[RFI] NOISE GADGETS

[Jim Brown]

On Wed, 10 Nov 2004 19:40:40 -0500, Tom Rauch wrote:

>A telco line runs at 300-3500 Hz, has reasonably high
>impedance (600 ohms), and that is very load impedance and
>balance critical.

You might want to take another look at this, Tom. I just did, looking at 
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! 
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!   

Another point. Unless you happen to be hanging a broadcast hybrid on 
it, a typical phone line is NOT all that impedance critical. I haven't 
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. Doing the math 
on cable construction tells us that. We all know what it takes to achieve 
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.

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?  

I love this list -- if you keep your brain ready to accept new input, you 
can learn something new every day!

73,

Jim Brown  K9YC