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What parameters are needed for an oscillator?

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devop

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I did the AC analys on the oscillator and meatured the phase/gain on VO,the phase shift is only 180 when the oscillator circuit works .
my question is that if we want the circuit osc we need :
1,a 360 degree shift at VO
2, Gain=1 at VO
Where am I wrong?Many thanks
 

Re: oscillator?

i think the gain should larger than 1.
 

Re: oscillator?

use a one more set of M1 & M2 at the output...

with series feedback...
 

oscillator?

I think
1. 180 degree phase shift.
2. gain at 180 degree phase shift should be large than 1.
 

Re: oscillator?

leg1234 said:
I think
1. 180 degree phase shift.
2. gain at 180 degree phase shift should be large than 1.
Click to expand...
you are right ,I made a mistake ,Gain should larger than 1

in the real circuit ,
VO will directly connect to the left side of the rs,
so I think It's a positive feedback system
we need a totally 360 degree shift or 0 degree shift to make it oscillate, where is my mistake?can anyone give me some explain?thanks
 

Re: oscillator?

devop said:
leg1234 said:
I think
1. 180 degree phase shift.
2. gain at 180 degree phase shift should be large than 1.
Click to expand...
you are right ,I made a mistake ,Gain should larger than 1

in the real circuit ,
VO will directly connect to the left side of the rs,
so I think It's a positive feedback system
we need a totally 360 degree shift or 0 degree shift to make it oscillate, where is my mistake?can anyone give me some explain?thanks
Click to expand...
can someone help me?thanks a lot
1,I connect B directly to the point A ,it is a negative feedback or a positive feedback???why
2,I did the AC analysis by the following way : connect the Ac source at A and get the gain and phase shift at B ,am I right?
3,if it's a positive feedback ,the system H should have 360 phase shift at the osc freq?but I got 180.
 

oscillator?

you can try another "open" point and architecture
 

Re: oscillator?

Its negative feedback because you have inverter. It needs 180º shift with gain >1 to oscillate. If it's buffer then you have positive feedback and no ammount of shift will make it oscillate.
 

Re: oscillator?

Sinisa said:
Its negative feedback because you have inverter. It needs 180º shift with gain >1 to oscillate. If it's buffer then you have positive feedback and no ammount of shift will make it oscillate.
Click to expand...
180 degree shift include all the circuit in H or besides the inverter?
 

oscillator?

For linear system consideration, model the inverter as a nagative gain, and model the other blocks in the loop as what it is (transfer function). Make the loop open at the high impendence node (exp: input of the inverter); excite ac signal with bias voltage at input break node, measure the output at the other break node. If the gain at total phase shift at 360 is larger than 1, system may oscillate. On the otherhand, if you try to calculate close loop transfer function from all the noise nodes to the output, if the pole locations are the conjugate unstable poles, then system will oscillates.
 

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    devop

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Re: oscillator?

use orcad 9.2 for this simulation and u will exactly know about your problem...
 

Re: oscillator?

leg1234 said:
For linear system consideration, model the inverter as a nagative gain, and model the other blocks in the loop as what it is (transfer function). Make the loop open at the high impendence node (exp: input of the inverter); excite ac signal with bias voltage at input break node, measure the output at the other break node. If the gain at total phase shift at 360 is larger than 1, system may oscillate. On the otherhand, if you try to calculate close loop transfer function from all the noise nodes to the output, if the pole locations are the conjugate unstable poles, then system will oscillates.
Click to expand...
thanks all
leg1234,you helped me,It's just what I want to know,
and there is one more question:I did the simulation and there are two points that the phase shift is 360,and both Gain >1, how can I know at which Frequency the system will oscillate?
 

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