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.
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).
--
Regards, Joerg
http://www.analogconsultants.com