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