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Passive or active resistor in on-die Xtal circuit?

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ricklin

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What's the difference between Passive resistor and long-channel transmission gate resistor?
An example, on-die inverter parallel with a long-channel transmission CMOS gate which is 800k ohms typically, and parallel xtal.
The oscillator fail to work some condition and some case, while no problem at all when parallel an off-chip 1Meg ohms resistor.:?:
 

braudelk

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This resistor is used to startup the osillation. Typically it can provide an appropriate operation point for later Gm stage. Maybe you should provide some detail information.
 

ricklin

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braudelk said:
This resistor is used to startup the osillation. Typically it can provide an appropriate operation point for later Gm stage. Maybe you should provide some detail information.

Yeah, the oscillator fail to startup, while there is no difference between DC operating point before and after add 1meg ohms off-chip resistor.
 

rfsystem

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The reason is probably that the MOS resistor is not only a resistor. It is adding some capacitance to the nodes and it has a phase shift of input voltage to output current. So the transconductance is not real. It act more like a distributed RC line.
 

ricklin

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rfsystem said:
The reason is probably that the MOS resistor is not only a resistor. It is adding some capacitance to the nodes and it has a phase shift of input voltage to output current. So the transconductance is not real. It act more like a distributed RC line.

Thanks for your reply.
But still problems, actually, there is no great different between simulation results of xtal startup with active load and with passive 800kohms.
When measure real chip, a spike on xtal out can be observed shortly after power up the chip and before the xtal start to oscillating. And no spike observed if a 1Meg ohms external resistor parallel to the xtal circuit. And some times the circuit fail to startup without the external resistor.
:?::?::?:
 

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