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Question Related to VCO

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shaikhsarfraz

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Hi,
Why the input resistance of the VCO should be ideally infinite and the input capacitance of VCO ideally should be zero.

Thatnks in advance

Sarfraz
 

the question is unclear. I wonder how VCO can have an input. it would be helpful if you post a reference to your question.
 

I guess, that's to let the maximum amount of control voltage (if this is what u mean by input of VCO) transferred to it from the control line, and happen quickly without cap charge delay.
 

shaikhsarfraz said:
Hi,
Why the input resistance of the VCO should be ideally infinite and the input capacitance of VCO ideally should be zero.

Thatnks in advance

Sarfraz


i think bcoz VCO is voltage control and definitely it need high input impedance to make sure the voltage transfer fully from the previous stage to VCO.
 

to prevent leakage current from the PLL's filter
 

VCO is for "voltage controlled oscillator"
Charge is transferred from charge-pump into loop filter after integration, to form voltage to VCO. If your VCO input is not having infinite input impedance, charge will definitely leak and VCO input range will be degraded. Or put in another word, when the charge pump delivers current to loop filter (charge pump has finite output impedance Ro), if it sees another impedance from VCO, they will form a voltage divider which limits the VCO input voltage range.
Output cap is always need to small enough for having wider and higher operating frequency range as more cap, lower frequency can oscillate
 

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