[Collins] Re: {Collins] Colpitts oscillator

[Dr.Gerald Johnson]

Its not necessarily excess C that keeps the frequency down, it could be 
excess L. Generally when tracking, its important to set the slugs in the coils at 
the LF end of the scale first. Because the variable capacitor is at or near 
maximum, the trimmer C has least effect. Then use the trimmer to set the 
high end. There will be some interaction so you repeat.

You can add a capacitor in series with the variable, but then you sacrifice 
its maximum range rather drastically because to have effect at the HF end 
of the tracking range, the capacitance of the capacitor is small, maybe 20 
pf, so to cut that down significantly you might need a 20 pf series capacitor. 
Then at the 400 pf end you still would have that 20 pf capacitor and the 
effective tuning C would be only 19 pf. So you would have reduced the C 
swing from 20 to 400 pf down to 10 to 19 pf. That won't cover more than a 
1.4:1 frequency range where it did wider before.

It might be necessary to reduce stray C by shortening leads around the 
switch and the active device.

73, Jerry, K0CQ, Technical Advisor to the CRA

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.