The oscillator obviously works at a critical point and Quality Factor of the inductor is not sufficiently high. Or the oscillator has a second parasitic oscillation point ( probably near to principal one ) and the oscillator "va-vient" between two points.
I think that the input impedance of the buffer is changing as the oscillator passes through some critical level or frequency. It could be that its trying to oscillate with the 100microH choke. Try a "base stopper", say 100 ohms in series and as close to the buffers base as possible. or put a resistor in series with the emitter bypass capacitor (50 ohms).
Frank
I think that the input impedance of the buffer is changing as the oscillator passes through some critical level or frequency. It could be that its trying to oscillate with the 100microH choke. Try a "base stopper", say 100 ohms in series and as close to the buffers base as possible. or put a resistor in series with the emitter bypass capacitor (50 ohms).
Frank
As a coil in a can does not show this effect, could it be that the test coil is coupling with the choke?
Frank
That's a good idea, try turning the audio up from your receiver, if its microphony , it should get worse. Try wearing phones, that should cure it.
Frank
Possibly it's "squegging". That is when the oscillations build up very rapidly and there is gate rectification, driving it negative enough to stop oscillations. The gate resistor now discharges the capacitors until it oscillates again and the cycle repeats. Try reducing the gate resistor, the supply voltage or both. The output buffer will have enough gain to compensate the lower amplitude.
If it is squegging then the you need to either limit the RF on the gate or reduce the gain of the FET. Now the pot reduces the Id of the FET, so its gain should fall but this is also altering the capacitive tap within the oscillator circuit as the input impedance to the source increases as the FET Id reduces. I wonder if putting a low value resistor in series with the source to capacitors junction would swamp this effect, though the Id would have to be higher to make up the gain. Or failing that, then a pair of back to back fast diodes connected from the gate to the source would limit the RF to about .7V peak.
Frank
Squegging is caused when the amplitude of the oscillations are enough to cut the device off completely. After some time when the capacitor storing the negative voltage discharges, the device starts to conduct and oscillations start again.
The oscillator circuit is a PI network with an inductor in the series arm. At one end there is a capacitor to RF earth feeding the gate. At the other end there is a capacitor to RF earth feeding the source, whose input impedance is 1/gm. As the current through the FET increase the gm rises, so the input impedance falls and the tuned circuit gets more damped. The voltage transformation across the PI network is the ratio of the two terminating capacitors.
As the DC current through the FET increases, more negative voltage is required on the gate to cut the FET off.
Frank
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