biff44 said:The main reason you need a loop filter is to keep the PLL stable. In terms of control theory, an oscillator has a transfer function of K/S, which has 90 degrees of phase shift. The return path has an integrator, or a quasi-integrator if passive, that also has a 90 degree phase shift as frequency goes higher. Add them both up and your open loop transfer function GH(s) has a 180 degree phase shift in it at some (or many) frequencies. Since the closed loop transfer function is
G(S)/(1+GH(S)), where the angle of GH(S) is now 180 degrees, you can see that the closed loop transfer function will have infinite gain at the frequency where the magnitude of GH is unity. That is, the PLL is guaranteed to oscillate/be unstable.
That is the only significant reason there is a loop filter. Other things like spur reduction, etc, are a secondary benefit only.
The loop filter is designed so that the angle of GH(S) goes from near 180 degrees back up to something closer to 90 degrees right around the frequency range where GH(S) magnitude is going to unity. This is a requirement for the PLL to meet if it is going to be a stable control loop.
biff44 said:sure sport!
biff44 said:It is a expression of incedulity.
Make yourself a PLL without a loop filter--For instance, a charge pump with a capacitor but no accompanying resistors. Turn it on, make some perturbation, like a phase step in the reference frequency, and tell me what the response is.
biff44 said:Well, if you wish, try to make a pll with a charge pump but without a capacitor. Let me know how it goes!
A charge pump plus capacitor forms a phase detector. A charge pump without a capacitor is useless, as it does not detect phase.
Let me know what other fairytale circuitry you wish to consider.
biff44 said:Well, if you wish, try to make a pll with a charge pump but without a capacitor. Let me know how it goes!
A charge pump plus capacitor forms a phase detector. A charge pump without a capacitor is useless, as it does not detect phase.
Let me know what other fairytale circuitry you wish to consider.
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