"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