Stanford Research Systems SR844, User Manual

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2-16 SR844 Basics
SR844 RF Lock-In Amplifier
expand is to expand the measurement resolution around some value which is not zero. For
example, suppose a signal has nominal value of 0.9 mV and we want to measure small
deviations, say 10
µ
V or so, in the signal. The sensitivity of the lock-in needs to be 1 mV
to accommodate the nominal signal. If the offset is set to -90% of full scale, the nominal
0.9 mV signal will result in a zero output. The 10
µ
V deviations in the signal only provide
100 mV of output. If the output is expanded by 10, these small deviations are magnified
by 10 and provide 1 V of output.
The SR844 can expand the output by 10 or 100 provided the expanded output does not
exceed full scale. In the above example, the 10
µ
V deviations can be expanded by 100
times before they exceed full scale (1 mV sensitivity).
Outputs proportional to quantities measured in dBm (R[dBm] and Ynoise[dBm]) may
also be expanded. The expanded output scales are 2 dB/V and 0.2 dB/V for
x
10 and
x
100
expands.
The phase output may also be expanded on CH2. The expanded output scales are 1.8
°
/V
and 0.18
°
/V for x 10 and x 100 expands. The phase output can not be offset. Instead use
the Reference phase to adjust the detection phase to yield a measured phase of zero.
Display Scales
Offsets are reflected in the displayed values. For example, if CH1 is displaying X, the X
offset is applied to the displayed value. When X is offset to zero, the displayed value will
drop to zero also. Any display which is showing a quantity which is affected by XY
offsets will display a highlighted
XYOffs indicator below the value. If the quantity is
affected by an R offset, the ROffs indicator will be on. Note that both indicators may be
on at the same time.
Output expands do not increase the displayed values. Expand increases the resolution of
the displayed value (not the size of the displayed signal). When X is expanded, the
display is shown with more digits but has the same non-expanded value. Any display
which is expanded will display a highlighted
Expand
indicator below the value.