Over the past few years there has been a considerable effort in the
superconductor electronics community to demonstrate a competitive
digital auto- or cross-correlator for radioastronomy applications
[46, 47, 48, 49]. A 256-stage
(4350 Josephson junctions) correlator was reported in
[46]. Being a remarkable demonstration of the SFQ
technology, this design does not fully employ its potential because it
uses global clocking and analog output which limits the high-frequency
performance and severely reduces the accuracy. Also, the complete
verification of the correlator was not performed due to the
limitations of the experimental setup. In [47] a
possible design of a 1024-stage correlator (comprising 512,000
Josephson junctions) for space-borne sub-mm receiver is presented. One
of disadvantages of this project is the absence of double oversampling
which is essential for 1-bit approach [28] because it
significantly decreases the minimum integration time. Another design
with global clocking of the delay line based on a circular shift
register was recently proposed in [48] and further
improved in [49] (see Chapter 
), where only
prescaling of channel outputs is done on-chip while accumulation is
performed by room-temperature electronics. For the current fabrication
technology [7] the resulting simplification of the delay
line and channel counters offers an advantage as reducing the
Josephson junction count while future technologies (see, e.g.,
[2]) might make asynchronous designs with on-chip
accumulation more attractive as offering a potentially better
high-frequency performance and simpler room-temperature interfaces.
In this chapter we present a detailed description of the design of an
all-digital asynchronous RSFQ correlator and report experimental
results for its major building blocks implemented using HYPRES'
- 
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Nb-trilayer technology.