Jim,
What we have here is a case where a few people think their
particular experience with lines and filters somehow means
we can misapply filters for the rest of the world without
any consequence.
> You might want to take another look at this, Tom. I just
did, looking at
I've looked at it dozens of times, if not hundreds. My
two-way business had repeaters all over the Great Lakes, I
did a lot of studio work at BC stations.
I can tell you right now the better quality line filters
will absolutely ruin most telco lines. A line filter sure
as heck won't work at my house.
Now it MIGHT work if my house was 500 feet to the A/D
conversion and the line amps driving the line had almost no
resistance between them and the filter, and there was only
500ft or less parallel distance to pick up AC from the power
lines ten feet above the phone line.
> the circuit analysis of some of the line filters in my
junkbox. It never
> occurred to me to try it, but I think Dave is right -- a
lot of power line
> filters ought to work pretty well as a line filter for a
phone line!
....and even more will hurt many lines.
> Depending on their design and circut values, some will
obviously work
> a lot better than others. I'm headed out to the Ft. Wayne
hamfest
> Saturday, and I'll likely pick up some of these filters
for exactly that use!
It is also telco line critical. Not all lines are a few
hundred feet long from the D/A conversion.
> Another point. Unless you happen to be hanging a broadcast
hybrid on
> it, a typical phone line is NOT all that impedance
critical.
Not true Jim. If I simply TOUCH one terminal of my line with
my finger while I am laying on a carpet on a wood floor, it
unbalances my line. Even 100K ohm induces severe hum.
To null hum in my lines, I added a 1.2 meg resistor in
series with a .01uF cap. The phone company can even tell it
is there when they run tests.
> measured the Zo of any phone lines, but I would be very
surprised if any
> of them have a Zo much higher than about 120 ohms.
The industry standard for equipment is 600 ohms.
Doing the math
> on cable construction tells us that. We all know what it
takes to achieve
The impedance of the wires has almost nothing to do with the
design impedance of the system, because the wires are not
large fractions of a wavelength long. The critical parameter
is the source and load impedance. In a short line, standing
waves have no place to stand.
> Zo on the order of 600 ohms -- look at typical ladder
lines designed for
> RF, and they're all a LOT wider spacing than any twisted
pair cable I've
> ever seen. If you were going to match the Zo of the
telephone line, you
> would be matching to 100-120 ohms, not 600 ohms.
Conductor spacing and diameter does not set Z0 of the system
unless the line is many wavelengths long.
If the line Z0 matched the equipment, the phone company
wouldn't have to worry about adding shunt L to compensate on
long lines. The load and source is designed for 600 ohms.
> 600 ohms used to be used a lot 40 years ago as a build-out
and load
> on audio circuits, but that interconnection protocol (600
ohm source,
> 600 ohm load) hasn't been used for anything other than
testing
> performance of output stages and the passive equalization
of
> broadcast loops for decades! And when's the last time
you talked to
> anyone at a telco who even knows what equalize a loop
means, let
> alone knows how to do it?
A few days ago.
73 Tom
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