Modem Phone line filter

[TimPerry]

That's bad advice.

Just exactly why you think the radio station or FCC is going to care
about RF getting into the premises wiring or telephone cable? The
station engineer may make a filter recommendation, but is under no
obligation to do any more than that.

The issue is... What non-linear device is causing the RF to be
rectified so it is audible. It could be a bad joint, a telephone, or
whatever..

Don

Ken, time have changed in that the engineer nowadays has multiple stations
to maintain. when i get a RFI complaint like this my first question is to
ask if Telco has been called. if the problem is before the customer demarc
no filter in the world will help as the interference is audio at this point.

if it clean at the demark and occurs only when the inside wiring is hooked
up then something can be done with filters. sometimes the problem can be
traced to a single plastic phone.

often the telco can test the line without sending anyone out.

all too often after a storm a telemetry line at a transmitter site will go
bad or partly bad. no amount of filtering will be very effective.

the charicteristic impedance of a "copper" line is about 150 ohms. the Z
was brought up to 600 ohms in dedicated circuits by 1:4 transformers.
nowdays it's all digital.

in the last 6 years i have not gotten a call back after referring a
complainant to the telco.

 

[Michael A. Terrell]

Anybody got a good resource on how to build a phone line filter?
I am near a AM radio tower and its really killing my dial up modem
at times. I tried a DSL filter in line. but i think the AM band is
not getting attenuated enough to do much good.

Its hard to peg characteristics. Would a modem be considered a 200 ohm
nominal load on the phone line?

What type of filter for AM radio?

Dual inline inductors?
parallel capacitor with series resistor maybe?

Any ideas or links?

Make a tuned trap for the station's carrier frequency, and put it
across the phone line. You can use the antenna coil and variable
capacitor from a junk am radio, or buy the parts from Mouser or one of
the places selling parts for crystal radios. Connect the antenna coil
and variable capacitor in series, then connect the filter across the
line. Tune it for minimum RF on the line. (Scope or AC coupled AC
voltmeter)

If you have a lot of outlets on that phone line you may need more
than one filter. The cheap "electronic network" used in cheap, modern
phones can rectify the RF and give you fits. The old transformer and
capacitors didn't cause these problems.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

Does your telephone company have a duty in contract to provide you a
clean phone line connection? Let them sort out filtering for you, and
maybe boost the line perhaps?

More signal, to make the problem worse? Something is being driven
into a non-linear operation, rectifying the RF.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

Anybody got a good resource on how to build a phone line filter?
I am near a AM radio tower and its really killing my dial up modem
at times. I tried a DSL filter in line. but i think the AM band is
not getting attenuated enough to do much good.

Its hard to peg characteristics. Would a modem be considered a 200 ohm
nominal load on the phone line?

What type of filter for AM radio?

Dual inline inductors?
parallel capacitor with series resistor maybe?

Any ideas or links?

Unplug the house wiring from the CO at the network interface, and see
if the line is clean at that point. The problem may be in the Telco
side.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[bz]

IIRC, phone lines are a nominal 600 ohm impedence.



--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[Eeyore]

IIRC, phone lines are a nominal 600 ohm impedence.

Isn't the bell circuit about 2k ?

Graham

 

[Don Bowey]

IIRC, phone lines are a nominal 600 ohm impedence.

The impedance of a non-loaded phone line is "nominally" 900 Ohms. That's
the compromise value used by the Telcos. The actual value could be higher
or lower depending on line length. Good modems will adjust themselves to
get the best match.

Modern phones should have a off-hook DC resistance of 430 Ohms if they
contain a tone dialer, or 330 Ohms if the have a pulse dialer. The 100 Ohms
difference is to provide the voltage drop to power the tone generator.
Which leaves me wondering what a modem DC resistance would be. Since they
are not line-powered, they could be designed to 330 Ohms, but if they were
designed to 430 Ohms it would be ok too. By way, this true of U.S. And
Canadian Standards.

Don

 

[jasen]

Isn't the bell circuit about 2k ?

near infinite DC resistance, 2k AC resistance sounds about right given
that you're allowed to parallel three of them.

 

[Don Bowey]

near infinite DC resistance,

In the U.S. And Canada the DC resistance must not be lower than 5 MegOhms.
That is with 5 phones connected, with each having a Ringer Equivalence of
one (total REN=5). You can have more phones connected, but the total REN
must not be greater than 5.

2k AC resistance sounds about right given
that you're allowed to parallel three of them.

See above. Where did the quantity 3 come from?

Don