What is an RF Frequency Synthesiser: technology & types

What is an RF Frequency Synthesiser: technology & types

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Frequency synthesizers are used in a host of different RF equipment - almost anywhere that a stable RF source is required.

RF frequency synthesizers provide high levels of performance in terms of stability, programmability and general convenience. As most equipment requiring the use of an RF synthesizer also has digital circuitry for other elements of the device, RF synthesizers lend themselves particularly well to being used.

Accordingly RF frequency synthesizers are used in equipment from simple Bluetooth transmitters and receivers, to Wi-Fi routers and mobile phones right up to very high performance communications systems, satellite links and the like. In fact anything that uses RF communications in almost any form is likely to use an RF synthesizer.

RF frequency synthesizer types / categories

There are several different types of categories of synthesizer. Each of them obviously has its own advantages and disadvantages. There are often choices that can be made about which type to choose

  • Direct: The direct forms of RF frequency synthesizer, are as the name suggests implemented by creating a waveform directly without any form of frequency transforming element. Direct techniques including forms of oscillator and mixer are used.
    • Direct Analogue Frequency Synthesis: This form of RF frequency synthesizer was sometimes called a mix-filter-divide architecture. The direct analogue frequency synthesizer gained this name because it accurately defines one of the more popular architectures for this form of synthesis.

      The direct analogue frequency synthesizer had several drawbacks: it required a considerable amount of critical circuitry which today does not lend itself to integration; the successive mix processes introduced significant numbers of spurious signals; the spurious signals required considerable levels of filtering, again adding to the cost. As a result, this type of RF synthesizer was only used as a last resort before the widespread availability of RF ICs and the possibility of utilising other forms of frequency synthesis.

    • Direct Digital Frequency Synthesis: Direct digital synthesizers, DDS are widely used now. They create the signal by having a stored version of the waveform required in digital format, and then advancing the phase in fixed increments. At each increment, the value for the instantaneous voltage of the waveform is looked up in the memory and converted to an analogue format. By advancing the phase and hence taking successive samples, the waveform is built up. The phase advance increments determine the signal frequency that is generated, as this determines how quickly the increments advance along the waveform and hence how quickly the waveform is repeated.
  • Indirect: Indirect frequency synthesis is based around phase locked loop technology. Here the output signal is generated indirectly. In other words the final signal is generated by an oscillator that is controlled by other signals. In this way the signals used in creating the output are indirectly replicated by the output oscillator, thereby giving the name to this technique.
    • Indirect Analogue Frequency Synthesis: Indirect analogue frequency synthesis uses phase locked loop technology with a mixer placed between the voltage controlled oscillator and phase detector. This enables and offset frequency to be introduced into the loop.
    • Indirect Digital Frequency Synthesis: The indirect digital frequency synthesis techniques introduce a digital divider into the RF phase locked loop between the voltage controlled oscillator and the phase detector. The VCO runs at a frequency equal to the phase comparison frequency times the division ratio. By altering the division ratio, it is possible to alter the frequency of the output signal. Typically the comparison frequency is equal to the channel spacing required. This could be 100 of 50 kHz for an FM tuner, 25 or 12.5 kHz for professional mobile communications systems, etc. It could be much smaller for general radio applications.

With RF synthesizers being so widely used within all forms of electronic equipment, their use is very widespread. With many new remote sensors for the Internet of Things using RF links, the use of RF frequency synthesizers of all forms is only going to increase.

Additionally many RF synthesizer chips are available, or frequency synthesizer functionality is included in other application specific chips and this makes the ability to generate accurate and controllable radio frequency signals very easy.

Watch the video: Frequency synthesizer (May 2022).