[GreenKeys] AH-1 and AK-2 AFSK oscillators.

[ad7i]

Hello John --

I don't know if you prefer to reconstruct a historical AFSK design, or if
you are simply looking for AFSK functionality (perhaps via a new design).

I'd be happy to do a layout of a modern AFSK oscillator board for you.
I've been working with KiCad for about the last two years and am now pretty
good at doing board layouts.  I get boards from China, five boards for less
than $6 total, delivered to my mailbox.  The only hitch is that the boards
can't be larger than 3.9" in either dimension in order to maintain the
prototyping pricing.  I haven't found that size limitation to be much of a
problem.  If the board layout does go over 3.9" the cost isn't a lot more,
but it's enough to try to keep things to 3.9" or less.

I've also been doing a lot of work with the AVR-DB and AVR-DD series of
microcontrollers ($1 to $4).   To quickly boil it down I have a working
design for a two-tone paging generator that implements an audio frequency
(4 Khz or less) Direct Digital Synthesizer in software that outputs
step-sine wave on the internal DAC.  Any change in frequency (AFSK) is
phase continuous thanks to the DDS.  Also, because I use a crystal for the
timebase of the AVR, the audio frequencies are accurate to about 1/100 Hz.
To clean up the step-sine wave I follow the DAC output with an 8th order
Butterworth low pass filter, which sounds like it would be complicated, but
it's just one 14 pin quad op-amp package and a surround of 8 Rs and 8 Cs.
The Rs are 1% and the Cs are 10%.  If I only need low current AC output the
last opamp can deliver several milliamps at 3vpp.  If I need big current
output then I follow the last filter opamp with an LM386 speaker driver,
which is good for about 700+ mA at 3vpp.

If it was desired to really go fancy, a socket for a micro-SD card could be
added, and the SD-card could hold PCM files of audio modem tones to be sent
out the DAC.  I haven't tested that with tones, but I have with human
speech and it works pretty well with that.

Let me know if this, or something similar, might be of interest.

Paul, ad7i



On Fri, May 12, 2023 at 10:47 PM Ralph Mowery <rmowery42 at charter.net> wrote:

> Not to discourage you, but check the prices of the ICs and Crystals now if
> they can even be found .
>
> Ralph ku4pt
>
>
>
> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net
> [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of John W9DDD
> Sent: Friday, May 12, 2023 7:37 PM
> To: greenkeys at mailman.qth.net
> Subject: [GreenKeys] AH-1 and AK-2 AFSK oscillators.
>
> Hi all,
>
> Way back when I built the AK-1 on perfboard.  Used it with a KWM-1 circa
> 1971.  I haven't fired it up yet to see if it still works.  I was
> considering upgrading it to a AK-2.
>
> The RTTY journal mentions K8JUG having PCB for the AK-1,  I didn't find any
> reference to there being boards for the AK-2.  Does anyone know there were?
> Or perhaps gerber files exist somewhere.
>
> It just occurred to me as I type that the ST-6 set of boards probably
> included.  I'll have to go research that.  I know at one time images or
> gerber were floating around at the time.
>
> I would consider doing my own layout and boards if I don't find anything.
>
> Pointers appreciated.
>
> John, W9DDD
>
>
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