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5-19
Chapter 5, FRONT PANEL OPERATION
are steady, implying the signal phase is stable, 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
signal. 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 and Y outputs to zero.
3) Re-establish the source of input signal. The X and Y channel outputs will now
indicate the true level of the 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 output
value.
5) Remove the source of input signal again.
6) Execute a second Auto-Offset operation, which will reduce the X and Y 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 effectively removing
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 levels required to give zero values at both the X and the Y
outputs. Any small residual values remaining after the initial Auto-Offset operation
can normally be removed by calling the function 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.
AUTO MEASURE
This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations: