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Subject: Re: Effect of Bandwidth on PLL stability

Date: 08/09/01 at 10:10 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Effect of Bandwidth on PLL stability posted by arun sharma on 08/08/01 at 3:24 PM:

Hello, Arun.

I'm not a phase locked loop (PLL) expert by any means, but a PLL is constrained from a stability standpoint like any other feedback system.

Each component of the PLL's feedback path has some particular degree of slowness about it which is going to contribute to the overall frequency response of the entire loop.

Consider for example, the VCO. If the oscillator uses a very high-Q tank circuit, getting the VCO to change its frequency quickly in response to whatever loop imperatives pertain will be difficult. The high-Q circuit will represent a low-frequency lowpass pole in the total feedback path.

If with such a VCO, you try to increase PLL bandwidth elsewhere, such as in the error amplifier, you may find yourself with more than a 6 dB / octave roll-off through zero dB loop gain and the result of that will be loop instability or loop oscillation.

A lower-Q VCO circuit may gain you some additional PLL bandwidth, but then that VCO may not be as spectrally pure as the high-Q one.

A tricky bit of business, wouldn't you say?

Good luck.

John Dunn - President
Ambertec, Inc.

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