PLL Frequency Synthesizer Tutorial

- 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.

Note on Phase Locked Loops:

Phase locked loops form the basis of many RF systems. They are use the concept of minimising the difference in phase between a reference signal and a local oscillator to replicate the reference signal frequency. Using this concept it is possible to use these loops for many applications from FM demodulators to frequency synthesizers.

Read more about the Phase locked loop PLL

PLL frequency synthesizer basics

A phase locked loop, PLL, needs some additional circuitry if it is to be converted into a frequency synthesizer.

The loop is broken and additional blocks added to provide the frequency synthesizer action. These blocks add a frequency offset into the loop in one way or another.

The basic action of the loop remains. The phase detector produces an error voltage proportional to the phase difference between its two input signals. This means that the voltage controlled oscillator will run at a different frequency to that of the phase detector or comparison frequency.

There are two main ways in which frequency synthesizers can be made from phase locked loops:

  • Digital PLL synthesizer:   This is the concept that is at the root of most single loop synthesizers. It involves placing a digital divider in the loop between the voltage controlled oscillator. This means that the voltage controlled oscillator frequency will be divided by the division ratio of the divider, e.g. n, and the VCO will run at n times the phase comparison frequency. By changing the division ratio of the divider, the output frequency of the oscillator can be changed. This makes the frequency synthesizer programmable.

    The basic block diagram of a frequency synthesizer using a phase locked loop with a divider placed between the VCO and the phase detector
    Basic digital frequency synthesizer

    These digital frequency synthesizers are ideal for many applications on their own. They perform well where the differences between channels are relatively high. Where virtual continuous tuning using steps of 1 Hz or 10Hz may be needed, this requires very high division ratios and this can degrade the phase noise performance and give rise to other issues. To achieve the required performance, it may ne necessary to combine a digital PLL synthesizer with some analogue techniques as described below.

    Read more about the digital PLL synthesizer
  • Analogue PLL synthesizer:   This form of frequency synthesizer introduces a mixer into the PLL between the voltage controlled oscillator and the phase detector. By introducing an external signal into the other terminal of the mixer, a fixed offset equal to that of the external frequency is introduced into the loop.

    The basic block diagram of an analogue frequency synthesizer using a phase locked loop with an RF mixer or multiplier placed between the VCO and the phase detector
    Basic analogue frequency synthesizer

    Care is needed when designing analogue synthesizers as there can be issues with the image signal. Although phases for the phase detector are reversed, it is still necessary to ensure that only the correct mix scenario is seen by the system. Sometimes steering voltages may be applied to the VCO to ensure the correct operation.

    Read more about the analogue PLL synthesizer

Many synthesizers use a combination of both these techniques to be able to produce high quality frequency synthesizers very cost effectively and with high levels of performance.

When designing a frequency synthesizer, careful analysis of the requirements is needed, especially those of the phase noise performance as this can affect the overall topology design. This can often be undertaken using computer tools, although manual graphical analysis can even provide some useful insights.

By Ian Poole

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