Frequency Synthesiser Types

- frequency synthesizers are now the accepted way of generating stable accurate RF signals for use in everything from radios to mobile phones and signal generators. There are several different types that can be used: indirect / direct, analogue / digital.

Frequency synthesizer technology is now an accepted part of RF development and RF equipment.

Synthesizers enable the flexible operation of high performance oscillators used in a variety of equipment from radio receivers and transmitters of all sorts to highly stable and flexible signal generators.

Frequency synthesizers were not widely used until the 1970s. The reason for this was that prior to the introduction of RF capable integrated circuit technology, frequency synthesizers required a considerable amount of circuitry, and this meant that costs were very high. This put them out of the range of most applications.

With the introduction of RF capable ICs, frequency synthesizer technology using phase locked loops became feasible, and with their advantages, their use became widespread.

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 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 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 frequency synthesis 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, and then advancing the phase in fixed increments. The phase advance increments determine the signal frequency that is generated.
      Read more about Direct Digital Frequency Synthesis
  • 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.
      Read more about Indirect Analogue Frequency Synthesis
    • Indirect Digital Frequency Synthesis:   The indirect digital frequency synthesis techniques introduce a digital divider into the 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 he 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.
      Read more about Indirect Digital Frequency Synthesis

Today's frequency synthesizers may use a variety of techniques and technologies. Typically the direct analogue approach is not used these days, but the other three may be used in various applications, the choices being dependent upon the requirements of the application.

By Ian Poole

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