boy said:
I have a comment on openloop. I dont think i will work with any series feedback scheme since we cannnot break the loop in that case. The example is, the oscilator with an inductor connect to gate and gnd. we use this inductor as a feed back element in order to make the device unstable. but this kind of feedback is series. I dont think we can use openloop to analy this scheme. any comment?
dear nandopg, boy and all
that's what i meant in my previous post. i think there isn't anyway we can find the open loop characteristic, as the feedback was cause by the transistor internal component and parasitic. the only way we can analyse this type of circuit is by negative resistance analysis.
to boy, having external feedback for 9GHz oscillator was a pain, unless u are designing a mmic oscillator where the lumped component can be realised.
for negative resistance analysis, as was mentioned by nandopg
>In spite of the negative resistance approach to design and analyze oscillators is popular, that is not the best one. This approach fails....
under what circumstances does the negative resistance approach fail?
>....to guarantee the start up conditions. A negative resistance at the input terminal doesn't means your oscillator will necessarily start up. You need to analyze S21 in open loop instead.
why a negative resitance at the input terminal doesn't mean the oscillator will start up? isn't the startup can trigger by surrounding noise?
>...to predict the loaded Q of the resonator used and so to estimate the phase noise. The loaded Q is readily available analyzing the group delay of S21 in open loop.
how to find loaded Q from group delay of S21?
>If you try to use the open loop approach instead of the negative resistance approach you will get an oscillator with a reasonable agreement with your simulations.
what simulation to u use to compare with ur measured result? transient or harmonic balance?
thanks for the help
cray