Research And Implementation Of 5MHz-400MHz Local Oscillator With Low Phase Noise

The local oscillator is the key to the phase noise measurement system,which determines the sensitivity of the measurement system to a certain degree.This sensitivity can be quantified by the phase noise,which is a key indicator of the local oscillator.It is necessary to develop a local oscillator with low phase noise for a high-sensitivity phase noise measurement system.According to the requirements of the phase noise measurement system,the output frequency range of the local oscillator is 5MHz-400 MHz,and the frequency resolution is 1mHz.Frequency synthesis techniques are used to realize the local oscillator.In this thesis,the synthesis theories and realization methods of low phase noise local oscillator are studied,and a circuit module which meets the system's functions and performances requirements is designed.The phase noise generated by the frequency synthesis system is divided into two parts for analysis in the thesis.One part is caused by the frequency change.It is strictly in accordance with the theoretical calculation and its absolute value is determined by the phase noise of the reference source(including other oscillators in the synthesis system).The other part is generated by the composite devices,including white phase noise,flicker phase noise,and ambient source phase noise.Phase white noise is additive noise,which is independent of other noise and obtained by linearly adding the white noise of all devices.It is the noise floor of the phase noise curve and is the most difficult noise to handle in the design.Compared with white phase noise,it is convenient and flexible to use the correlation between the other phase noises to achieve low phase noise frequency synthesis.Based on the two sources of phase noise,the phase noise equivalent model of the local oscillator source is established and used as the theoretical basis for the design of the local oscillator system.In order to develop a low phase noise frequency synthesis source,a high resolution narrowband signal with low phase noise is generated by using a 10 MHz reference source to excite the direct digital synthesis(DDS)and it is spread to a broadband signal that meets the system requirements by the direct analog frequency synthesis method in the thesis.A 10 MHz ultra-low phase noise reference source is designed at first in the thesis.With a new oscillating circuit and high-precision temperature control circuit,the output phase noise of the reference source can reach-170dBc/Hz@1kHz and the Allan variance can reach 6E-13@1S.On this basis,a method to continue the performance of the reference source is sought in the subsequent frequency synthesis.In the thesis,comb spectrum generator,regenerative frequency divider and frequency multiplier are designed with analog devices,which can help minimize the using of digital devices with high noise to achieve the purpose of reducing the noise floor of the local oscillator.The comb spectrum mixing technique and the triple mixing structure are introduced in the frequency expansion,which not only reduces the system complexity,but also guarantees the system phase noise performance.The output phase noise of the local oscillator circuit module made in the thesis reaches-148dBc/Hz@1kHz(5MHz)?-147dBc/Hz@1kHz(10MHz)?-142dBc/Hz@1kHz(100MHz)?-130dBc/Hz@1kHz(400MHz),and the other indicators also meet the design requirements.