Modem Phone line filter

[Bob Urz]

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?

 

[Meat Plow]

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?

http://tinyurl.com/y7wzgu

 

[Joerg]

Hello Bob,

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?

If a series connection of DSL blockers doesn't work you might want to
try a common mode choke. Either one off the shelf or take a #77 core and
wind twisted pair through it as many times as it'll comfortably fit
through there. In a pinch the core of a discarded flyback transformer or
the big ferrite from a scrapped AT power supply can do. Mains
transformer cores would not be so good here.

 

[Aratzio]

http://tinyurl.com/y7wzgu

shpxurnq

 

[Meat Plow]

shpxurnq

fgnyxre

 

[Ancient_Hacker]

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.

A DSL filter should do an excellent job, knocking down the AM band by
at least 25 decibels.

Try plugging in a phone instead of the modem and listen carefully, with
and without the DSL filter. The DSL filter should make a huge
difference. If it helps, but you can still hear the radio station, put
a second DSL filter inline.

If the filter doesnt make much difference, then the audio detection is
happening somewhere else, perhaps in another phone or your phone entry
box signal limiters. Try unplugging the other phones one at a time and
see if that makes any difference.

If that doesnt find anything, you might have to put a dsl filter
BEFORE the entry box or AFTER the entry box.

 

[Aratzio]

fgnyxre

Nope, finger out my usual addy in here.

Neener.

 

[Adrian C]

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.

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?

 

[BOB URZ]

Hello Bob,


If a series connection of DSL blockers doesn't work you might want to
try a common mode choke. Either one off the shelf or take a #77 core and
wind twisted pair through it as many times as it'll comfortably fit
through there. In a pinch the core of a discarded flyback transformer or
the big ferrite from a scrapped AT power supply can do. Mains
transformer cores would not be so good here.

I have tried some snap on split cores and not had much luck.
As well as a single DSL type in line filter. This Am transmitter antenna is
within 1000 feet of my house.

After some research, i found most of the split cores are more for the MHZ
region rather than the Kilo hertz AM radio band.

I have some salvaged cores from my junk box. I would hate to use
a flyback sized core. i think i might have a small input ac donut and
maybe some inter stage stuff. I will have to poke around. Other than
application, i wonder how you can ID the cores for there type or frequency
range?

I am curious how you do model this as a filter network. What load
does a average modem put on the phone line? If you were trying to design a
filter, you would have to know impedance's to be in the ballpark. It seems
like a pie type with two inductors in line with
the balanced modem leads with a cap/resistor across them would
give a deeper cut in the unwanted frequencies.


Bob

 

[James Sweet]

A DSL filter should do an excellent job, knocking down the AM band by
at least 25 decibels.

Try plugging in a phone instead of the modem and listen carefully, with
and without the DSL filter. The DSL filter should make a huge
difference. If it helps, but you can still hear the radio station, put
a second DSL filter inline.

If the filter doesnt make much difference, then the audio detection is
happening somewhere else, perhaps in another phone or your phone entry
box signal limiters. Try unplugging the other phones one at a time and
see if that makes any difference.

If that doesnt find anything, you might have to put a dsl filter
BEFORE the entry box or AFTER the entry box.

Or as close to the computer as possible. I believe they should work fine
in either direction, so you could plug it right into the computer then
use a normal phone cord to the wall. The AM signal may be picked up
right there in the cord into the modem.

 

[Joerg]

Hello Bob,

I have tried some snap on split cores and not had much luck.
As well as a single DSL type in line filter. This Am transmitter antenna is
within 1000 feet of my house.

That's way too little inductance. You have to wind the twisted pair line
through there many times. Also, after several uses the core halves often
don't have a snug fit anymore and then the inductance drops rapidly.

After some research, i found most of the split cores are more for the MHZ
region rather than the Kilo hertz AM radio band.

It's a matter of inductance. A split core snapped onto a line is only
one turn. Not enough below 1.6MHz, usually.

I have some salvaged cores from my junk box. ...

Take the largest one you find and wind twisted pair through there until
it's hole is stuffed good.

... I would hate to use
a flyback sized core. i think i might have a small input ac donut and
maybe some inter stage stuff. I will have to poke around. Other than
application, i wonder how you can ID the cores for there type or frequency
range?

Unknown core ID: Wind ten turns around it, measure inductance, calculate
Al value. Now measure I.D., O.D. and thickness, then go to the Amidon or
Fair-Rite catalog to find which type matches. Most EMI ferrites will be
#43 material. For AM band higher perm stuff works better but in a pinch
you have to try to make do with what's there. BTW, don't use the
Fair-Rite web catalog if you are on a slow internet connection. It's
humongous PDF files.

