  We report a novel radio autocorrelation (AC) search for extraterrestrial
intelligence (SETI). For selected frequencies across the terrestrial microwave
window (1-10 GHz) observations were conducted at the Allen Telescope Array to
identify artificial non-sinusoidal periodic signals with radio bandwidths
greater than 4 Hz, which are capable of carrying substantial messages with
symbol-rates from 4-1000000 Hz. Out of 243 observations, about half (101) were
directed toward sources with known continuum flux > ~1 Jy over the sampled
bandwidth (quasars, pulsars, supernova remnants, and masers), based on the
hypothesis that they might harbor heretofore undiscovered natural or
artificial, repetitive, phase or frequency modulation. The rest of the targets
were mostly toward exoplanet stars with no previously discovered continuum
flux. No signals attributable to extraterrestrial technology were found in this
study. We conclude that the maximum probability that future observations like
the ones described here will reveal repetitively modulated emissions is less
than 1% for continuum sources and exoplanets, alike. The paper concludes by
describing a new approach to expanding this survey to many more targets and
much greater sensitivity using archived data from interferometers all over the
world.
