Frequency synthesis technology originated in the 1930s and has a history of more than 70 years. 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.
Using a high-precision crystal oscillator as a reference, a series of high-definition frequency sources with a certain frequency interval can be generated by a synthesis technique, which is divided into direct synthesis and phase-locked loop synthesis.
A frequency synthesizer is a device that utilizes one or more standard signals to generate a large number of discrete frequency signals through various technical approaches. Direct digital frequency synthesis (DDS) technology is a third-generation frequency synthesis technology that has developed rapidly with the development of digital integrated circuits and microelectronics technology following direct frequency synthesis and indirect frequency synthesis. Based on the theory of digital signal processing, it performs frequency synthesis based on the amplitude-phase relationship of the signal. It has extremely high frequency resolution, extremely short frequency conversion time, wide relative bandwidth, continuous signal phase during frequency conversion, and arbitrary waveform. The output capability and digital modulation functions are widely used in various fields such as instrumentation, remote telemetry, radar, electronic countermeasures, navigation, and broadcast television. Especially in short-wave frequency hopping communication, the signal is constantly changing over a wide frequency band, and it is required to quickly switch the frequency and phase in a small frequency interval. Therefore, the local oscillator signal source using DDS technology is an ideal choice. This method is simple, reliable, easy to control, and has high frequency resolution and conversion speed, which is very suitable for fast frequency hopping communication.
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 with a phase-locked loop combined frequency. 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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