There are three ways to implement a frequency synthesizer: direct analog frequency synthesis, indirect frequency synthesis, and direct digital frequency synthesis.
According to the chronological order of occurrence, it can be divided into 3 generations.
The first generation: direct analog frequency synthesis technology. Using one or more different crystal oscillators as reference sources, multiple discrete frequency output signals are directly generated by frequency multiplication, frequency division, mixing, etc., called direct frequency synthesis. The signal obtained by this method has the characteristics of high long-term and short-term stability of frequency and fast frequency conversion, but the debugging is difficult and the spur suppression is difficult.
The second generation: phase-locked frequency synthesis technology. In the 1950s, phase-locked frequency synthesizers, also known as indirect synthesizers, appeared. It uses one or several reference frequency sources to generate a large number of harmonics or combined frequencies by harmonic generator mixing and frequency division, etc., and then locks the frequency of the voltage controlled oscillator to a certain harmonic or combined frequency with a phase-locked loop. The desired frequency output is indirectly generated by the voltage controlled oscillator. The advantage of this method is that since the phase-locked loop is equivalent to a narrow-band tracking filter, the signal of the desired frequency can be well selected, the spurious component is suppressed, and a large number of filters are avoided, which is advantageous for integration and miniaturization.
The third generation: direct digital frequency synthesis technology. Since the 1970s, with the development of digital integrated circuits and microelectronics technology, a new synthesis method, direct digital frequency synthesis (DDS) technology, has emerged. It performs frequency synthesis from the concept of phase, adopts digital sampling storage technology, and has the advantages of accurate phase, frequency resolution, and fast conversion time and so on.
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