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Design >> Analog Design >> question about crystal oscillator
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Message started by hyy95 on Mar 17^th, 2009, 7:25pm

Title: question about crystal oscillator
Post by hyy95 on Mar 17^th, 2009, 7:25pm

Hi,
I have some questions about the feedback resistor in the pierce oscillator setting:
why can it force the transistors into active region?
why it's value should be high?
how does a low value Rf degrade the frequency stability?
how does a low value Rf increase the current?
how can we find the best value of Rf?

Thanks

Title: Re: question about crystal oscillator
Post by pancho_hideboo on Mar 17^th, 2009, 9:10pm

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

why can it force the transistors into active region?

Since it forces input and output to same voltage.

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

why it's value should be high?

Since it is negative feedback resistor.

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how does a low value Rf degrade the frequency stability?

Rather if Rf is too small, oscillation will not start up.

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how does a low value Rf increase the current?

It Rf is too small enough not to start up oscillation, static DC current is consumed at inverter where both NMOS and PMOS are on state.

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how can we find the best value of Rf?

It depends on oscillation margin and drive current for crystal.

Title: Re: question about crystal oscillator
Post by hyy95 on Mar 17^th, 2009, 9:25pm

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

why it's value should be high?
Since it is negative feedback resistor.

I don't get this one, can you explain more details? Thanks!

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how does a low value Rf degrade the frequency stability?
Rather if Rf is too small, oscillation will not start up.

Why won't the oscillation start up, can you explain in details? Please!

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how can we find the best value of Rf?

It depends on oscillation margin and drive current for crystal.
[/quote]
can you share more details on this one? How do you derive the Rf value based on oscillation margin and drive current for crystal. Or can you point me to a paper that discuss this in details? Thanks!

Title: Re: question about crystal oscillator
Post by pancho_hideboo on Mar 17^th, 2009, 10:12pm

hyy95 wrote on Mar 17^th, 2009, 9:25pm:

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

why it's value should be high?
Since it is negative feedback resistor.

I don't get this one, can you explain more details? Thanks!

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how does a low value Rf degrade the frequency stability?
Rather if Rf is too small, oscillation will not start up.

Why won't the oscillation start up, can you explain in details? Please!

hyy95 wrote on Mar 17^th, 2009, 7:25pm:

how can we find the best value of Rf?

It depends on oscillation margin and drive current for crystal.
can you share more details on this one? How do you derive the Rf value based on oscillation margin and drive current for crystal. Or can you point me to a paper that discuss this in details? Thanks!

These are too basic issues. So read circuit theory text book for beginner.

I'm not kind and patient enough for explaining such too basic issues.
Maybe someone who is very kind and patient will help you.