Stanford Research Systems SR844, User Manual

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CH1 Display and Output 3-19
SR844 RF Lock-In Amplifier
Dividing the signal by an Aux Input less than 1.0 V is equivalent to multiplying the signal
by a value greater than 1.0. Small Aux Inputs can cause the ratioed outputs to overload.
The
UNDERFLOW indicator is on whenever the ratio Aux Input falls below
±
50 mV.
In both cases it is necessary to change the setup. In general, the Aux Input ratio signal
should be conditioned to be close to
±
1 V.
Expand
This key selects the Output Expand for the current CH1 display quantity. This function
expands the display quantity by
×
1 (no expand), x10 or x100 and expands both the
display and on the corresponding analog CH1 OUTPUT. The Expand for the displayed
quantity is shown by
x10
and
x100
indicators above the
Expand
key. Neither indicator
is on when the expand is
×
1.
Expand amplifies the CH1 analog output by
×
10 or
×
100. If the output overloads, the
OVLD indicator above the output BNC turns on.
The value shown on the display remains the same, but is shown with greater resolution; 1
extra digit at
×
10, or 2 digits at
×
100. The
Expand
indicator within the display is on
whenever the display is expanded. Expanding a quantity can cause the display to
overload, indicated by
OVLD
within the display.
Expand is an output function and has no effect on the internal values used for
computation, i.e. expanding X will
not affect R and
θ
. Expand is applied after offsets and
ratios.
The typical use of the Expand function is in conjunction with the Offset function, to
magnify variations of the measured quantity about a nominal value. Remember, in order
to expand a quantity it must be less than 10% (
×
10) or 1% (
×
100) of full scale.
Example : Suppose the X component of the input is 12.345 mV. The SR844 is set to
100 mV sensitivity with Offset, Ratio and Expand off. The CH1 display reads
X=12.34 mV. The analog CH1 OUTPUT is 1.234 V (10 V is full scale). The offset is
now turned On and set to –10%. Since the offset is a percentage of 100 mV full scale, the
offset is -10 mV and the display now reads 2.34 mV. The
XYOffs
indicator within the
display turns on. The analog CH1 OUTPUT is now 0.234 V. If Expand is turned on at
x10
, the display will read 2.345 mV (extra resolution) and the analog CH1 OUTPUT will
be 2.345 V (amplified).
Output
This key switches the analog CH1 OUTPUT between the
DISPLAY
quantity and
X
.
When set to Display, the analog CH1 OUTPUT provides a signal proportional to the
Display quantity (as selected by the
Display
key). An output of
±
10 V corresponds to
±
full scale on the display. The CH1 output has the same offset/ratio/expand that is
applied to the display.
When set to X, the CH1 analog CH1 OUTPUT provides a signal proportional to X. An
output of
±
10 V corresponds to
±
full scale sensitivity. The output has the
offset/ratio/expand that was selected for X, either from the front panel keys or via remote
interface. Note that if the Output is set to X and the Display is set to
another quantity,
their Expands may be different, and there is no indication of the output expand for the
analog output X.
You cannot have different offset/ratio/expand for X on the display and X on the analog
output. But you
can have different offset/expand for R (on display) and X (analog out).