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why injection locked oscillator do not oscillate at the LC tank resonate freq? (Read 266 times)
tulip
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why injection locked oscillator do not oscillate at the LC tank resonate freq?
Apr 26th, 2020, 1:32am
 
A injection locked oscillator can oscillate at the injected signal's frequency, I can understand this.

When the injected signal disapears, the same oscillator will oscillate at the oscillator's LC tank's resonate frequency, since at the LC tank's resonate frequency, the circuit satisfies the oscillation condition.

My question is : why the injection locked osc only oscillate at the injected signal frequency when the injected signal appears. In my opnion, at this condition, there are two frequencies satisfies oscillation criteria, the injected frequency and the LC tank resonation frequency, so the circuit can either oscillate at one of the two frequencies or oscillating with dual frequencies.

Thank you in advance.
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Ken Kundert
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Re: why injection locked oscillator do not oscillate at the LC tank resonate freq?
Reply #1 - Apr 26th, 2020, 11:45am
 
When in injection lock the circuit is not autonomous, so the oscillation criteria need not be satisfied. The injection signal injects energy that makes up for the loss that results from the circuit not operating at its normal oscillation frequency.

-Ken
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tulip
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Re: why injection locked oscillator do not oscillate at the LC tank resonate freq?
Reply #2 - Apr 26th, 2020, 5:24pm
 
Thank you very much.

You explained why injection locked oscillator can oscillate at the injected frequency, but whether the injection signal exists or not, at the LC tank resonation frequeny, the circuit satifies the oscillation criteria, why the circuit do not oscillate at this frequency? why the injection of signal prevents the circuit oscillating at the LC tank resonation frequency?
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Ken Kundert
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Re: why injection locked oscillator do not oscillate at the LC tank resonate freq?
Reply #3 - 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
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tulip
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Re: why injection locked oscillator do not oscillate at the LC tank resonate freq?
Reply #4 - Apr 28th, 2020, 12:08am
 
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.
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Horror Vacui
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Re: why injection locked oscillator do not oscillate at the LC tank resonate freq?
Reply #5 - Jun 1st, 2020, 4:37am
 
There is a well cited (1159 times according to google scholar) article about injection locking:

B. Razavi, "A study of injection locking and pulling in oscillators," in IEEE Journal of Solid-State Circuits, vol. 39, no. 9, pp. 1415-1424, Sept. 2004, doi: 10.1109/JSSC.2004.831608.
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