Phase locked loop simulations
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PHASE LOCKED LOOP SIMULATIONS. By, R.Vikram Reddy(0104445). Talk Outline. History Introduction PLL Basics PLL Types Loop Components -Phase Detectors -Voltage controlled Oscillators -Loop Filters Applications.

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R.Vikram Reddy(0104445)

Talk Outline



PLL Basics

PLL Types

Loop Components

-Phase Detectors

-Voltage controlled Oscillators

-Loop Filters



  • Coincides with invention of “coherent communication” (DeBellescize, 1932).

  • The earliest widespread use of PLLs was to the horizontal and vertical sweeps used in television, where a continuous clocking signal had to be synchronized with a periodic synch pulse.

  • PLLs were critical to development of color television

  • The first PLL IC arrived around 1965. This created an explosion in the use of PLLs.

PLLs Today

– PLLs in every cell phone, television, radio, pager, computer, all telephony, ...

– The most prolific feedback system built by engineers.

– At low end: all software PLLs implement entire PLL functionality on sampled data.

– At high end: optical PLLs used in clock recovery for 160 Gbps data (OFC 2002).

PLL Basics

  • Definition:

    A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal.

Basic idea of a Phase Locked Loop

Components Of PLL

  • Phase Detector (PD):A nonlinear device whose output contains the phase difference between the two oscillating input signals.

  • Voltage controlled oscillator (VCO): Another nonlinear device which produces oscillations whose frequency is controlled by a lower frequency input voltage.

  • Loop filter

  • General sinusoid at reference input can be written as:


  • Assume VCO output signal is

    Vosc=Eoscsin(ωt- Ød+90)

    =Eosccos(ωot - Ød)……(2)

  • Phase Detector output:


    where Km is the multiplication constant

  • Vpd=KmEinEoscsin(ωt)cos(ωt-Ød)

  • Using the familiar trigonometric identity in terms of the PLL:

    Vpd=0.5KmEinEosc[sin(Ød) + sin(2ωt-Ød)]

  • The output of the low pass filter :


    For small Ød


How A PLL Works:

Capture Range:

  • The maximum difference between the input signals frequency and oscillators free running frequency where lock can eventually be attained is defined as the capture range.

  • The Phenomenon of “Beating”.

Lock Range:

  • The maximum frequency excursions over which the output remains locked with the input is called the lock range.

  • The maximum output of the low pass filter is given by

    Vcntl-max=0.5 KlpKmEinEosc

    when Ød=90.

    Therefore lock range is given by


Types Of PLLs

  • Analog or Linear PLL (LPLL)

    • Digital PLL (DPLL)

    • All digital PLL (ADPLL)


  • The LPLL (Best) or analog PLL is the classical form of PLL. All components in the LPLL operate in the continuous-time domain.


  • The phase detector is typically some form of analog multiplier.

  • The phase error function is of the form

    φ(t) = KmK1Asin [θ(t) − θ^(t)]

    ≈ KD[θ(t) − θ^(t)]

  • The loop filter may be active or passive, but it typically results in the loop being either first-order or second-order.

  • The design/analysis of the loop filter makes use of the Laplace transform.


The digital PLL is just an analog PLL with a digital phase detector.

  • The DPLL is a hybrid system

  • The DPLL is very popular in synthesizer


In the below figure the optional digital divider, and variations on it, are used in frequency synthesis applications.

  • Popular types of digital phase detectors include:

    – Exclusive or gate (EXOR)

    – Edge-triggered JK-flip flop

    – Phase frequency detector (PFD)


  • The all-digital PLL (classical all-digital) is distinctly different from the other two PLLs

  • The ADPLL is a digital loop in two senses:

    – All digital components

    – All digital (discrete-time) signals

  • There are many ADPLL building blocks, and many variations on putting them together.


  • The VCO is replaced by a Digitally Controlled Oscillator (DCO) or also called a Numerically Controlled Oscillator (NCO)


Phase Detector

  • A phase detector is a circuit that normally has an output voltage with an average value proportional to the phase difference between the input signal and output of VCO.


  • Phase detectors can be a simple EX-OR gate, a sample and hold, an analog multiplier or a combination of D-flip flops.


  • The simplest phase detector is xor

  • An exclusive OR gate gives a high output when the signals are of opposite sign and a low output when they are of the same sign.

Phase Frequency Detector

  • It is a combination of tri state phase frequency detector and a charge pump.

Sample and Hold Phase Detector

  • A sample and hold circuit samples an input signal and holds on to its last sampled value until the input is sampled again.


Design Circuit:XOR (TTL Logic) gate:


VCC 4 0 5V

VINA 1 0 PULSE(0V 5V 0US 0.1US 0.1US 2US 5US)

VINB 9 0 PULSE(0V 5V 0US 0.1US 0.1US 3US 5US)

RBA 4 3 4K

RBB 4 8 4K

RCSA 4 5 1.9K

RCSB 4 11 1.9K

RSDA 7 0 1.2K

RSDB 12 0 1.2K

RCX 4 14 3K

RC 4 15 1.6K

RCP 4 16 120

RX 18 0 1K





DX 17 19 DIODE

Q1A 2 3 1 QM

Q1B 10 8 9 QM

QS2A 5 2 7 QM

QS2B 11 10 12 QM

QSDA 6 7 0 QM

QSDB 13 12 0 QM

QX1 14 13 6 QM

QX2 14 6 13 QM

QS 15 14 18 QM

QP 16 15 17 QM

QO 19 18 0 QM


.MODEL QM NPN (IS=1E-14 BF=50 BR=1 RB=70 RC=4 TF=0.1NS TR=1NS)

.TRAN 0.1US 15US

.PLOT V(1) V(9) V(19)


Voltage Controlled Oscillators (VCOs)

  • The function of a VCO is to generate a stable and periodic waveform whose frequency can be varied with the applied control voltage.

