Stanford Research Systems SR865A Operation Manual page 65

Chapter 2
θ ranges from −180° to +180° regardless of the sensitivity or range. When CH2 outputs a
voltage proportional to θ, the output scale is 18°/Volt or 180°=10 V. The phase bar graph
and numeric readout scales are also unaffected by the Sensitivity.
X, Y and R Output Offset, Ratio and Expand
The SR865A has the ability to offset the X, Y and R outputs. This is useful when
measuring deviations in the signal around some nominal value. The offset can be set so
that the output is offset to zero. Changes in the output can then be read directly from the
display or output voltages. The offset is specified as a percentage of the Sensitivity and
the percentage does not change when the Sensitivity is changed. Offsets up to ±999% can
be programmed.
The X, Y, and R outputs may also be rescaled with a ratio function. This allows signals to
be normalized to some experimental parameter being monitored with an auxiliary input
voltage. Whenever enabled, ratio is indicated by the
the display.
The X, Y and R outputs may also be expanded. This simply takes the output (minus its
offset) and multiplies by an expansion factor. Thus, a signal which is only 10% of
sensitivity can be expanded to provide 10 V of output rather than only 1 V. The normal
use for expand is to expand the measurement resolution around some value which is not
zero. For example, suppose a signal has a nominal value of 0.9 mV and we want to
measure small deviations, say 10 µV or so, in the signal. The Sensitivity of the lock-in is
set to 1 mV to accommodate the nominal signal. If the offset is set to 90% of full scale,
then the nominal 0.9 mV signal will result in a zero output. The 10 µV deviations in the
signal only provide 100 mV of dc output. If the output is expanded by 10, these small
deviations are magnified by 10 and provide outputs of 1 VDC.
The SR865A can expand the output by 10 or 100 provided the expanded output does not
exceed 10 V. In the above example, the 10 µV deviations can be expanded by 100 times
before they exceed full scale (at 1 mV Sensitivity).
The analog output with offset, ratio and expand is
where Offset is a fraction of 1 (50%=0.5), Expand is 1, 10 or 100, and the output cannot
exceed 10 V. When enabled, Ratio In is the voltage on either Aux 3(for X or Y) or Aux 4
(for R); if disabled, the Ratio In factor is simply 1. In the above example (without ratio),
Output = (0.91mV/1mV − 0.9) × 10 × 10V = 1V
for a signal which is 10 µV greater than the 0.9 mV nominal. (Offset = 0.9 and
expand = 10).
ª
X, Y, or R
«
Sensitivit
«
Output
§
«
Ratio
¨
«
©
¬
1.000
RATIO
º

Offset
»
y
»
u
Expand
·
»
In
¸
»
¹
¼
V
Basics
LED and "Ratio" labels on
u
10V
SR865A DSP Lock-in Amplifier
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