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4-22
saved as well. When a trace is recalled from disk,
the marks are recalled as well. Existing marks are
replaced with the recalled marks.
[CURSOR MAX/MIN]
Pressing [CURSOR MAX/MIN] will center the
cursor region around the maximum or minimum
data value on the active chart display. The Cursor
Seek mode in the CURSOR SETUP menu choos-
es whether this key finds the max or min. If the
cursor seeks the mean, then the [CURSOR
MAX/MIN] key finds the maximum on-screen
point. The cursor will be positioned at the Min,
Max, or Mean of the data within the region,
depending upon the cursor seek mode. The
[CURSOR MAX/MIN] key only searches the data
which is on the screen. If the max/min value
occurs at more than one location, then the one
closest to the left edge is found.
[AUTO RESERVE]
Pressing [AUTO RESERVE] will adjust the dynam-
ic reserve to the minimum reserve required. To do
this, the reserve is decreased until a RESRV over-
load is detected, indicating that the analog input
amplifier is overloaded. The reserve is then
increased to remove the overload.
Auto Reserve will work only if the overloading
noise source has a frequency greater than a few
Hz. Lower frequency noise sources may overload
so infrequently that Auto Reserve can not detect it.
[AUTO RESERVE] does not change the notch pre-
filter settings.
[AUTO GAIN]
[AUTO GAIN] will adjust the sensitivity so that the
detected signal magnitude is a sizable percentage
of full scale. If the present value of R is a very
small percentage of full scale when [AUTO GAIN]
is pressed, then Auto Gain will respond very quick-
ly. If R is very much greater than full scale, then
Auto Gain may take several time constants to
reach the optimum gain setting. Auto Gain will
stop before reaching the optimum sensitivity if a
RESRV overload is detected.
[AUTO GAIN] does not adjust the time constant or
roll-off.
[AUTO PHASE]
[AUTO PHASE] adjusts the reference phase shift
so that the measured signal phase is 0°. This is
done by subtracting the measured value of θ from
the programmed reference phase shift. It will take
several time constants for the outputs to reach
their new values during which time θ will move
towards 0°. Do not press [AUTO PHASE] again
until the outputs have stabilized. When the meas-
urement is noisy or if the outputs are changing,
Auto Phase may not result in a zero phase.
AUTO SETUP
There is no truly reliable way to automatically
setup a lock-in amplifier for all possible input sig-
nals. In most cases, the following procedure
should setup the SR850 to measure the input
signal.
1. Press [AUTO GAIN] to set the sensitivity.
2. Press [AUTO RESERVE].
3. Adjust the time constant and roll-off until
there is no FILTR overload.
4. Press [AUTO PHASE] if desired.
5. Repeat if necessary.
At very low frequencies, the auto functions may
not function properly. This is because very low fre-
quency signals overload very infrequently and the
time constants used tend to be very long.
[AUTOSCALE]
Chart displays and bar graphs may be Auto
Scaled. Auto Scaling only affects the displays - the
data values and the front and rear panel analog
outputs are not affected.
Pressing [AUTOSCALE] when a bar graph is the
active display sets the range of the graph so that
the present reading will be a major percentage of
the range. The center will always be set to zero. If
the data is rapidly changing, then Auto Scaling
may not result in the optimum scaling. Both the
range and center may be manually adjusted in the
DISPLAY/SCALE menu.
Keypad