I am curious how you do model this as a filter network. What load
does a average modem put on the phone line? If you were trying to design a
filter, you would have to know impedance's to be in the ballpark. It seems
like a pie type with two inductors in line with
the balanced modem leads with a cap/resistor across them would
give a deeper cut in the unwanted frequencies.

It's very little differential load but there is a DC current that can
saturate inductor cores. That won't be much of a problem for a common
mode choke.

Another method is to notch out the frequency of that station if it gets
in differentially (that is rare though).

 

[Meat Plow]

Nope, finger out my usual addy in here.

Neener.

shpxurnq

 

[Meat Plow]

Nope, finger out my usual addy in here.

Neener.

qba'g xabj jung lbhe hfhny nqql vf

 

[Ancient_Hacker]

Hmm, funny that the DSL filter didnt help.


The relevant frequencies are 100 x apart, so the filter is super non
critical.

A couple of 2.5 mH chokes in series and two 0.01uf capacitors in
parallel will make a dandy filter, at least 40dB down. That's
roughly what's inside a DSL filter.

But again, I'd follow the investigative steps from my previous msg. it
won't do any good to place a good filter in a bad place.

 

[Eeyore]

Hello Bob,


If a series connection of DSL blockers doesn't work you might want to
try a common mode choke. Either one off the shelf or take a #77 core and
wind twisted pair through it as many times as it'll comfortably fit
through there. In a pinch the core of a discarded flyback transformer or
the big ferrite from a scrapped AT power supply can do. Mains
transformer cores would not be so good here.

I have here a BT RF2 filter.

It simply consists of a 2.2mH choke in both lines.

Like these
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=432167

I'll post a pic in abse.

I'd expect a common mode choke to be better of course.

Graham

 

[Ken Weitzel]

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?

Hi Bob...

Spent my working lifetime in broadcast, so can virtually guarantee
that you don't have to do a thing

Just call the station, ask for the Chief Engineer, and tell him of
your problem.

If your country is anything like Canada, he'll be there in a flash,
and all of your problems will be gone

If you want him there even faster'n a flash, just ask him if
you're supposed to be telling him about the problem, or should
it be the DOT/FCC/whatever country you're in

Take care.

Ken

 

[Franc Zabkar]

I think it's more like 600 ohm.

- Franc Zabkar

 

[Don Bowey]

Hi Bob...

Spent my working lifetime in broadcast, so can virtually guarantee
that you don't have to do a thing

Just call the station, ask for the Chief Engineer, and tell him of
your problem.

If your country is anything like Canada, he'll be there in a flash,
and all of your problems will be gone

If you want him there even faster'n a flash, just ask him if
you're supposed to be telling him about the problem, or should
it be the DOT/FCC/whatever country you're in

That's bad advice.

Take care.

Ken

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

 

[Ancient_Hacker]

It simply consists of a 2.2mH choke in both lines.

Like these
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=432167

I'll post a pic in abse.

I'd expect a common mode choke to be better of course.

I *hope* they use a more appropriate choke than that one-- if you read
down a bit the specs say it resonates at around 1.35 MHz. That means
it's more of a capacitor than anything else above that frequency, and
not a very good inductor in the AM band. Although likely to be good
enough at the lower DSL frequencies.

A good 2.5mH RF choke is pie wound to minimize capacitance and is good
up to 30MHz at least.

Opening up a DSL filter here shows a somewhat better design -- a two
solenoid-wound chokes and two mylar capacitors across each end.

But that still is not quite optimum-- the DSL frequencies that filter
tries to choke out are not quite as high as the AM BC band. SO a pair
of *good* RF chokes will probably work better than the DSL filter type
of chokes.

 

[Eeyore]

I *hope* they use a more appropriate choke than that one-- if you read
down a bit the specs say it resonates at around 1.35 MHz. That means
it's more of a capacitor than anything else above that frequency, and
not a very good inductor in the AM band. Although likely to be good
enough at the lower DSL frequencies.

A good 2.5mH RF choke is pie wound to minimize capacitance and is good
up to 30MHz at least.

Well, they look like those parts. See the pic in a.b.s.e.

Opening up a DSL filter here shows a somewhat better design -- a two
solenoid-wound chokes and two mylar capacitors across each end.

But that still is not quite optimum-- the DSL frequencies that filter
tries to choke out are not quite as high as the AM BC band.

No ? I thought they coincided with low end of AM quite accurately in fact.

SO a pair
of *good* RF chokes will probably work better than the DSL filter type
of chokes.

Probably so.

Graham