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.
Finally found somewhat of a data sheet.....think I'll try the RX pin and see what happens.
Great design technique, eh?
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