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?
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.