"bigorangebus" wrote in message
oups.com...
We dont seem to have places where you can get cheap stuff in Britain.
I tried to buy a spectrum analyser from ebay last week, good job I
didnt pay it was a con artist. Problem is with kit like spectrum
analysers is that you need something capable like an HP, otherwise its
can be next to useless. And everyone seems to know the value of an
HP856xrange unit, being $4000+!

I did once go to a ham radio show just outside of london, nothing
cheap there though. Though I will remember two enthusiastic bearded
men who were each looking through boxes of junk at opposite ends of a
stand, they had their head in their boxes and were talking to each
other on radios..saying "bill theres some good stuff in this box,
over", "george..yes Ive found some great stuff in here, over"!

Meanwhile...I have a 612 based colpitts oscillator which on test works
at 110Mhz with 10pf base/emmiter and 22pF to ground on the emmiter.
But curiously jumps up to 210Mhz when I double the 22pF cap to 44pF.
Ive got a frequency counter via a FET buffer showing those
frequencies, and (with the counter turned off) I can verify the
frequency with my portable scanner set close by. So increasing the
capacitance doubles the freq of operation!? I do wonder whether the
spectrum has just spread into harmonics...but i cant find them on my
scanner...and wheres that HP spectrum analyzer..hmm

If anyone out there knows why increasing the emitter ground capacitor
in a colpitts will increase the frequency I would be very happy to be
educated! Given that the freq should be the emitter ground cap in
series with the emmiter base, in parallel with the inductance. Which
is 100nH, with a 1/2PI ROOTLC tuned circuit relationship.

Happy happy happy...



10p-22p-100nH should run 200MHz!. The 44pF cap sounds a bit dodgy for HF
osc operation.
Oscillator caps I use for the (later version) SA602 mixer are usually
5pF-5pF.
A 110MHz-210Mhz design earlier this year used 5pF-5pF-100nH and 2 back to
back SMV1259 varicaps.
And yes, above about 30MHz concrete account has to be taken of the layout
wiring and parasitics. Modelling will get you maybe 70% of the way there,
invisible factors the other 30% .
Most important of all is minimising or allowing for the effect of test gear
and it's connections. From bitter experience I realise it's a skilled
artform that can only be acquired through doing.
And yes, the more test gear the better. Come what may, buy that speccy
analyser!.
If you ever come across designs where the author indicates possession of
only basic test gear then be wary. Very wary!.






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