> Anyone has familiarity with this:
> http://cyberdyne.homeip.net/public/FM-AirDryer.pdf
> ?
>
> I have to implement such a protocol on a board, basically to make it
> behave as a peripheral of such.
>
> Nothing really hard - 1000-2000Hz sine wave, 2sec steps, 1.5 duration ...
> I just have some doubts on what it really means the usual "0dB / 600ohm"
> :) I never got it through in depth. I will pick an amplifier suitable to
> that.
>

This looks like a standard audio output level that was originally defined
in the telephone industry. It's 0 dBm into a 600 ohm load, or 1mw into 600
ohms. It's something like 770mV. Originally the source resistance was also
600 ohms, but in the past 20 or 30 years the source resistance has been
low (50 to 100 ohms or lower). On short lines, the inputs bridge the line
with a 10k or higher input resistance. On long line, though (several
kilometers), the line is still loaded with 600 ohms. A lower load
resistance equalizes the loss over a wider audio frequency range. In fact,
it's fairly common in broadcast to use a pair of Western Electric 111-C
transformers to convert the 600 ohm source and load impedances at each end
of the line to 150 ohms so the line is driven with 150 ohms and loaded
with 150 ohms.

We use balanced line driver chips from TI or THAT to drive these lines
now. You can also use a pair of op amps, one driving one line, the other
inverting the signal and driving the other line. This is similar to
driving with a transformer with the center tap grounded. But the line
driver chips from TI and THAT have a servo output that will improve CMRR
over the two op amp approach. They act as a floating transformer secondary
so you can short one side to ground to drive unbalanced inputs.

Good luck!

Harold



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