PLL

1. UNIT IV - SPECIAL ICs
Functional block, characteristics &
application circuits with
565-phase lock loop IC

2. Phase Locked Loops
• Is an important building block of linear
system
• Application
• This technique for electronic frequency
control is used today in
– Satellite communication system
– Air borne navigational system
– FM communication systems
– Computers, etc.,

3. Phase Locked Loops - Basic Principle

4. Phase Locked Loops - Basic Principle
• PLL system consists
– Phase detector / Comparator
– A Low Pass Filter
– An error Amplifier
– A Voltage Controlled
Oscillator(VCO)
• VCO is a
• free running multivibrator and
• operates at a set frequency f0 called free running
frequency, depends of capacitance and resistance
(externally)
• It can also be shifted to either side by applying a
dc control voltage VC
• Frequency deviation depends on VC (dc), hence it
is called “Voltage Controlled Oscillator”.

5. Phase Locked Loops - Operation
• If the input signal VS , fS is
applied to PLL.
1) Phase detector (Basically
multiplier) compares phase,
frequency of VS & V0
2) Ve – error voltage is
generated
3) Phase detector produces sum (fS + f0), difference (fS -
f0)
4) (fS + f0) is removed by LPF & (fS - f0) is amplified
and applied as VC to VCO

6. Phase Locked Loops - Operation
5) VC shifts the VCO frequency in tern to reduce the
difference (ie., fS - f0), When this process starts
the signal is in “Capture Range”
6) The VCO change frequency continues till fS = f0,
then this is called “Locked”, Now fS = f0 but
phase difference φ exists
7) Phase difference generates, VC
to shift VCO frequency to maintain Lock
8) If Locked PLL tracks the input signal.

7. Phase Locked Loops - Basic Principle
• Three stages of PLL are,
– Free running
– Capture.
– Locked (or) Tracking
Important Definitions
•Lock-In Range
•The range of frequencies over which the PLL can maintain lock
with input is called lock-in range (or) tracking range.
•Expressed as percentage of f0.
•Capture Range
•The range of frequencies over which the PLL can acquire lock
with input is called capture range.
•Also expressed as percentage of f0.
•Pull-in time
•Total time taken by the PLL to establish lock is called pull-in time.

8. Phase Locked Loops – Analog Phase Detector
• Phase Detector /
Comparator
– Most important part of PLL
– Types are analog and digital
• Analog Phase Detector
• The principle of analog phase
detection using switch type phase
detector is shown in figure
• Electronic switch S is opened and
closed by VCO input (square wave)
• The input signal chopped at a
repetition rate (determined by VCO
frequency)

9. • Figure c). Shows the
input VS is in phase with
V0 , ϕ=00.
• Switch S closed when V0
is +ve, so output Ve will
be half sinusoidal
• For ϕ=900, ϕ=1800
shown in figure (c), (d)
• This type is called half
wave detector
Phase Locked Loops – Analog Phase Detector

10. • The output of LPF gives error signal,
which is average value of output as
shown in dotted line
• From output waveform for perfect
lock the VCO output should be 900
out of phase with respect to the input
signal.
• Closed loop analysis
• Phase comparator is basically a
multiplier, so, output
– K is the gain of the phase comparator (or)
attenuation constant
– φ is the phase shift between input (VS) and
VCO output (V0)
Phase Locked Loops – Analog Phase Detector

11. • At lock fs = f0
• It contains two terms
– Double frequency term
– DC term
– Both are function of φ
• Double frequency term is
eliminated by LPF
Phase Locked Loops – Analog Phase Detector

12. • DC signal is input to VCO, in
perfect locked state
• fS = f0, phase shift 900 (i.e.,
cos900 = 0), to get error signal
ve = 0
• Problem with switch type are
– ve is proportional to
amplitude of supply voltage
– ve is proportional to cosφ (non
linear)
• Both can be eliminated by
limiting the amplitude of input
• (converting constant amplitude
square wave)
Phase Locked Loops – Analog Phase Detector

13. • Figure shows a digital type XOR phase
detector
• The output of XOR gate is high when
only one of the input signals fS (or) f0 is
high.
• This type can be used when both input
are square waves
• At fS = f0 is shown in figure(b), fs is
leading f0 by φ
• DC output with φ is shown in figure(c)
• The slope of curve gives the conversion
ratio Kφ of the phase detector.
– Kφ for VCC = 5V is Kφ = 5/π = 1.59 V/rad
Phase Locked Loops – Digital Phase Detector

14. • Another digital phase detector is
an edge triggered phase detector
• Circuit is an R-S Flip-Flop
mode.
• This circuit is useful when both
input f0, fS should be square
waveform and duty cycle <50%
• this type of detector has better
capture tracking and locking
characteristics as output voltage
is linear up to 3600
Phase Locked Loops – Digital Phase Detector

15. • Common type VCO in IC form
is NE/SE566.
• Timing capacitor CT is linearly
charged (or) discharged by
constant current source / link
• The current can be controlled
by VC applied at pin 5 (or)
changing the timing resistor RT
• The voltage at pin6 is held at
same voltage as pin 5 to reduce
the charging current.
PLL – Voltage Controlled Oscillator (VCO)

16. • The voltage across CT is
applied to the inverter input
terminal
– Schmitt trigger A2 via
buffer amplifier A1
– Output of Schmitt trigger
varies from VCC and 0.5
VCC
• If Ra = Rb voltage at input A2
varies from 0.5 VCC to 0.25
VCC
PLL – Voltage Controlled Oscillator (VCO)

