PLL Bandwidth definition and calculation?

gavin114

1,
. B& t  j' e, K4 iWhen designing PLL, it is always said that PLL's bandwidth should be set to be smaller than Fref/10. what's the PLL bandwidth? Is it Wc(open loop unit gain bandwidth), Wn( Natural frequency) or W-3db ( closed loop -3db frequency)?
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, \9 N& e% i0 s+ w6 m9 P  kand I know Wc means the frequency where gain=1. W-3db means the frequency where gain decrease 3db. what does natural frequency mean?

3, If all the parameters are know, like Ich, Kvco, R,C1,C2. How to get Wc, Wn,
W-3db, Phase Margin, Damping factor. can anyone give the formulars ? I found some thing for internet:
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" V$ e9 q, j6 E; h8 X- NNatural Freq: Wn=sqrt[(Kvco*Icp)/(2*pi*Cbig*N)]
* }/ H7 j" F; X& K! HDamping Factor: zeta=(Rlpf/2)*sqrt[(Icp*Cbig*Kvco/2*pi*N)]
Open loop bandwidh Freq: w0=(Icp*Kvco*Rlpf)/2*pi*N
3dB Bandwidth: w3dB=wn*sqrt(1+2*zeta^2+sqrt(2+4*zeta^2+4*zeta^4))

* b5 P( Z$ U" p9 f9 t" h# hAre these equations are correct?
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Regards,

gavin114

Hi, renzhq,
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; F+ G# f" n, E$ ?3 ^# KI don't think Wc=W-3db?
Assume forward transfer is H(s), Closed loop transfer is Close(s), feedback divider ratio is N.
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) G! Y- E  c. e. j/ u/ ESo Closed(s)=H(s)/(1+H(s)/N),  W-3db is the root when 1+H(s)/N=0, and Wc is the root when |H(s)/N/=1. Are their roots same?

: g8 X% b+ I4 i2 D1 i5 s' UThank you?

renzhq

it is Wc, if we make an approximate，it equals W-3db