[SI-LIST] Re: Questions on PLL response
- From: Geoff Stokes <gstokes@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 5 Feb 2004 11:40:47 -0000
Dear Partha
Answering question 2 first, the PLL bandwidth depends on the loop gain
parameters and their base band frequency responses. It's all described in
F.M. Gardner "Phaselock techniques".
The optimal practicable damping factor will give the fastest response time
with minimal overshoot and ideally no ringing, so that the settling time
window is minimised for which you can expect phase disturbance and any
associated timing uncertainty. For a second order loop, a damping factor
zeta=1 gives the fastest settling time with no overshoot. Zeta=0.707 gives
the fastest settling time with a single overshoot, and slightly faster than
for zeta=1. In addition, the 0.707 value gives a second order Butterworth
type filter response to any input phase disturbances. If the response is
underdamped, that settling time window is lengthened. If the response is
overdamped, the settling time is also lengthened, but also the stop-band
phase-filter rejection is reduced. In practice, because of VCO
characteristic non-linearity and other uncertainties, the damping factor
cannot be controlled precisely. If you can get the value between about 0.5
and 2 you're doing quite well. I would aim for a value of around 1 because
you have less margin for error on the low side.
Cheers
Geoff
> -----Original Message-----
> From: Parthasarathy Sampath [mailto:parthsv@xxxxxxxxx]
> Sent: 05 February 2004 10:17
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Questions on PLL response
>
>
> Hi All,
> 1. The PLL response time depends on 'damping
> factor'. The common value of it used is '0.707', which
> makes the system underdamped since for damping factor
> < 1, the system is underdamped.
> My question is why not make the system overdamped
> since the response will be smooth and straight?
>
> 2. What are the parameters on which PLL bandwidth
> depend?
>
> Thanks in Advance,
>
> Regards,
> Partha!
>
>
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