I also ran RTTY and had a DXer a few houses away (actually
about 750-900 feet...), only mine was 2m FM and he was
2m SSB.
My audio oscillator was a one-transistor oscillator with an
88mh toroid in the collector, and a feedback coil I added
that fed back to the base. A second added winding was
the output to the next stage. A capacitor resonated at
1975, and a polar relay contact added a second cap in
parallel to drop it to 2125.
Anyway, the solution was to switch the added cap
with a FET, which was controlled by a flip-flop, which
was triggered by the output of a comparator chip (the
part number of LM311 comes to mind).
The comparator chip generated a blip at zero crossing.
End result was that the audio tone changed frequency
at zero crossing instead of at random points in the
audio waveform.
Your method sounds more elegant.
Mike WA6ILQ
At 12:18 PM 04/22/08, you wrote:
Back in the '60s when we were still using mechanical teleprinters, I
operated RTTY on the same bands a notable local DXer operated. He
was really upset if any clicks and pops were present to upset his DX
band. Using a polar relay to key the AFSK oscillator frequency
really screwed up the band.
I came up with a 555 circuit running in the astable mode and applied
the keying to the timing resistor in the 555 circuit. I found that
generating the signal at 16 times the output frequency (2125/2975)
would allow shifting the frequency very smoothly and eliminated the
clicks and pops the direct keying generated. I applied the polar
relay contacts directly to the frequency shift resistor in the 555
circuit, and any bounce generated by the relay was absorbed in the
divide by 16 circuit.
A very versital device indeed -
73 - Jim W5ZIT
skipp025 <[EMAIL PROTECTED]> wrote:
The 555 is one of the most useful electronic circuit chips ever
made. It has a fairly large number of possible simply configured
mono & astable (cycle) circuit operations. And it's just as easy
to get lost when trying to use it for specific tasks.
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