Ken Kundert wrote on Apr 26th, 2020, 8:08pm:When the injection signal is small, it shows up as a perturbation of the normal oscillation signal. As the injection becomes larger the behavior becomes somewhat chaotic as the two signal fight each other. Once the injection signal becomes large enough, it dominates over the oscillator's tendency to self oscillate.
To see why this happens, remember that oscillators only exhibit stable oscillations if they satisfy the Barkhausen criterion: that there must be a frequency f0 where the loop gain T is -1. To get robust start up behavior oscillators are designed to have the magnitude of the loop gain being greater than one at the oscillation frequency with the recognition that this will cause the oscillation to grow until compression reduces the effective loop gain to the point where the the magnitude is 1.
In the case of an injection-locked oscillator the circuit responds at the frequency of the injection signal, and the response is large enough for the circuit to go into compression. If the signal is large enough, then the magnitude of the loop gain will always be less than one, which prevents the circuit from oscillating at its natural frequency.
-Ken
Your explanation is very clear, thank you so much.
From your explanation, I think designers should be careful when designing injection locked oscillator. To prevent the oscillator from oscillating at the natural frequency, the injection signal amplitude must be large enough.
Thanks again for your help.