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Stability Analysis of PLL with Active Loop Filter

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nagamlan

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If I want to put this LF in PLL, how can I do the stability analysis. As there is no dc path from i/p of amp to o/p of amp, the LF(amplifier) wont get its operating points in STB or AC analysis.
 

biff44

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Yes, that integrator stage will go to one of the power supply rails after turn on.

A stability analysis is useless unless you also include the feedback path, i.e. the oscillator Kvco and the divider network 1/N. So a closed loop analysis will include a restorative force to keep the op amp from going to the rails.

However, if you wish to just analyze the open loop circuit as you drew it (or better with the VCO and divider added) , you would need to do so in the frequency domain (i.e. draw a bode plot) using the standard methods of drawing the open loop transfer function 1+GH(S)

https://en.wikipedia.org/wiki/Closed-loop_transfer_function
 

nagamlan

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As in PLL with all practical components we cant run ac analysis, we need to model it. If I use practical amp in active LF, we also need to model amp. As I have seen its impossible to give proper dc voltage to amp. So how to model amp for PLL
 

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