Ametek 7280 Instruction Manual page 70

Chapter 5, FRONT PANEL OPERATION
5-24
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.
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