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Chapter 3, TECHNICAL DESCRIPTION
3-16
Auto-Phase
In an Auto-Phase operation the value of the signal phase is computed and an
appropriate phase-shift is then introduced into the reference channel so as to bring
the value of the signal phase to zero. The intended result is to null the output of the Y
channel while maximizing the output of the X channel.
Any small residual phase can normally be removed by calling Auto-Phase for a
second time, after a suitable delay to allow the outputs to settle.
The Auto-Phase facility is normally used with a clean signal that is known to be of
stable phase. It usually gives very good results provided that the X channel and Y
channel outputs are steady when the procedure is called.
If a zero error is present on the outputs, such as may be caused by unwanted coupling
between the reference and signal channel inputs, then the following procedure should
be adopted:-
1) Remove the source of input signal, without disturbing any of the connections to
the signal input which might be picking up interfering signals from the reference
channel. In an optical experiment, for example, this could be done by shielding
the detector from the source of chopped light.
2) Execute an Auto-Offset operation, which will reduce the X channel and Y
channel outputs to zero.
3) Re-establish the source of input signal. The X channel and Y channel outputs
will now indicate the true level of input signal, at the present reference phase
setting.
4) Execute an Auto-Phase operation. This will set the reference phase-shifter to the
phase angle of the input signal. However, because the offset levels which were
applied in step 2 were calculated at the original reference phase setting, they will
not now be correct and the instrument will in general display a non-zero Y
channel output value.
5) Remove the source of input signal again.
6) Execute a second Auto-Offset operation, which will reduce the X channel and Y
channel outputs to zero at the new reference phase setting.
7) Re-establish the source of input signal.
This technique, although apparently complex, is the only way of removing the effect
of crosstalk, which is not generally in the same phase as the required signal.
Auto-Offset
In an Auto-Offset operation the X offset and Y offset functions are turned on and are
automatically set to the values required to give zero values at both the X and the Y
outputs. Any small residual values can normally be removed by calling Auto-Offset
for a second time after a suitable delay to allow the outputs to settle.
The primary use of the Auto-Offset is to cancel out zero errors which are usually
caused by unwanted coupling or crosstalk between the signal channel and the
reference channel, either in the external connections or possibly under some
conditions in the instrument itself. Note that if a zero error is present, the Auto-
Offset function should be executed before any execution of Auto-Phase.
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