Design Of High-Resolution And Low Phase Noise Frequency Synthesizer

Frequency synthesizer has developed nearly 80 years, and it has been a crucial position in modern electronic systems. It is usually used as LO or an excitation signal in electronic system and it decides the merits of the entire system. Modern communication system and electronic measurement industry is growing rapidly, so the request of communication quality and accuracy becomes higher and higher. That means the phase noise, spurious, lock time and frequency range of frequency synthesizer also put forward higher requirements.At begin of this paper, it describes the background and basic theoretical knowledge of the frequency synthesizer, and then discuss structures, and the structures of frequency synthesizer discussed from simple to complex. First, it analyzes the simplest single loop PLL, and points the weaknesses of this structure. Then it use the structure by mixing frequency outside loop, the structure by mixing frequency outside loop can only optimize 2.5dB, it not obviously work. Finally it choose the structure by mixing frequency in the loop, this structure can reduce the frequency division in the loop, and can optimize 14 dB than single loop. This design can reflect the advantage of three generations of frequency synthesizer: frequency multiplier provides an ultra-low phase noise's local oscillator frequency for the mixer, and the part of DDS provides fine resolution, and the phase lock loop section make wide frequency range. The article use the four frequency instead of a single frequency, and make the output frequency become wide, it also use an auxiliary locking device to solve the wrong lock problem because of non-linear of the mixer.The frequency synthesizer can divide into three part: the frequency multiplication's board, the DDS part and the PLL part. Each part test respectively, finally three parts combine together. The frequency synthesizer phase noise is-99dBc/Hz@1kHz,the spur is-65 dBc, frequency resolution less than 500 Hz, it has reference value for future's designers.