17. • When voltage on CT > 0.5VCC
(during charging) the output
of A2 goes low (0.5 VCC)
• The capacitor now discharges
and when at 0.25 VCC, output
of A2 goes HIGH (VCC)
– Since the source and sink
currents are equal,
– Capacitor charges and
discharges for the same amount
of time
• Triangular wave form across
CT, available at pin 4
PLL – Voltage Controlled Oscillator (VCO)

18. • Output of A2 is inverted by A3
is available in pin3
• The f0 can be calculated as
voltage on C changes from
0.25VCC to 0.5VCC
ΔV = 0.25 VCC
• C charges with constant
current source
• For triangular waveform T =
2Δt
PLL – Voltage Controlled Oscillator (VCO)

19. • But
• VC , control voltage at pin 5
• Output frequency of VCO (f0) can be
changed by
(1). RT, (2). CT, (3). VC at
modulating input pin 5
• VC can be varied by R1, R2 as shown
in figure.
• RT, CT first selected, so that f0 lies in
centre of operating frequency range,
then VC is varies.
PLL – Voltage Controlled Oscillator (VCO)

20. Voltage to Frequency Conversion Factor (KV)
• Important parameter of VCO
– Where, ΔVC is the modulation voltage required to produce
frequency shift Δf0
• Assume original frequency = f0, New frequency = f1, Δf0 =
f1 – f0

21. • LPF not only removes the high
frequency components and
noise, but also controls the
dynamic characteristics of the
PLL.
• Dynamic characteristics
– Capture and lock range
– Band-width
– Transient response
• If BW reduced,
– response time increased.
– but also reduces the capture
range.
PLL – Low Pass Filter(LPF)

22. • The charge on filter C gives a
short time memory to PLL
• If signal < noise for few cycles.
– Vdc on C shift the frequency of
VCO up to pick up
• This produces high noise
immunity and locking
stability.
PLL – Low Pass Filter(LPF)

23. • PLL are available as independent IC
packages and are externally
interconnected to make PLL , IC
PLL 565 is the most commonly used
PLL
Is available in 14pin DIP package and
10pin metal can package
• The output frequency f0 of VCO
(both inputs 2,3 are grounded) is
– Where, RT, CT are external
resistance and capacitance
connected to pin 8, 9
– RT – recommended value is
between 2KΩ to 20KΩ.
Monolithic Phase Locked Loop - IC PLL 565

24. • The VCO free running frequency is
adjusted with RT, CT.
• A short circuit between pin4 and 5
connects VCO output to the phase
comparator.
• A capacitor connected between pin7
& 10 to make LPF with Rinternal
(3.6KΩ)
Monolithic Phase Locked Loop - IC PLL 565

25. • The output from a PLL system can be obtained as
either voltage signal VC(t) (ve at FB),
or frequency signal (VCO output)
• Voltage output used in frequency discriminator
applications
• Frequency output in
– Signal conditioning
– Frequency synthesis
– Clock recovery applications.
PLLApplications

26. • A divide by ‘N’ network is inserted between VCO
output and phase comparator input.
• In locked state f0=Nfs
• Multiplication factor can be obtained by selecting
a proper scaling factor ‘N’ of the counter
PLLApplications - Frequency Multiplication / Division

27. – VCO can directly locked to the n-th harmonic of the
input signal.
– But n should be <10
• The same circuit can be used for frequency
division, since the VCO output is rich in
harmonics, it is possible to lock the mth harmonic
of VCO output with fs, f0 = fs / m.
PLLApplications - Frequency Multiplication / Division
• Frequency multiplication
can also be obtained by PLL
in harmonic locking mode.
• If input is rich in harmonics
example, square wave , pulse
train etc.,

28. • A multiplier / mixer & LPF are connected
externally to the PLL
• Input fs (has to be shifted) & f0 are applied as
input to mixer,
• Output contains sum and difference of fs & f0
– but output of LPF contains only (f0-fs).
PLLApplications - Frequency Translation

29. • The translation (or) offset frequency f1 (f1 <<fs) is
applied to the phase comparator.
• When PLL is in locked state
f0 – fs = f1 → f0 = fs + f1
• We can shift an incoming frequency fs by f1
PLLApplications - Frequency Translation

30. • PLL is locked to the carrier frequency of the incoming AM
signal.
• The VCO output has
– Same frequency as carrier
– Unmodulated (fed to multiplier)
– Always 900 out of phase
• AM input signal also 900 phase shift before multiplier, So
in multiplier both are in same phase
PLLApplications - AM Detection

31. • Output of multiplier contains both the sum and the
difference signals
• High frequency components are filtered by LPF
• Since PLL responds only to carrier frequency which are
very close to VCO output, PLLAM detector exhibits a high
degree of selectivity and noise immunity
PLLApplications - AM Detection

32. • If PLL is locked to a FM signal
– the VCO tracks the instantaneous frequency of
the input signal
• The filtered error voltage
– which controls the VCO and
– maintains lock with input signal is the
demodulated output.
PLLApplications - FM Demodulation

33. • In digital data communication etc,. binary data is
transmitted by means of carrier frequency, this is
shifted between two preset frequency.This type of
data transmission is called FSK technique.
• The binary data can be retrieved using a FSK
demodulator at receiving end.
PLLApplications - Frequency Shift Keying (FSK) Demodulator

34. • The figure shows FSK demodulator using PLL for tele typewriter signals
of 1070Hz and 1270Hz
• When signal appears at input loop
– Locks to the input frequency
– Tracks it between the two frequency
• A three stage filter removes the carrier component and
output signal is made logic compatible by a voltage
comparator.
PLLApplications - Frequency Shift Keying (FSK) Demodulator