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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 MEASURE
This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations:
The instrument is set to AC-coupled and input FLOAT mode. If the reference
frequency is more than 10 Hz the output time constant is set to 10 ms, otherwise it is
set to the lowest synchronous value, the filter slope is set to 12 dB/octave, output
expand is switched off, the reference harmonic mode is set to 1, the X offset and Y
offset functions are switched off and the Auto-Sensitivity and Auto-Phase functions
are called. The Auto-Sensitivity function also adjusts the AC Gain if required.
The Auto-Measure function is intended to provide a means of setting the instrument
quickly to conditions which will be approximately correct in typical simple
measurement situations. For optimum results in any given situation, it may be
convenient to start with Auto-Measure and to make subsequent modifications to
individual controls.
NOTE: The Auto-Measure function affects the setting of the AC Gain and AC
Gain Automatic controls during execution. Consequently, it may not operate
correctly if the AC Gain Automatic control is turned off. In this case, better results
will be obtained by performing Auto-Sensitivity followed by Auto-Phase functions.
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 channel and
Y channel 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.
Chapter 5, FRONT PANEL OPERATION
5-27
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