What is the PLL Frequency Synthesizer?

- an introduction or tutorial to the essentials of phase locked loop, PLL frequency synthesizer operation and design

PLL frequency synthesizers are widely used in all forms of radio communications equipment today.

These frequency synthesizers are found in a variety of items from cellular phones to all forms of wireless products and domestic radios and televisions to professional radio frequency equipment like signal generators and spectrum analyzers as well as professional radio equipment and much more.

PLL frequency synthesizers offer very many advantages over the use of other forms of oscillator.

Frequency synthesizers not only offer high levels of stability and accuracy (determined by the reference which is normally a crystal oscillator); they are also easy to control from digital circuitry such as microprocessors. This enables facilities such as keypad frequency entry, channel memories and more to be implemented - all of which are expected as basic functionality in today's equipment.

In view of all their advantages, PLL frequency synthesizers are usually the preferred form of radio frequency oscillator for most applications. Accordingly synthesizers are included in many radio chip-sets from cellular phones to radio and televisions.

PLL Basics

Most frequency synthesizers are based around a phase locked loop or PLL.The PLL uses the idea of phase comparison as the basis of its operation. From the block diagram of a basic loop shown below, it can be seen that there are three basic circuit blocks, a phase comparator, voltage controlled oscillator, and loop filter. A reference oscillator is sometimes included in the block diagram, although this is not strictly part of the loop itself even though a reference signal is required for its operation.

The basic diagram of a phase locked loop used as the basis of many frequency synthesizers showing the phase detector, loop filter and voltage controlled oscillator
Phase locked loop basic diagram

The phase locked loop, PLL, operates by comparing the phase of two signals. The signals from the voltage controlled oscillator and reference enter the phase comparator Here a third signal equal to the phase difference between the two input signals is produced.

The phase difference signal is then passed through the loop filter. This performs a number of functions including the removal of any unwanted products that are present on this signal. Once this has been accomplished it is applied to the control terminal of the voltage controlled oscillator. This tune voltage or error voltage is such that it tries to reduce the error between the two signals entering the phase comparator. This means that the voltage controlled oscillator will be pulled towards the frequency of the reference, and when in lock there is a steady state error voltage. This is proportional to the phase error between the two signals, and it is constant. Only when the phase between two signals is changing is there a frequency difference. As the phase difference remains constant when the loop is in lock this means that the frequency of the voltage controlled oscillator is exactly the same as the reference.

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