After Joachim's experiments with the RM80 frequencies and MEPT, using the varicap, giving some problems with chirp I decided to try another approach, and we will see what the results are.
In place of the varicap I mounted a trimmer capacitor, in parallel with the R10 (feeding the varicap through I mount a 2.2nF capacitor, so that Q2 creates a RF-short of the trimmer capacitor, giving rise to the lower frequency RX LO of about 3579.350.
Just trying, and as expected the oscillator does not oscillate on the stray capacitance of the circuit without any capacitor in stead of the varicap.
First test was with a 2-10pF trimmer, giving rise to a frequency above 3580 (even with full capacitance), and stopping oscillation at low cap. values. Not good enough.
A 2-18 trimmer with a parallel fixed capacitor of 10pF is better : the (high) frequency range is now about 3579.75 - 3580.10. I noticed that the oscillation on the higher frequency is weaker on the higher frequency by about 10 dB, still the oscillation holds. A lower value coupling capacitor to the buffer, as Joachim indicates, is probably in order.
This makes me think that it should be possible to replace the trimmer with another varicap with lower capacitance than the one supplied., and with a zener stabilized voltage make the tx frequency tunable. A range of about 250 Hz should be attainable with one or two BB105 diodes - and I should have some of those somewhere.
On with the experiments, I will leave the RM80 with the trimmer cap for a while and see if the TX frequency remains stable when transmitting.
The lower value coupling capacitor to the buffer increases the oscillator frequency by 50 - 100 Hz - the oscillator now tunes down to .830 .
Parallel capacitor changed to 22pF, trimmer still 2-18pF, tunes (upper frequency) about .760 - just above 3580.000. nice range.
"The Art of Electronics" #8 -- Why Not a Simple Emitter Follower as The AF Output Circuit? - Click on the image for an easier read *Back when Dean KK4DAS and I were trying to find a suitable AF amplifier circuit for our High School Direct Conversi...
4 hours ago