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Subject: Re: Effect of Bandwidth on PLL stability

Date: 08/15/01 at 11:20 AM
Posted by: Ron Harrison
E-mail: bogus@bogus.net
Message Posted:

In Reply to: Re: Effect of Bandwidth on PLL stability posted by John Dunn - Consultant on 08/09/01 at 10:10 PM:

John, I know this is counter-intuitive, but the resonant frequency of a high-Q tank circuit CAN be changed "instantaneously". If you change the capacitance or inductance of the resonant circuit abruptly, the resonant frequency will change abruptly.A good mechanical analogy is a pendulum. Imagine a huge pendulum which has such a low period that you can lengthen it in a short time relative to its period. The pendulum may have very high Q, but its period will still change abruptly. Electrical oscillators work the same way.You are correct about loop stability being a standard control system design problem. If the OP is concerned about the effect of input phase noise on the output, that will certainly be improved by reducing loop bandwidth, unless he is using a low-Q VCO and has a big frequency divider in the feedback loop.Ron

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