Applications Information
In designing with phase locked loops such as the LM565, the
important parameters of interest are:
FREE RUNNING FREQUENCY
Lag-Lead Filter
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.”
DS007853-12
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:
The loop gain of the LM565 is dependent on supply voltage,
and may be found from:
Associated with this is a damping factor:
f
o
= VCO frequency in Hz
V
c
= total supply voltage to circuit
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.
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
1
C
1
<
K
o
K
D
, the damping factor for the loop be-
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
f
o
= free running frequency of VCO
V
c
= total supply voltage to the circuit
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:
Simple Lead Filter
R
2
is selected to produce a desired damping factor
δ,
usually
between 0.5 and 1.0. The damping factor is found from the
approximation:
δ
)
π τ
2
f
n
These two equations are plotted for convenience.
Filter Time Constant vs Natural Frequency
DS007853-11
DS007853-13
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