On Nov 1, 11:42 am, Tim Shoppa wrote:
On Oct 30, 9:40 am, "dave.harper" wrote:

I'm using the wein bridge to generate audio tones that are fed to the
transmitter for digital radio communication (basically a homebrew ASK
radio modem). I'd be open to using that, but I'm not aware of any
radio modems that output square waves.

This oscillator also has 2 digital pots attached: 1 to trim the space
tone, and 1 to trim the mark tone. There's a high speed switch
between the two pots to rapidly switch between mark and space tones.
I could use this same setup with a 555 and trim the resistor to vary
the pulse duration, but I'm not sure what impact a square wave would
have on transmission, reception, decoding, etc...?

Has anyone heard of a square wave being used as an audio tone for
digital radio communication?

I think you mean "AFSK". At least that's what I think you mean. ASK =
Amplitude Shift Keying.

Something in the transmitter chain stops (or should stop!) the square
wave from being square with all the odd harmonics going out to
infinity.

This something may be in the audio stage (example: low pass audio
filter), or the IF stage (example: bandpass filter), or in the RF
stage (example: tuned circuits).

It is very doubtful that after going through the transmitter and
receiver that it'll come out as a square wave on the other end.
Choosing to transmit a square wave audio waveform is usually a poor
choice because you know that it can't come out looking that pretty on
the other end. This sort of design decision might be made for a very
low-end radio control transmitter of the 60's or 70's out of
ignorance, but today we know how to do far better with little extra
effort.

Most designs make a conscious choice to be a friendly transmitter, and
limit splatter and unnecessary bandwidth that would be in violation of
the FCC rules, by running any square wave through a low pass audio
filter AND additionally using a rational choice for the IF filtering
too. Way better than nothing, is a simple RC low pass in the audio
stage. Still to be nice the resulting audio level has to be carefully
set to not cause splatter in subsequent stages.


Hey OM

I looked at this guys profile.

I would say he's into telemetry.

If you want to stay away from EMI take it to the 2.4Ghz band.

I don't think he's a ham..

I think he cross forumed this post.

My best guess is he is running into trouble on the VHF/UHF spectrum
where there is tonnes o EMI. QRN and QRM.

73 OM
de n8zu