  Correlation radiometers make true differential measurements in power with
high accuracy and small systematic errors. This receiver architecture has been
used in radio astronomy for measurements of continuum radiation for over 50
years; this article examines spectroscopy over broad bandwidths using
correlation techniques. After general discussions of correlation and the choice
of hybrid phase, experimental results from tests with a simple laboratory
multi-channel correlation radiometer are shown. Analysis of the effect of the
input hybrid's phase shows that a 90 degree hybrid is likely to be the best
general choice for radio astronomy, depending on its amplitude match and phase
flatness with frequency. The laboratory results verify that the combination of
the correlation architecture and an analog lag correlator is an excellent
method for spectroscopy over very wide bandwidths.
