On 10/6/2014 12:33 PM, Ralph Mowery wrote:
"rickman" wrote in message
...
Gareth really had me going on this one. I was actually thinking of
building this. But there is only one problem with the idea. To use the
Si570 as the VFO for a receiver it would need to output a sine wave.
However the Si570 outputs square waves for digital circuitry. I don't
think the mixer would appreciate all the harmonics produced in a square
wave would it?


I don't know how it would work at RF, but as a square wave is made up of all
odd harmonics, if you use a low pass filter after it you should get a good
sine wave.

This was done years ago in a teletype tone generator to generate pure sine
waves to modulate a SSB transmitter with a single tone. The filter was made
of coils and capacitors. It may be made more compact now there are many op
amps and other active devices in production.

Just something to think about.

That is fine if your frequency is fixed, but where would you put the
corner frequency for a tunable VFO? Also, it is hard to get enough
attenuation of the second harmonic since it is the closest to the
fundamental and the one most likely to give you trouble in the mixer.

--

Rick