The circuit below is the beat frequency oscillator in a superhet receiver. Q1 is a Colpitts oscillator whose output is buffered by Q2. C3 is a trimmer to fine-tune the frequency.
The circuit oscillates at the expected frequency of about 20MHz, but the output amplitude is very unstable and jumps around randomly by at least an order of magnitude, seemingly without provocation. It's also too low: I'm getting about 3-30 mV(rms), and I'd need about 300 mV at least. How can I predict the output amplitude of this oscillator, and what can I do to obtain a more stable and higher amplitude?
Another thing I've noticed is that the oscillator around Q1 produces a relatively pure sine wave if I cut it off at R2, but as soon as I add the amplifier stage, it gets significantly distorted. Can I do something to fix that? And Q2 runs quite hot.
This question is related, but I'm not sure the advice given there applies in this situation:
How to increase the amplitude of a colpitts oscillator circuit?
Some experiments based on KD9PDP's suggestions. I got a bit more consistent amplitudes of about 200mV today, so there could be a problem with some of the earlier measurements. It's possible I didn't always have the correct load on the output (if it wasn't just the weather). Nevertheless, I still need to increase the amplitude by at least 50%.
Increasing C1 has little effect on the output up to at least 570 pF (100+470 parallel). The amplitude changes by no more than about 10%. After adding 2.2 nF in parallel to C1, the oscillation doesn't start.
After increasing R6 to 1k, Q2 goes into cutoff and distorts the negative half wave. However, I can get away with raising R6 to 390\$\Omega\$ before distortion sets in, and then Q2 stays noticeably cooler.
Eliminating R2 also causes cutoff and distortion for all values of R6 I tested.