upgrade
in system ground at some sites can eliminate telephone RFI. At other
sites, a better ground can reduce interference noticeably, requiring
less work and fewer filters.