Enhancing time-stamp counter phase modulation measurements

This paper demonstrates that digital signal processing techniques can enhance the quality of phase modulation measurements produced by a time-stamp (phase digitizing) frequency counter. A typical time-stamp counter utilizes a digital divider to reduce signal frequency to the desired sample rate. Unfortunately, division also reduces phase modulation to the point where useful information may be obscured by counter measurement uncertainty (jitter). An analogy between an analog-to-digital converter (ADC) and a time-stamp counter predicts that the counter induced modulation can be modeled as random noise which is white in phase. The noise magnitude is directly related to the instrument's resolution specification. Fourier analysis, subject to some restrictions, can compute the power spectra of phase or frequency modulation, revealing even low level responses. A number of techniques can be used to reduce the amount of counter induced noise that appears on time domain plots of phase and frequency modulation. Experimental data, generated by a prototype counter, illustrates the type of results that can be expected from Fourier analysis and various noise reduction techniques