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Author Topic: ADSL Susceptibility  (Read 1900 times)
G4EDY
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Posts: 10




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« on: July 02, 2003, 09:13:16 AM »

I have been trialling ADSL for a month using a US Robotics Connectsure USB modem and an iMac. It works well given that I am 5km from the exchange and the telecom wiring is ancient and awful!!

I have not noticed any increase in noise on 80-10m but have now got an RFI susceptibility problem in that my transmissions are audible on the telephone.

There is no change to my equipment or telephone, just the addition of the necessary passive filters.

I can only assume that the high local RF field when I transmit is producing a differential signal on the phone line (which is mounted on poles outside the property) of sufficient magnitude to overload the electronics at the exchange which demodulates my signal to produce an audio signal that happily travels back down the line to my phone!!

I wonder what sort of filtering is used at the exchange? What happens if you have a local AM broadcast station within the ADSL 0.1 to 1.1MHz passband?

Any comments appreciated, 73s Martin
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KA5S
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Posts: 229




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« Reply #1 on: July 02, 2003, 02:49:07 PM »

Since your telephone RFI is new, the ADSL modem is probably rectifying RF.

An ADSL filter (I worked on this stuff at my last employer) is usually built into the modem and into the telco equipment.  A series capacitor blocks audio from coupling into ADSL circuitry. However, protective diodes are included to deal with ring voltages and transients, and these WILL rectify RF. Since their forward voltages are not well matched, the rectified RF is likely to show up as a differential output at audio frequencies.

Since ADSL uses spectrum up to 1.1 MHz, a low-pass filter before your ADSL modem, with a cut-off around 2 MHz, might well reduce this effect.

There is another possiblity, which is that RF is entering the modem by way of the mains conductors.  I observed such a problem on an early ADSL modem myself, but as it was not our equipment, I could do little but advise the manufacturer. However, a sturdy common-mode powerline filter dealt with it.


Cortland
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N6AJR
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Posts: 9910




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« Reply #2 on: July 11, 2003, 07:52:57 PM »

Thry a K-com phone filter in line with the dsl filter, it fixed mine..
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KE4ZHN
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Posts: 139




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« Reply #3 on: July 20, 2003, 12:21:45 PM »

I fixed mine with a Kcom filter as well, BUT I discovered that it worked MUCH better if installed in the phone box itself rather than at the phone. Just hook it up inside the phone boxe where your phone wires enter the house and it probably will fix the problem.
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