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# How to predict the locking time of a PLL?

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#### shaq

##### Full Member level 5
Dear all,

How to predict the locking time of a PLL?

Thanks!

The lock time can be used in the design of a PLL - you need to specify the period of time needed to aproach the final frequency with a specified error.
From this, by using a graph, you can determine the damping ratio, ξ coresponding to that lock time. Using the damping ratio you can determine the component values for the filter.

Maybe there are more elaborated methods, but this document explains this method very clear, with an example.

www.freescale.com/files/rf_if/doc/app_note/AN535.pdf
www.circuitsage.com/pll/pll_fundamentals.pdf

lock time of an integer N PLL is approximately given 4/FB ,where FB is the loop bandwidth of the PLL.

AMARNATH

amarnath said:
lock time of an integer N PLL is approximately given 4/FB ,where FB is the loop bandwidth of the PLL.

AMARNATH

In my opinion, the locking time relates the type of lthe loop filter (2nd order, 3rd order).

But, I don't know how to predict the locking time of differential type of the loop filter.

Iam talking about an approx expression for a PLL using a second order loop filter(two caps with a resistor in series with one cap).This is the approx expression you get once you make some approximations. This is good enough for rough calculations. I think for a differential lpf, you need to take care that the opamp poles does not infulence the pole of the LPF, iam not very sure since i have not designed one.

amarnath

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