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question on telephone line interface circuit

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farrokhiyan

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Dear friends,

the below image shows the line interface of a dial up modem. the DC loop, ring detector, isolation transformer and hook relay are clear. the question is what does the RC circuit (shown in red) do?
when modem is not in operation and a ring signal comes, don't you think the high AC voltage of ring can go to other side of isolation transformer through this RC circuit?

Tnx

 
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goldsmith

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Hi
I think that RC network is a simple Snubber network , to prevent some transient responses and of course to remove some un wanted noises too .
Best Wishes
Goldsmith
 

betwixt

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Line impedance matching, that's all. Different countries use different line impedances, the R & C value will be set for the country where the modem is to be used (or should be...).
If the line impedance isn't matched, the modem would be unable to fully isolate the incoming and outgoing signals from each other.

Brian.
 

farrokhiyan

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Dear goldsmith,
frankly I've noticed that it can be a snubber but the way it is being used can provide a path for ac voltage of ring signal that was my second question.

Dear Betwixt,
the important thing is the path which the ring signal can induce to other side of transformer. can't we use them after the hook relay?
 

betwixt

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The ring signal is detected via R10 and C7.

In most countries, data is sent down the phone lines while the phone is still on-hook, for example caller ID information so the impedance needs to be fairly constant while the line isn't seized. Puting them after the relay (hook switch) woould change the impedance as it operated.


Brian.
 

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