Phase Locked Loops

1. Unit 6:
Phase-lockedLoops
(PLLs)

2. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Voltage-Controlled Oscillator
The oscillator output is a variable frequency square wave or triangular
wave. The output frequency depends on the modulation input voltage (VC).

3. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
566 Voltage-Controlled Oscillator
The output frequency can
be calculated as shown in
the graph.
Note that the formula also
indicates other circuit
parameters that affect the
output frequency.

4. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Phase-Locked Loop
The input signal is a
frequency and the
output signal is a
voltage representing
the difference in
frequency between the
input and the internal
VCO.

5. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Operating Modes of the
Phase-Locked Loop
Three operating modes:
Lock
fi = fVCO
Tracking
fi  fVCO, but the fVCO adjusts until fVCO= fi
Out-of-Lock
fi  fVCO, and they never will be the same

6. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Phase-Locked Loop: Lock Mode
The input frequency and the
internal VCO output
frequency are applied to the
phase comparator.
If they are the same, the
phase comparator output
voltage indicates no error.
This no-error voltage is
filtered and amplified before it
is made available to the
output.
The no-error voltage is also applied to the internal VCO input to maintain the
VCO output frequency.

7. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Phase-Locked Loop: Tracking Mode
If the input frequency does not
equal the VCO frequency then
the phase comparator outputs an
error voltage.
This error voltage is filtered and
amplified and made available to
the output.
The error voltage is also applied
to the VCO input. This causes
the VCO to change output
frequency.
This looping continues until the VCO has adjusted to the new input frequency
and they are equal again.

8. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Phase-Locked Loop: Out-of-Lock Mode
When the input frequency
does not equal the VCO
frequency and the resulting
error voltage does not cause
the VCO to catch up to the
input frequency, the system
is out of lock.

9. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Phase-Locked Loop Frequency Ranges
Lock Range—The range of input frequencies
for which the VCO will track.
Capture Range —A narrow range of
frequencies into which the input frequency must
fall before the VCO can track. If the input
frequency falls out of the lock range it must first
enter into the capture range.

10. Electronic Devices and Circuit Theory
Boylestad
© 2013 by Pearson Higher Education, Inc
Upper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.13 Summary
Phase-Locked Loop
Applications
FM demodulator
Frequency Synthesizer
FSK decoder

11. Introduction
Basic Operation
Applications
2
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Outline

12. Introduction 1
• A phase-locked loop (PLL) is an electronic circuit that consists of a phase
detector, a low-pass filter, and a voltage-controlled oscillator connected as
shown.
• The closed-loop operation of the circuit is to maintain the VCO
frequency locked to that of the input signal frequency.

13. Introduction 2
• Common applications of a PLL include:
• Frequency synthesizers that provide multiples of a reference signal
frequency.
• FM demodulation networks for FM operation with excellent linearity
between the input signal frequency and the PLL output voltage.
• Demodulation of the two data transmission or carrier frequencies in
digital-data transmission used in frequency-shift keying (FSK) operation.
• wide variety of areas including modems, telemetry receivers and
transmitters, tone decoders, AM detectors, and tracking filters.

14. Basic Operation 1
• Capture and Lock operation:
• Within a capture-and-lock frequency range, the dc voltage will
drive the VCO frequency to match that of the input.
• While the loop is trying to achieve lock, the output of the phase
comparator contains frequency components at the sum and
difference of the signals compared.
• A low-pass filter passes only the lower frequency component of
the signal, so that the loop can obtain lock between input and
VCO signals.

15. Basic Operation 2
• lock operation:
• input signal frequency is the same as that from the VCO .
• Best operation is obtained if the VCO center frequency fo is set
with the dc bias voltage midway in its linear operating range.
• The amplifier allows this adjustment in dc voltage from that
obtained as output of the filter circuit.
• When the loop is in lock, the two signals to the comparator are
of the same frequency, although not necessarily in phase.
• A fixed phase difference between the two signals to the
comparator results in a fixed dc voltage to the VCO.
• Changes in the input signal frequency then result in change in
the dc voltage to the VCO.

16. Basic Operation 3
• Owing to the limited operating range of the VCO and the feedback
connection of the PLL circuit, there are two important frequency
bands specified for a PLL.
• The capture range of a PLL is the frequency range centered about
the VCO free-running frequency fo over which the loop can acquire
lock with the input signal.
• Once the PLL has achieved capture, it can maintain lock with the
input signal over a somewhat wider frequency range called the lock
range.

17. Applications:
FrequencyDemodulation 1
• The PLL center frequency is selected
or designed at the FM carrier
frequency.
• The filtered or output voltage is the
desired demodulated voltage,
varying in value in proportion to the
variation of the signal frequency.

18. Applications:
FrequencyDemodulation 2
• R 1&C1  set the
center frequency of
the VCO.
• C2  sets the LPF
passband
• Lock range:
• Capture range:

19. Applications:
FrequencySynthesis 1
• A frequency divider is inserted between the VCO output and the phase
comparator so that the loop signal to the comparator is at frequency fo and
the VCO output is Nfo.
• This output is a multiple of the input frequency as long as the loop is in lock.

20. Applications:
FrequencySynthesis 2
• example using a 565 PLL as frequency multiplier and a 7490 as
divider.

21. Applications:
FSK Decoders
• The decoder receives a signal at
one of two distinct carrier
frequencies, 1270 Hz or 1070 Hz,
representing the RS-232C logic
levels or mark (-5 V) or space
(+14 V), respectively.
• As the signal appears at the input,
the loop locks to the input
frequency and tracks it between
two possible frequencies with a
corresponding dc shift at the
output.
• The RC ladder filter
is used to remove
the sum-frequency
component.

22. • For more details, refer to:
• Chapter 13, Boylestad, Electronic Devices and Circuits, 11th
edition.
22