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Oscillators are interesting beasties. Their power demands are not linear and not resistive. Start/stop phenomena can cause hysteresis and they can exhibit a negative resistance function. In other words, unless you've been careful with their HT feed decoupling, the little beggers can superimpose a low frequency relaxation oscillation on top of their RF oscillation. If you can trigger your scope on the LF component, you'll see the RF component turning on and off. This has long been known about and has the name 'Squegging'. It works the same way as the old neon bulb + capacitor + resistor oscillator, only with the RF oscillator playing the part of the neon.
There are some other possibilities, but this is the one to check first.
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Thanks. Interesting! Squegging looks like "christmas tree" in the spectrum analyzer.
Indeed HT feed decoupling cap was my first prime suspect. I checked it repeatedly and it was fine.
Previously i encountered feedback, "sqealing" of the oscillator when the oscillator coil was physically too close to the mixer coil in a 3-gang FM tuner with cascode amp. I solved the problem by adding a grounded partition wall between the coils. A long time ago in my MW homebrew valve superhet radio, I encountered squegging and motorboating when I built the valve set using PCB strip board without a good RF ground plane. From there onwards, I avoid PCB strip board for my valve radio project completely.
this is a nice demo of the neon relaxation oscillator:
https://youtu.be/ZNYwIEouvxA
Here is a bit more about group delay for filters of different orders:
https://www.microwaves101.com/encycl...lay-in-filters