Design and operation of a pulse-number multiplier for a high-precision RSFQ D/A converter

A pulse-number multiplier (PNM) is a principal subsystem of our rapid single flux quantum digital-to-analog converter for metrology applications. The PNM circuit generates a high-speed pulse train from an input of a precise low-frequency clock signal. We designed, fabricated, and successfully tested a PNM test circuit. The PNM test circuit was implemented by using our standard cell library on 1.6 kA/cm/sup 2/ Nb trilayer technology. Fully functional operation was confirmed by low-speed testing. Experimental bias margins were wider than /spl plusmn/20%, and these results agreed with a simulation. We also tested the circuit by average-voltage measurements for estimating the frequencies of output pulse trains. Results of average-voltage measurements implied that the output frequency of the PNM circuit was tunable from 5 GHz to 17 GHz. The bit error rate (BER) of the PNM circuit was measured, and extrapolation of the error-function fits suggested a sufficiently small BER at the optimum bias point.

I. Introduction

Rapid single flux quantum (RSFQ) circuits can simultaneously carry out ultrafast digital processing and metrologically precise quantization and multiplication of electrical signals. We are developing a high-precision RSFQ-based digital-to-analog (D/A) converter system for metrology applications [1]. The D/A converter is designed to operate with a 10-MHz reference clock from an external stabilized signal source. The 10-MHz clock is internally converted by the pulse-number multiplier (PNM) into an effective 10-GHz train of single flux quantum (SFQ) pulses, and then into a voltage corresponding to a set of input digital codes. This approach does not require external microwave equipment, so the whole system becomes simple and inexpensive. The PNM circuit is a principal subsystem of the D/A converter. In order for the D/A converter to generate an exact voltage, the PNM circuit needs to operate correctly.