Design And Experimental Study Of Phase Repeatable Sweep Frequency Source

The quality of frequency source determines the performance of electronic measuring instrument.In the electronic measurement instrument based on down conversion,when the sweep source is used as the local oscillator signal source to participate in down conversion,the unknown and randomness of the local oscillator signal will introduce random phase interference into the intermediate frequency component,which leads to the incoherence of the phase measurement results of different spectrum components for a single receiver.So design a sweep source to make its output phase repeat,then the random phase introduced by local oscillator signal source can be eliminated through the internal calibration technology of the system in electronic measuring instruments.Therefore,the phase repeatability of sweep source is the main research direction of this project.The frequency synthesis technology used in this paper is PLL frequency synthesis technology,which has the advantages of simple structure,multi band,low phase noise.In this paper,according to the technical specifications of the sweep source,the scheme of the sweep source system is designed.After the selection of devices,the feasibility of the system scheme is discussed.After the design of the sweep source circuit,a phase difference measurement system based on down conversion is built to measure the phase repeatability of the sweep source.After testing,the frequency sweep source has a wide frequency band of 200 MHz ～ 6GHz,and the phase noise of 1GHz output frequency point is-95.5dbc/ Hz@10k Hz The standard deviation of phase repetition is less than2.142°.The temperature stability experiment shows that the phase repeatable standard deviation of the system is less than 1° from 5℃ to 35℃.The main work of this paper is as follows:1.The basic module,working principle and performance parameters of PLL frequency synthesis technology are described.The relationship between time jitter and phase noise is obtained.The noise performance of PLL technology is discussed,which provides a theoretical basis for the design of frequency sweep source.2.According to the performance index of sweep source,the scheme of sweep source system is determined.The selection of PLL chip,low phase noise crystal oscillator and clock buffer is completed.The loop filter is designed and simulated.In order to solve the problem of random phase output,a look-up table of wideband VCO frequency control word is established,and the output divider is set in the PLL.Finally,the specific design process,schematic diagram and PCB layout are given.3.In order to verify that the output phase of sweep source is repeatable,a phase difference measurement system based on analog down conversion and digital down conversion is built,including the design and selection of mixer,signal conditioning circuit,ad module,digital mixer and digital filter.4.The performance index of the sweep source is tested,and then the repeatability of the output phase of the sweep source is verified by using the phase difference measurement system.The results show that the sweep source meets the requirements of the design.