Design Of Digital Counter Based On Under-Sampling

At present, digital and virtual instrument technology are not on application inthe field of high-precision frequency measurement. The main reason lies in thelimitations of the sampling rate. In the field of high-precision frequencymeasurement, the frequency of signal under test is higher, and there are highdemands for measurement accuracy. Due to the limitations of the Nyquist theorem,digital and high-precision frequency measurements that require extremely highsampling rate can be achieved signal under test,. it is difficult to realize underexisting conditions,The digital counters based on under-sampling raised in this article, is ahigh-precision frequency measurement equipment based on digital and virtualinstrument technology. It is able to overcome the limitations of the Nyquist theorem,and,measure the frequency accurately of the signal under test.The author’s main duties include the following aspects:1, The author study on the digital frequency measurement techniques currentlyin use, and find the most important reason for which digital technology is difficult tobe used in high-precision frequency measurement is sampling rate restrictions.2, The author propose the two-process precision frequency measurementmethod, which include the preliminary measurement process based on the frequencyspectra analysis and the precise measurement process based on the under-samplingmethod. This method can be able to overcome the limitations of the Nyquistcriterion to measure the signal accurately with low sampling rate3, With the Data Acquisition Board and The high-performance computers, theauthor build an efficient, reliable and stable hardware platform for the digital counterbased on under-sampling,.4The author develop the software part of the digital counter based onunder-sampling by the Labwindows/CVI. The software uses a multi-threadeddesign, in clouds three modules: panel module, data acquisition module andfrequency measurement module. This software can measure the frequency of signalunder test by the preliminary measurement, the precision measurement and theautomation measurement to complete treatment of the measured signal precision frequency measurement.5, The paper take the equipment performance testing to the digital counterbased on under-sampling, using the equipment noise floor test to verify theperformance of the digital counter based on under-sampling. Comparison with themeasurement results of the SR-620and the TSC MMS to verify the correctness ofthe device measurement results. Been tested for measurement accuracy of theequipment is better than1e-11middleweight, measurement stability can be achieved5.6e-12, and measurement results with high correctness.