I am conducting ongoing experiments with regenerative radio
receivers. One problem I have repeatedly encountered is low frequency
squigging/motorboating. Particularly when using somewhat higher CE
transistor currents. Oddly, changing the values of any of the
capacitors in the circuit did not change the frequency of the
squigging. There was nothing wrong with the decoupling and even
adding additional decoupling capacitors had no effect. Almost
certainly the unwanted low frequency oscillations are due to
electrothermal effects.
Directing hot air from a hairdryer at the transistors for less that a
second stopped the low frequency oscillation. After stopping the
hairdryer the circuit started squigging again after a couple of
seconds.
The fact that regenerative receivers are operated with the amount of
positive feedback (regeneration) so close to the threshold of
oscillation makes the extremely sensitive to the gain of the
sustaining amplifier.
Even slight changes in the gain of the transistors (caused by self-
heating or the environment) can then have a big effect. This is
almost certainly the basis of the low frequency squigging in the
circuit.