On Thu, 2006-11-23 at 06:18 -0800, Jerry Volpe wrote:
> I've done a preliminary search of the Internet on this subject but
> surprisingly came up dry....
>
> Does anyone have notes describing successful modifications for the PTO
> style transceivers?
>
> And yes, I know about AFSK but I'd prefer to avoid it. I recently set up
> my Orion for FSK and would like to modify one of my other transceivers
> to incorporate FSK. I suppose it might be as easy as providing a
> calibrated voltage control to a small varactor operating in parallel to
> the one used for RIT/XIT.... yet you would also have to unbalance the
> balanced mixer to insert carrier OR operate in CW mode without the
> normal CW offset taking effect. So although simple in concept adding FSK
> creates some interesting challenges. So wondering if anyone tackled this
> in the past.
>
> Happy Turkey Day to all!
>
> Jerry, KG6TT
> Fairfield, CA
>
AFSK doesn't work all that badly unless you have a lot of filter blow
around in transmit, over drive the radio, or pick tones that have
harmonics in the transmit pass band. I used symmetrical triangular waves
for a while, not sawtooth, and it worked without complaints of spurious
outputs. But being a modified relaxation oscillator, frequency shift was
rapid, just a change of slope. A truly symmetrical triangular wave has
only 8% total harmonic distortion.
You can wiggle any oscillator, starting with the BFO and its CW shift,
the mixer crystals, or the PTO. You can inject a bit of current into the
existing shifter control line or add one of your own. Way back when RTTY
was young and based on clanking machinery, it was thought best if the
shifter used a saturated diode rather than a voltage variable capacitor
for better VFO stability whether on RTTY or some other mode with the
modified transmitter. Then you set the shift with a variable capacitor.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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