Ametek 7124 Instruction Manual page 80

Chapter 5, FRONT PANEL OPERATION
5-26
AUTO SENSITIVITY
This function only operates when the reference frequency is above 1 Hz. A single
Auto-Sensitivity operation consists of increasing the full-scale sensitivity range if the
magnitude output is greater than 90% of full-scale, or reducing the range if the
magnitude output is less than 30% of full-scale. After the Auto-Sensitivity function
is called, Auto-Sensitivity operations continue to be made until the required criterion
is met.
In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
necessarily what is really required.
AUTO PHASE
In an Auto-Phase operation the value of the signal phase with respect to the reference
is computed and an appropriate phase-shift is then introduced into the reference
channel so as to bring the difference between them 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 difference 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 which 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 instrument 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.