# how to calculate the natural frequency for PLL?

1. ## calculate natural frequency

Hi, guys, If we already have the parameters for passive third-order LPF, how to calculate the natural frequency and damping factor?

we can find the equations to get these for second-order LPF. but I do not know how to get those for third-order passive LPF.

thanks.

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2. ## pll natural frequency

Originally Posted by gavin168
Hi, guys, If we already have the parameters for passive third-order LPF, how to calculate the natural frequency and damping factor?
we can find the equations to get these for second-order LPF. but I do not know how to get those for third-order passive LPF.
1.) Do you know that using a 3rd order LPF the PLL will be of 4th order ?
2.) As a good approximation you certainly can use the formulas for 2nd order PLL´s as all higher order poles must be not dominant for stability reasons.

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3. ## how to calculate natural frequency

I am just curious to know why you would want to use a 3rd order loop filter?Is it because of jitter considerations(since the VCO ripple is reduced?).
As far as I know, all the papers on PLLs have used only a 2nd order LPF with the 3rd pole being far enough so as to not interfere with stability.

Further, may be you can derive the transfer function of PLL by handanalysis, plug in the formula in MATLAB and see if the response(various transfer functions) is as desired.

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4. ## natural frequency pll

I also want to know how to calculate the natural frequency and damping factor
for type II 3rd loop filter ( LPF is 2nd order). Most papers and book get the these
two parameters used 2nd loop filter modeling.
Actually, most PLL use 3nd and 4th order loop filter in industry.

Can anybody help us?

5. ## calculating natural frequency

actually, for ur 2nd order LPF, the second capacitor C2 which parallel to C1 and Rp are very small. for simplity approximation, C2 is << 1/10C1 like A_U_J reply.

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