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March 19th 04, 04:44 PM

On 18 Mar 2004 16:01:29 -0800, (Tom Bruhns) wrote:

BTW, I just built the suggested circuit, on one of those old white
plug-boards which is NOT a great idea at RF. It worked fine. Easy to
tune just with a scope. Performance was as I expected: full -0.6V to
+5.6V or so swing at 18MHz at the input of the "18MHz" gate, and an
18MHz rectangular wave at its output. 74HC04 hex inverter; 3 stages
as a simple RC oscillator to generate the 3.7MHz input. Duty cycle of
that 3.7MHz was fairly close to 50%, but definitely not right on. The
performance tells me that the 'HC04 really does have a fairly high
input impedance at 18MHz, and requires under a milliwatt to
drive--perhaps well under. There's AMPLE fifth harmonic power in that
3.7MHz square wave out of an 'HC04, and you just need to filter and do
whatever impedance transformation is appropriate to get the desired
result.

Hi Tom,
Thanks for taking the trouble to cobble together this circuit. I've
done likewise here but am sorry to report that all I'm getting is the
ubiquitous 3rd harmonic on 10.whatever Mhz as usual, regardless of
what the variable cap's tuned to. I made the cap to ground variable as
well, but no dice.:-(. The output looks as if it may well have some
higher frequency content and could be filtered to recover a useable
level of 5th., however. I don't think there's a quick and dirty way to
achieve this - for you perhaps with your longer experience, but sadly
not at this end. Looks like I'm going to just have to swallow hard and
build some proper filtering and use a few more amplifying stages: no
big deal but I just wish I'd known it from the outset as it would have
saved me a lot of time and trouble. I don't think you can get away
with short cuts at RF, especially if you're a non-expert.

--

The BBC: Licensed at public expense to spread lies.