Frequency synthesis technology is to generate a low-frequency standard reference signal with high spectral purity and high stability through a crystal oscillator, and then perform linear operation in the frequency domain, and obtain one or more signals with the same stability and low phase noise that meet the requirements of various indicators through frequency doubling, frequency mixing, frequency division and other technologies.
From the development history of frequency synthesis, frequency synthesis methods have experienced direct analog synthesis, phase-locked technology and direct digital synthesis in turn.
In the design of microwave generator, single-loop phase-locked frequency synthesis or multi-loop phase-locked frequency synthesis is usually adopted. Multi-loop phase-locked feedback network uses harmonic mixing and microwave sampler to down-convert the frequency output of microwave master oscillator to RF frequency band to form a loop, and finally realizes the locking of microwave master oscillator. There is also a widely used new oscillator and indirect frequency synthesis technology to design microwave generators.
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Microwave generators can be divided into feedback type and negative resistance type from the circuit structure. Feedback oscillator is mainly used in low frequency circuit system, and negative resistance oscillator is mainly used in high frequency circuit system. Therefore, the negative resistance oscillation circuit is more suitable for high frequency range such as radio frequency and microwave, and the microwave oscillation circuit can be analyzed and designed by using the negative resistance principle.
In a certain circuit structure, microwave transistor can be regarded as a two-port device. When the specific terminal of the transistor is grounded, a two-port negative resistance oscillator is formed due to the nonlinear negative resistance characteristic. The equivalent network of two-port negative resistance oscillator includes active device (BJT), feedback circuit, resonance network and output network.
Modern frequency synthesis technology combines analog technology, digital technology, optical technology and calculation method. According to the technical index of frequency synthesizer, it reasonably combines mature frequency synthesis technologies such as direct frequency synthesis technology, phase locked loop (PLL) and direct digital frequency synthesis technology with new oscillators and new technologies.
The technical indexes of microwave generator such as spectrum purity, frequency switching speed and output frequency range can meet the requirements of different occasions.
In particular, the method of generating microwave by light based on microwave photonics is also developing rapidly. The methods that can be realized are: optical harmonic method, photoelectric oscillator method, optical heterodyne method, phase modulator method and so on. These frequency synthesis technologies provide a broader space for synthesizing microwave, millimeter wave and submillimeter wave frequency signals.
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