Estimation of analog-to-time and time-to-digital conversion efficiency in analog optical communication system testbed

This paper presents theoretical calculations and preliminary test results obtained to estimate the efficiency of using analog-to-time and time-to-digital converters for data transmission in analog pulse position modulated (PPM) optical communication system testbed designed in the time measurement lab. The structural design of an analog-to-time converter (PPM modulator) involves modified uniformly sampled pulse width modulation (PWM) technique. To encode information into the time domain, narrow-width pulses (PPM symbols or representative events) are generated at the intersections between uniformly sampled analog input signal and the falling half-waves of the continuous reference sine-wave. Information is transmitted as the analog time position of the single pulse in each PPM frame. It is assumed that complete synchronization between time-scales of transmitter and receiver is maintained at all times. Photo-detected PPM signal is digitized into time slots, by precise time measurement. The demodulation is performed by substituting the time slot values into expression of the reference sine-wave. High precision Event Timer A033-ET with single-shot measurement error less than 3ps rms is used for precise time registration. The standard deviation 8.4ps rms for the PPM time slot was experimentally obtained at the demodulator end. It corresponds to the resolution of 8.3 bits per symbol with data transfer rate of 83Mbps, when input signal range is 2.0V.