On 10/7/2014 10:53 AM, Ralph Mowery wrote:
"rickman" wrote in message
...
On 10/6/2014 1:31 PM, Ralph Mowery wrote:
.

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 thought we were going to start with a square wave. Nothing shapes the
square wave. You just run it through a low pass filter just above the
fundimental or 1st harmonic if you want to call it that..

The circuit I am thinking about is from an old RTTY audio tone generator I
built and was designed by Irv Hoff years ago, around 1970 or so.

He used a unijunction to generate a tone of 2125 or 2295 Hz (rtty tones)
then fed it into a 2 transistor multivibrator to generate good square waves.
Then into a low pass filter made of two coils and I think 2 or 3 capacitors.
This filtered out all the odd harmonics ( as there are no even harmonics in
the square wave) and just leaves the fundimental frequency as a pure sine
wave.

That way you could have a VFO going from 5 to 6 MHz and set the filter for
about 8 MHz and anything above that would be filtered out. No harmonics or
anything but a pure sine wave.

As I said, not sure if this would work at RF or not, just something to
think about.

You say the filter removes "all" of the harmonics... that is obviously
not correct. The filter may reduce them, but it does not and can not
completely remove them. The nearest tones (which are also the largest
amplitude tones) will only be reduced a small amount really. Or maybe
you are planning to use a brick wall filter?

My question intended to ask *WHY* would anyone design a circuit to
produce a square wave and then spend the time and trouble to filter it?
Earlier you mention that filters are easier now, but in reality analog
filters are still much more difficult than just generating a tone in the
first place.

I can use a single chip and a DAC to produce tones up to many MHz with
very high accuracy and purity.

I don't get it...

--

Rick