Using DSP Technique to Average ADC Samples for Improving the Frequency Measurement Resolution

摘要

Most present common counters can't be used to measure the frequency by using the gate time less than 0.01 second. In addition, the frequency measurement resolution of common counters is about 3X10~(-10) in 1 second gate. In fact, the frequency resolution is inversely proportional to the gate time. For improving the frequency resolution, using the high-speed digital signal processor (DSP) to average the signal samples is created. We use analog-to-digital converter (ADC) to sample the device under test (DUT), and the frequency standard is used to be the reference clock. The DSP averages every ADC's sample to represent one of the DUT's measurements in the time interval of the gate time. Because it is able to average the samples and record the measurements in the time interval of each gate time that is specialized from 1 microsecond to 1 second, the DSP can measure the continuous samples of the DUT. Additionally, the measurement noise can be reduced and the measurement resolution can be improved by averaging the samples. According to the measurements, the frequency difference between DUT and frequency standard can be evaluated. Since the resolution in our design is inversely proportional to not only the gate time but also square of the number of the samples for averaging, it is better than that of common counters when gate time is larger. Additionally, the frequency stability less than one second is able to be evaluated in our measurement system.