Fast RSFQ and ERSFQ Parallel Counters

Historically one of the most challenging high-speed rapid single flux quantum circuits to implement has been a parallel counter that sums a set of unweighted inputs and produces a binary-weighted word at the same clock rate. A 7-to-3 parallel counter that sums seven inputs has been designed and tested at the target clock frequency of 40 GHz and at frequencies up to 50 GHz using its own dedicated testbed. Yielded in both 10- and 20-kA/cm2 current densities using MIT Lincoln Laboratory's foundry, this 7-to-3 summing circuit has become a digital circuit benchmark. Most recently, a version with 15 parallel inputs producing a 4-bit output at the same target frequency was designed by combining 2 variants of 8-to-4 parallel counters. The first variant based on the 7-to-3 parallel counter, sums eight unweighted inputs, whereas the second variant sums two 4-bit binary-weighted words by pairwise summing of bits of equal weights. Design considerations for scaling this circuit will be discussed together with the circuit performance and yield.