**What is Phase Locked Loop? Its Concept Of Operation, Characteristics & Applications**

Table of Contents

**What Is Phase Locked Loop?**

The

phase locked looporPLLis an electronic circuit with avoltage controlled oscillator, whose output frequency is continuously adjusted according to the input signal’s frequency.

A **Phase locked loop** is used for tracking **phase** and **frequency** of the input signal. It is a very useful device for synchronous communication. **PLL** acquires the carrier frequency in suppressed carrier mode of communication and produces a coherent carrier signal inside the receiver for demodulation.

**Block Diagram**

A PLL operates like a typical **feedback system**. It updates the output frequency of **VCO** until it matches the frequency of the input signal i.e. **in sync** with the input signal.

**Concept Of Operation**

The operation of **Phase locked loop** is based on the **phase difference **between the input and output signals. The phase difference between of two signals can be understood by the figures given below.

The figure above shows two sinusoidal signals having a **constant phase difference**. When two signals have** constant phase difference** then they are said to have the **same frequency**. They only have shifted phase. Thus it shows that the two signals are at the **same frequency**.

However, this figure shows two signals having a **var****ying** phase difference. This changing difference shows that the two signals are **non-coherent**. It means that the two signals are totally different signals with different frequencies.

It is clear from the discussion above that the phase difference is related to the frequency of the signal. The **Phase locked loop** makes the phase difference constant between its input & output signal. And produce a stable frequency signal same as the input signal.

This key concept is put to use in PLL device.

**Operation Of PLL**

There are 3 components of PLL and the **operation** of each component is given below.

**1) Voltage Controlled Oscillator (VCO)**

The **Voltage Controlled Oscillator** often known as **VCO** is a type of oscillator that produces a sinusoidal signal whose frequency can be controlled by an external voltage. The frequency produced by **VCO** is varied linearly with respect to the input voltage. The sinusoidal output of the** VCO** is

**ω(t) = ω _{c} + c e_{o}(t)**

Where **c** is the constant of **VCO** & **e _{o}(t)** is the voltage proportional to the phase difference between the input signal’s frequency and

**VCO’s**frequency.

**ω**is the

_{c }**free running**frequency of the oscillator.

The output of **VCO** is **ω _{c}** if

**e**However, if

_{o}(t) = 0.**e**is

_{o}(t)**positive**, then the output frequency of

**VCO**will

**increase**& if

**e**is

_{o}(t)**negative**, then the output frequency will

**decrease**.

**2) Phase Detector**

The **phase detector** is an electronic circuit that compares two signals and generates a voltage signal which is proportional to the **phase difference** between the two signals.

Basically, a phase detector is a **multiplier**. It multiplies the two sinusoidal signals i.e. the input/reference signal & the output of **VCO**.

Suppose the input signal is **A sin(ω _{c}t+ϴ_{i})** & the output signal of

**VCO**is

**B cos(ω**

_{c}t+ϴ_{o})Then the output of phase detector **x(t) **is**:**

**x(t) = A sin(ω _{c}t+ϴ_{i}) B cos(ω_{c}t+ϴ_{o})**

**x(t) = AB sin(ω _{c}t+ϴ_{i}) cos(ω_{c}t+ϴ_{o})**

**x(t) = AB/2 {sin(ω _{c}t+ϴ_{i}+ ω_{c}t+ϴ_{o})+ sin(ω_{c}t+ϴ_{i}– ω_{c}t-ϴ_{o})}**

**x(t) = AB/2 {sin(2ω _{c}t+ϴ_{i}+ϴ_{o})+ sin(ϴ_{i}-ϴ_{o})}**

**x(t) = AB/2 sin(2ω _{c}t+ϴ_{i}+ϴ_{o})+ AB/2 sin(ϴ_{i}-ϴ_{o})**

The **high frequency** component is then removed by using the loop filter discussed below.

**3) Loop Filter**

The filter used in the loop of **PLL** is a **narrow band low pass filter**. It filters any high-frequency components from the output signal of the phase detector and provides a fixed voltage signal to **VCO**.

After passing the signal **x(t)** through loop filter it blocks the term of high frequency.

**x(t) = AB/2 sin(2ω _{c}t+ϴ_{i}+ϴ_{o})+ AB/2 sin(ϴ_{i}-ϴ_{o})**

The output signal **e _{o}(t)** of

**loop filter**is

**e _{o}(t) = AB/2 sin(ϴ_{i}-ϴ_{o})**

Where **AB=2 **&** (ϴ _{i}-ϴ_{o})** is the

**phase difference.**If there is a

**non-zero**phase difference, then the loop filter will produce a voltage signal proportional to the phase difference. Due to this voltage signal, the output of

**VCO**is maintained.

**How It Works**

As long as there is a phase difference it will keep maintaining its frequency until it is **locked**. In such condition, both signals have same frequency & constant phase. These both signals are said to be **phase coherent** or **in phase lock.**

Suppose the **PLL** is locked & the **VCO **is generating a stable frequency. Suddenly the input signal’s frequency **increases** i.e. **ω _{c}+k**

**A sin((ω _{c }+k)t+ϴ_{i}) = A sin(ω_{c}t+kt+ϴ_{i})**

Then the output of **loop filter** will be

**e _{o}(t) = AB/2 sin(kt+ϴ_{i}-ϴ_{o})**

This, in turn, increases the difference and the voltage output of loop filter increases. Due to which the frequency of the **VCO** output signal increase.

**Characteristic Of PLL**

**Phase-locked loop**s are designed for a Specific**range**of frequencies. This range of frequency is called**Capture Range**of**PLL**. A**PLL**can lock onto a signal if its frequency lies in its Capture Range.- When the
**PLL**is locked onto an input signal, the input signal can be changed. Suddenly, the input signal changes then the**PLL**will start tracking its phase and frequency in a fixed range. This tracking range is called**Lock Range**. - The most important feature of
**PLL**is the**noise filtering**. It completely blocks any unwanted low power frequencies from the input signal and provides a**pure and stable**sinusoidal wave.

**Applications of PLL**

It is the most widely used circuit in modern communication.

- It is used in
**demodulation**of Amplitude Modulated**suppressed carrier**signal. - It is also used for demodulation of
**Frequency**Modulated & Phase Modulated signals. - It can also be used in
**clock recovery**from a signal. - Because of its stable nature, it is used as
**frequency synthesizer**in almost every digital device. - A crystal oscillator can generate stable frequency up to 300Mhz. while PLL can generate signals of a high & stable frequency
**greater than 300 Mhz**. - It is used in different devices such as a
**signal analyzer**,**signal generator**, radar, cell phones & radio etc.

you may also read: