"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!.






--
Posted via a free Usenet account from http://www.teranews.com

This is really useful. I'd neglected to mention the other 22pF cap in
parallel with the inductor(in the test circuit, to be replaced by a
varicap with 1nF DC block in actual design), and with circuit
parasitics it runs at about 110Mhz. I actually doubled the 22pF cap
in the emitter/ground (two 22pF caps in parallel). So one 22pF was
110Mhz, and two in parallel was over 200Mhz! So theres the quandry.

My main circuit uses one varicap diode, can't use back to back as I
need the whole cap range. Ive realised that just connecting a 10k pot
(in my test circuit) to a 47 series resistor onto the diode (as seen
on several circuits on the web) is a bit dodgy as the effect is not
linear (presumably because the ends of the 10k pot create a lower rf
impedance).

I was also wondering whether my surface mount 100nH inductor has too
low a Q to be stable (49). Might be causing some of my more random
affects.

"bigorangebus" wrote in message
oups.com...
This is really useful. I'd neglected to mention the other 22pF cap in
parallel with the inductor(in the test circuit, to be replaced by a
varicap with 1nF DC block in actual design), and with circuit
parasitics it runs at about 110Mhz. I actually doubled the 22pF cap
in the emitter/ground (two 22pF caps in parallel). So one 22pF was
110Mhz, and two in parallel was over 200Mhz! So theres the quandry.

My main circuit uses one varicap diode, can't use back to back as I
need the whole cap range. Ive realised that just connecting a 10k pot
(in my test circuit) to a 47 series resistor onto the diode (as seen
on several circuits on the web) is a bit dodgy as the effect is not
linear (presumably because the ends of the 10k pot create a lower rf
impedance).

I was also wondering whether my surface mount 100nH inductor has too
low a Q to be stable (49). Might be causing some of my more random
affects.


At 44pF the emitter feedback factor is only about 0.2 and moving into an
area where a Colpitt may fail to oscillate and all bets are off.
Could be the osc' is running into a on/off squegging mode and the counter is
picking up the 2nd harmonic of a grossly distorted waveform. (a speccy would
show this instantly).
The 47k series R is fine. The diode can see as loading a min of 42k and a
max of 52k, which essentially is a zero load on the dynamic resistance of
the tuned circuit, which may only be a few kohm at best. (a variable R
loading of this nature has only trivial effect on an oscillator)
A Q of 49 is perfectly OK to run the Colpitt. In practice, higher Q's will
offer maybe a 10%-20% increase in maximum attainable oscillation frequency.
The 100nH coil I prototyped was 5 turns of thin tinned wire wound round a
pencil. Doesn't sound much but cost nothing and gave a Q of 180!.

(as an isolated side note, the same coil using Gold plated wire gave Q =180
and the same with Silver plated wire Q=200)




--
Posted via a free Usenet account from http://www.teranews.com

hmmm number 1 rule...make sure you're not chasing problems faced by
poor test equipment!

Well got rid of the scope probe and used a direct connection with some
50R coax, suddenly I can see the actual oscillator waveform (was
completely killed by all 10 sets of probes I pulled out of the box,
some are 100Mhz rated). So I now know when its oscillating or not.
Also, the 200 odd MHz reading on the frequency meter was caused by an
impedance mismatch doubling the frequency. With 50R input on the
frequency counter and a suitable output resistor on my oscillator
buffer amp I get the correct frequency.

Also I'd made a silly mistake which was throwing me (causing no low
mixer output)...I connected a 455kHz IF can filter can across the
output of the 612, but the impedance was too low, so it was just
hammering the output. With a suitable 1.5kR ceramic filter I can see
tuned carriers. (My excuse is that I was suckered into it by having
used a similar tuned can on the input of the 612 in the past!)

So...a bit of progress. Still looking out for a good HP spec with
300Hz res band option! Everytime I see a skip I look in
hopefully....

Thanks to everyone who provided their thoughts on this...its helped a
lot