Chapter 3, TECHNICAL DESCRIPTION
3-16
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.
3.4 General
3.4.01 Accuracy
When the demodulator is operating under correct conditions, the absolute gain
accuracy of the instrument is limited by the analog components in the signal channel,
and the absolute phase accuracy is limited by the analog components in both the
signal channel and the reference channel. The resulting typical accuracy is ±1.0
percent of the full-scale sensitivity and ±0.5 degree respectively.
3.4.02 Power-up Defaults
When the rear panel CONFIG Switch 1 is set to 1, all instrument settings are
retained when the unit is switched off. When the instrument is switched on again the
settings are restored but with the following exceptions:-
a) The curve buffer is cleared.
b) Any sweep that was in progress at switch-off is terminated.
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