On Tue, 7 Oct 2014, Ralph Mowery wrote:


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.

But the problem is, most rigs aren't using that kind of design now.

They are single conversion (with a 9MHz or so IF) or an upconversion to
above 30MHz, so the "VFO" covers a large territory, and since most rigs
are now general coverage (since now it's almost as easy as a hamband only
rig), you need a lot more low pass filters. You're right, if this was in
the old days, with a fixed range VFO, a low pass filter would be fine,
since only one would be needed.

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

One reason Irving Hoff divided that unijunction oscillator output was
because the unijunction put out a tiny pulse, which has a lot more
harmonnc content, dividing it by 2 gave the waveforme a 50% square wave,
which has harmonics, but not at every harmonic, so it's simpler to filter.

There are better ways now. With almost the same amount of division
(especially in this case since the unijunction oscillator frequency
doesn't matter, so long as it's low enough to be stable), you can generate
"stepped sinewaves" with a ripple counter and some weighted resistors,
giving you a cleaner waveform that requires less filtering. That sort of
thing was around not long after that Irving Hoff article, just a few
years.

Michael