Study On Methods Of Generating High-Precision Offset Frequency

In order to further enhance the measure capability of multi-channel digital frequency stabilityanalyzing system (DFSA) which was developed by National Time Service Center (NTSC),methods of generating offset frequency were studied. Two principle prototypes of offsetfrequency generator were successfully developed and their performances were also analyzed.The main research of this paper can be summarized as follows:After analyzing the method of phase-locked loop (PLL) for generating offset frequency,methods of generating10.0001MHz signal were researched and hardware circuits were designed.The key technologies of low noise mixer, low-pass filter, loop filter and isolation amplifier wereapplied. A principle prototype of offset frequency generator whose output signal is10.0001MHzwas successfully developed. Two methods were designed to test the performance of the principleprototype. When the results were measured using the method of measuring frequency, the Allandeviation of the10.0001MHz prototype is7.18e-13which is better than the frequency stabilityof8662A at the same frequency(8.43e-13). Where, the8662A is a frequency signal generatormade in Agilent, in USA. While the results were measured using the method of measuring timeinterval, the Allan deviation of the10.0001MHz prototype is7.72e-13which is better than thefrequency stability of8662A at the same frequency(9.30e-13). These results are measured with1second sampling interval.Based on the analysis of single side-band mixing method for generating offset frequency,hardware circuits were design such as low noise mixer, band-pass filter, isolation amplifier andso on. In order to solve accurate orthogonal phase shift, the scheme that consists of orthogonalsplitter, digital phase shift module using CPLD, digital delay chip and line delay was put forward.A principle prototype of offset frequency generator whose output signal is9.99999MHz wassuccessfully developed. Two methods were designed to test the performance of the principleprototype. When the results were measured using the method of measuring frequency, the Allandeviation of the9.99999MHz prototype is4.65e-13which is better than the frequency stabilityof8662A at the same frequency(9.21e-13). While the results were measured using the method ofmeasuring time interval, the Allan deviation of the9.99999MHz prototype is5.43e-13which isbetter than the frequency stability of8662A at the same frequency(1.01e-12). These results aremeasured with1second sampling interval.The results show that the frequency stability of the developed offset frequency generators at10.0001MHz and9.99999MHz are better than that of8662A at the corresponding frequency. Thedeveloped offset frequency generators can meet the requirement of DFSA. They also can supply offset frequency signal for other test systems using the dual-mixer time-difference method orbeat frequency method, and also can be applied to other fields as a high-precision frequencysource.