Applications Information

In designing with phase locked loops such as the LM565, the

important parameters of interest are:

FREE RUNNING FREQUENCY

LOOP GAIN: relates the amount of phase change between

the input signal and the VCO signal for a shift in input signal

frequency (assuming the loop remains in lock). In servo

theory, this is called the “velocity error coefficient.”

The loop gain of the LM565 is dependent on supply voltage,

and may be found from:

f

V

Loop gain may be reduced by connecting a resistor between

pins 6 and 7; this reduces the load impedance on the output

amplifier and hence the loop gain.

HOLD IN RANGE: the range of frequencies that the loop will

remain in lock after initially being locked.

f

V

THE LOOP FILTER

In almost all applications, it will be desirable to filter the sig-

nal at the output of the phase detector (pin 7); this filter may

take one of two forms:

A simple lag filter may be used for wide closed loop band-

width applications such as modulation following where the

frequency deviation of the carrier is fairly high (greater than

10%), or where wideband modulating signals must be fol-

lowed.

The natural bandwidth of the closed loop response may be

found from:

Associated with this is a damping factor:

For narrow band applications where a narrow noise band-

width is desired, such as applications involving tracking a

slowly varying carrier, a lead lag filter should be used. In gen-

eral, if 1/R

comes quite small resulting in large overshoot and possible

instability in the transient response of the loop. In this case,

the natural frequency of the loop may be found from

R

between 0.5 and 1.0. The damping factor is found from the

approximation:

δ ) πτ

These two equations are plotted for convenience.

Simple Lead Filter

DS007853-11

Lag-Lead Filter

DS007853-12

Filter Time Constant vs Natural Frequency

DS007853-13

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