Ametek 7280 Instruction Manual page 101

Chapter 5, FRONT PANEL OPERATION
This menu has six controls affecting the virtual reference mode, a single display, and
two keys to establish virtual lock. The left-hand side of the display is used for the
semi-automatic method of establishing Virtual Reference mode, with the right-hand
side being used for the manual mode.
The controls operate as follows:-
Semi-Automatic Method
START FREQUENCY
This control defines the start frequency from which the instrument will begin to try to
find the applied signal. It should be set to a value close to, but less than, the expected
signal frequency. If the start frequency is within 100 Hz of the signal frequency then
there is no need to set the stop frequency control. Adjusting the Start Frequency
control also sets the present Oscillator Frequency to the same value.
STOP FREQUENCY
This control defines the stop frequency at which the instrument abandons its search
for the signal, and should be set to a value greater than the expected signal frequency.
If the start frequency is within 100 Hz of the signal frequency then there is no need to
set this control.
STEP SIZE
This control defines the frequency step size used during the automatic search.
TIME/STEP
This control defines the time per point within the automatic search.
START AUTO SEARCH
Pressing the Start Auto Search key starts the internal oscillator at the defined start
frequency and increases it at a rate of 1 Hz per second. If the instrument detects a
signal greater than 50% of full-scale then the search stops, the unit locks on to that
signal, and the Main Display - Virtual Reference Mode appears, as shown in
figure 5-34.
Manual Setup
If the semi-automatic setup method does not work because the signal requires a time
constant longer than 100 ms then the initial search can be performed manually. This
can be done using the controls on the right-hand side of the display, and the Start
Frequency control on the left-hand side.
The controls operate as follows:-
TIME CONSTANT
This control sets the instrument's output time constant.
SENSITIVITY
This control should be set so that the amplitude of the expected signal will be at least
50% of full-scale. For example, if the signal is expected to be about 10 µV then set
the full-scale sensitivity to be either 5 µV or 10 µV.
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