Page 336 - Analog and Digital Filter Design

Filters for Phase-LocKed LOOPS 333

The capture range is defined as the frequency offset from the VCO’s center
frequency over which a phase-locked loop can lock onto a signal. This range
is determined by applying a signal having a frequency outside this range. then
altering the frequency until lock is obtained. This range is most important for
frequency synthesizer applications, because if the loop is unable to lock onto
an input signal, it cannot work. In FM demodulator applications the center
frequency of the input signal may not be the same as the VCO’s center fre-
quency; any offset must be within the capture range.

The lock range is defined as the frequency offset from the VCO’s center fre-
quency where lock is no longer possible. This range is determined by applying
a signal having a frequency that is within the capture range, and locked, and
then altering the frequency until lock is lost. The lock range is equal to, or
greater than, the capture range. In frequency synthesizer applications this is not
very important because the input signal does not normally change and, in any
case. input signals should be within the capture range. The lock range is very
important in FM demodulation systems because the input signal frequency is
being changed by the modulation. If the FM signal deviates beyond the lock
range. the loop will lose lock. The signal will be captured again as the signal fre-
quency returns to nearer the center frequency, but there will be an audible click
at the radio receiver’s output.

The lock range of a phase-locked loop depends on the device used. In the case
of a simple phase detector the lock range is given by K@. KO in rads. In the case
of a frequency and phase detector, the lock range is determined by the maximum
and minimum oscillator frequency. A frequency and phase detector is different
from a simple phase detector because the frequency and phase detector output
is oiily limited by the power supply voltage. If the frequencies of the two signals
are different, the frequency and phase detector output voltage will be set at one
of the power rails. The VCQ control range is less than the power supply voltage.
In the example, the power supply had a 5V rail but the VCQ input voltage range
was 0.9 V to 4.1 V.

The capture range depends on the loop filter, unless a frequency and phase detec-
tor is being used, in which case it is equal to the lock range. A simple RC filter
has a capture range given by:

The lead-lag network capture range is generally wider, and is given by: