The early "adding" interferometers operated by adding two voltages, then detecting their sum using a square law detector with the result
This has the disadvantage that the autocorrelated signals are often orders of magnitude stronger than the
cross-correlated signals. In the 1950's, Ryle developed the technique of phase switching, in which the phase of one of
the voltages is periodically flipped, so that the signal flips back and forth rapidly between
and
