On 10/6/2014 1:31 PM, Ralph Mowery wrote:
"rickman" wrote in message
...
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.


There will not be a second harmonic for a square wave, just the odd number
such as the 3,5,7 and so on.
That was one of the reasons for generating the tones and passing them
through a circuit that converted them into good square waves and then to
the low pass filter.

You are right, no even harmonics in a square wave. What circuit clips a
tone into a square wave just so it could be run through a low pass filter?


I don't recall if any frequency range of the VFO was mentioned. So if going
from a range of 5 to 8 MHz the corner frequency could start around 9 MHZ
and should be dropping off alot at 15 MHz where the 3 rd harmoinc of the
starting point of 5 MHz would be.

So is this a viable practice to shape a square wave with a filter to use
in a mixer? I just find it hard to imagine that the harmonics would not
create some real problems. I'd have to do the math, but I expect even a
third harmonic has potential of creating a lot of spurs. I guess I'm
used to dealing with people who want very selective receivers. But I
have to admit I have forgotten a lot of the little bit of receiver
design I did learn.

--

Rick