  • The actual clock from PLL is the VCO output.

  • VCO’s frequency is modulated by the input voltage

Classification of VCO

Depending on the type of output waveform, VCOs are classified as

  • Harmonic Oscillators

  • Relaxation Oscillators

Some of the commonly used VCOs-Ring Oscillators:Common in monolithic topologies and it uses odd number of inverters connected in feedback loop.-Other forms of VCOs, such as crytal oscillators and resonant oscillators essentially run on the same principle

VCO Circuit Design:

  • A simple design of VCO consists of a collector coupled astable multivibrator using n-p-n transistor with a control voltage.

    Step 1

Step 2f=1/2RCln (1+Vcc/V)

Step 3:

*Voltage controlled Oscillator

VCC 6 0 DC 5V

VI 7 0 PULSE(0 5 0US 30US 30US 30US 40US)

RC1 6 1 1K

RC2 6 2 1K

RE1 7 10 1K

RE2 7 11 1K

R3 7 8 2K

R4 7 9 2K

R5 8 0 4.7K

R6 9 0 4.7K

C1 1 4 150PF

C2 2 3 150PF

Q1 1 3 0 QM

Q2 2 4 0 QM

.MODEL QM NPN (IS=2E-16 BF=50 BR=1 RB=5 RC=1 RE=0 TF=0.2NS TR=5NS)

Q3 3 8 10 QM1

Q4 4 9 11 QM1

.MODEL QM1 PNP (IS=2E-16 BF=100 BR=1 RB=5 RC=1 RE=0 TF=0.2NS TR=5NS)

.IC V(1)=0 V(3)=0

.TRAN 0.1US 90US

.PLOT TRAN V(2) V(7)


Loop filter:

  • The loop filter may be active or passive, but it typically results.

  • In the loop being either first-order or second-order.

  • A 1st order filter having a low frequency pole and high frequency zero is recommended.

  • Loop filter system functions, F(s), include:

    Filter Type Filter F(s)

    perfect integrator 1+sτ2/sτ1

    imperfect integrator 1+sτ2/1+sτ1

    lag or low pass 1/1+sτ

Filter Design

  • Design circuit is a simple RC low-pass filter which can extract the average value from the output of the phase detector.

  • This average value is used to drive the VCO







low pass filter

R1 1 2 86K

C1 2 3 5PF

R2 3 0 2K

VIN 1 0 PULSE(0V 9V 0US 0US 0US 2US 5US)

.TRAN 0.01US 15US

.PLOT TRAN V(2) V(1)



  • The earliest widespread use of PLLs was for the horizontal and

  • vertical sweeps used in televisions.

  • PLLs were critical to development of color televisions also.

  • PLLs today:

  • –Cell phones, televisions, radios, pagers, computers, all

  • telephony,

  • – At low end: all software PLLs implement entire PLL functionality

  • on sampled data.

  • – At high end: optical PLLs used in clock recovery for 160 Gbps

  • data

  • Phase-locked loops are widely used for synchronization purposes

  • Space communications for coherent carrier tracking

  • and threshold extension

  • Bit and symbol synchronization

  • They are also used for:

  • Demodulation of frequency modulated signals

  • To synthesize new frequencies which are multiples of a

  • reference frequency, with the same stability as the

  • reference frequency.

  • Deskewing

  • the clock must be received and amplified

  • finite delay dependent on process,temperature and voltage between detected clock edge and the received data

  • delay limits the frequency at which data can be sent

  • deskew PLL on the receiver side phase-matches the clock at each data flip-flop to the received clock.

  • 2. Clock generation

  • multiplies lower-frequency reference clock (usually 50 or 100 MHz) up to the operating frequency of the processor that operate at Gigahertz

  • 3. Spread spectrum

  • All electronic systems emit some unwanted radio

  • frequency energy

  • limits on this emitted energy and any interference

  • caused

  • spreading the energy over a larger portion of the

  • spectrum

  • 4. Jitter and noise reduction

  • reference and feedback clock edges can be brought into very

  • close alignment

  • phase and frequency of generated clock is unaffected by rapid

  • changes in voltages of the power supply lines and of the

  • substrate on which the PLL circuits are fabricated

1. Carrier Recovery

• General block diagram of frequency recovery from a modulated


• When carrier has strong component in signal spectrum, PLL

can lock.

• When carrier is missing from signal spectrum, PLL must be

preceded by a nonlinear element.

2. Costas Loop

  • A Costas loop can both recover the carrier and demodulate the data from a

  • signal.

  • If there were no modulation, the upper arm is simply a PLL lock to a carrier.

  • The effect of the lower arm of the loop is to lock to the modulation and cancel it

  • out of the upper arm of the loop.

  • Does the same thing as squaring loop, but down converts signal to baseband &

  • does filtering there.

3. Clock distribution

4. Frequency Synthesis

In wireless applications, frequency synthesizers provide local oscillators ability for up and down conversion of modulated signals

A basic PLL synthesizer

5. Disk Drive Control

Amplitude encoded position error signal (PES) in a hard disk.

PLLs are used to time the acquisition

of the readback signal.

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