[Collins] Re: {Collins] Colpitts oscillator

[Dr.Gerald Johnson]

If turning the dial higher in frequency actually make the high end, move the dial 
pointer...

Tuned circuit range always is limited (when using a variable capacitor) by the ratio of 
max to min and both are increased by strays. You might be able to decrease the 
min of the variable capacitor by bending any ears that overlap when full open. 
Bend them out. You might be able to increase the max by pushing outer plates 
closer to the stator plates. The slots in many variable capacitor outer plates are for 
just such tracking effects.

There is a chapter in the Radiotron Tube Designer's Handbook covering such 
tracking designs.

The one capacitor has max of 400 so the 12 pf trimmer can have an effect and 
come close to having the same value for both capacitors so the feedback is fairly 
consistent over the wide tuning range. Oscillators do tend to have wildly varying 
output over a wide tuning range because of tuned circuit Q changes and 
feedback changes. A Colpitts with a triode works well with equal capacitors, but 
can work just as fine with a ratio as wild as 1:5. I found out one time that too much 
feedback in an oscillator in an electron coupled circuit where the output was taken 
off the plate could be so high that there was no RF in the plate current. That was 
when converting a command set VFO from triode to 6AG7 electron coupled for 
slightly better stability. Had to move the cathode tap from mid coil to only a turn or 
two from the bottom to have plate current and RF output.

73, Jerry, K0CQ, Technical Advisor to the CRA

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