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FCC phone line and transformer

I have a few modems laying around and noticed, (especially on the
lower cost ones), that there isn't even a transformer on the PCB for
interfacing to telphone line.

All of them are using DAA chips but I would assume that a transformer
would almost be mandatory in getting part 68? approval.

Guess not though?

I have one modem here, a motorola, that just goes straight into the
phone line from the DAA chip.
 
E

Eric Tappert

Jan 1, 1970
0
I have a few modems laying around and noticed, (especially on the
lower cost ones), that there isn't even a transformer on the PCB for
interfacing to telphone line.

All of them are using DAA chips but I would assume that a transformer
would almost be mandatory in getting part 68? approval.

Guess not though?

I have one modem here, a motorola, that just goes straight into the
phone line from the DAA chip.


The FCC only requires isolation between the phone line and the
modem/phone. Historically this has been done with a transformer, but
there are schemes that use opto-couplers and even high voltage
capacitors to provide the required isolation. Often these are
packaged as a single DAA (Data Access Arrangement) "chip" even though
they contain multiple devices..

ET
 
I'm trying to slap some audio onto the phone line via this
transformerless modem, without (possibly) using a transformer myself.

The audio is a low level speaker output so the low impedance is a plus
for driving anything.

I can do it with a cap right to one of the DAA pins that go to the
phone line (before those isolation caps) and it works fine but I was
wondering if there's an even better (and safer) place to put the
audio.

I see a QE pin, an RX pin, a DCT pin and a QB pin on the DAA itself.

Any ideas? RX maybe?

I'm not familiar with these chips at all.
 
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