Modem Phone line filter

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?

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----

formatting link
The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Reply to
Bob Urz
Loading thread data ...

On Thu, 16 Nov 2006 09:12:26 -0600, Bob Urz Has Frothed:

formatting link

Reply to
Meat Plow

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.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

On Thu, 16 Nov 2006 10:25:38 -0500, Meat Plow transparently proposed:

shpxurnq

--

Does the name Pavlov ring a bell?
Reply to
Aratzio

On Thu, 16 Nov 2006 16:21:23 +0000, Aratzio Has Frothed:

fgnyxre

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

 Click to see the full signature
Reply to
Meat Plow

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.

Reply to
Ancient_Hacker

On Thu, 16 Nov 2006 11:43:31 -0500, Meat Plow transparently proposed:

Nope, finger out my usual addy in here.

Neener.

--

Does the name Pavlov ring a bell?
Reply to
Aratzio

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?

-- Adrian C

Reply to
Adrian C

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

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----

formatting link
The #1 Newsgroup Service in the World! 120,000+ Newsgroups

----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Reply to
BOB URZ

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.

Reply to
James Sweet

Hello Bob,

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.

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

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

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.

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).

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

On Thu, 16 Nov 2006 18:30:46 +0000, Aratzio Has Frothed:

shpxurnq

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

 Click to see the full signature
Reply to
Meat Plow

On Thu, 16 Nov 2006 18:30:46 +0000, Aratzio Has Frothed:

qba'g xabj jung lbhe hfhny nqql vf

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

 Click to see the full signature
Reply to
Meat Plow

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.

Reply to
Ancient_Hacker

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

Reply to
Ken Weitzel

I think it's more like 600 ohm.

- Franc Zabkar

--=20 Please remove one 'i' from my address when replying by email.

Reply to
Franc Zabkar

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

Reply to
Don Bowey

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.

Reply to
TimPerry

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.
 Click to see the full signature
Reply to
Michael A. Terrell

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.
 Click to see the full signature
Reply to
Michael A. Terrell

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.