The discussion above is based around a single-phase
instrument. A development of this is the dual-phase lock-in
amplifier, which is not, as some people think, a dual channel
unit. Rather it incorporates a second phase-sensitive detector,
which is fed with the same signal input as the first but which is
driven by a reference signal that is phase-shifted by 90 degrees.
This second detector is followed by a second output filter and
amplifier, and is usually referred to as the “Y” output channel.
The original output being referred to as the “X” channel.
An advantage of the dual-phase unit is that if the signal
channel phase changes (but not its amplitude) then although
the output from one detector will decrease, that from the second
increases. It can be shown, however, that the vector
magnitude, R, remains constant, where:- R = Ö(X2
+ Y2
)
Hence if the lock-in amplifier is set to display R, changes in the
signal phase will not affect the reading and the instrument does
not require the adjustment of the reference phase-shifter circuit.
This capability has led to the dual-phase instrument becoming
by far the most common type of unit