Stanford Research Systems SR865A, Operation Manual

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48 Basics Chapter 2
SR865A DSP Lock-in Amplifier
The X and Y offset, ratio and expand functions in the SR865A are output functions and
do not a
ffect the calculation of R or θ. R has its own output offset , ratio and expand. θ has
no offset or ratio or expand capability. To offset
θ
, simply use Auto Phase.
X, Y and R Display Offset and Ratio
Output offsets and ratios
are
reflected in the displays. For example, when the X output is
offset to zero, the displayed value will drop to zero also. This means that the bar graph
and numeric readout both drop to zero. In addition, if X is being charted on the graph, its
graph will drop to zero. Any display which is showing a quantity which is affected by a
non-zero offset will display a highlighted Offset indicator within its display. Similarly,
any display which is showing a quantity affected by a non-unity ratio factor will display a
highlighted Ratio indicator within its display. Remote queries of offset and/or ratioed
quantities are also affected by the offset or ratio.
X, Y and R Display Expand
Output expands do not increase the displayed numeric values of X, Y or R. Expand
increases the resolution of the displayed X, Y or R numeric value. This is because the
expand function increases the resolution of the output, not the size of the input signal.
The displayed value will show an increased resolution but will continue to display the
original value minus the offset. Any display which is showing a quantity which is
affected by a non-unity expand will display a highlighted Expand indicator within its
display.
Output expands affect the bar graphs. The bar graphs are simply a visualization of the
BNC outputs and as such are expanded to provide more visual resolution.
Output expands do not affect the strip charts. The values being charted are already
floating point numbers with all of the resolution available. The strip charts do reflect the
offsets however.
What is Dynamic Reserve Really?
Suppose the lock-in input consists of a signal at f
ref
plus noise at some other frequencies.
The real world definition of dynamic reserve is the ratio of the largest noise signal to the
actual signal (at f
ref
). This ratio is usually expressed in dB. For example, if the f
ref
signal is
1 µV and the noise reaches 1 mV, then the dynamic reserve is 60 dB (noise is 1000 times
the signal).
Dynamic reserve is therefore a property of the input signal generated by the experiment;
that is, the ratio of noise to signal at the BNC is determined by factors outside the lock-in.
It is the job of the lock-in to measure the signal to the best of its ability, whatever the
ratio of noise to signal at the input. Of course, it is always better to have less noise and
more signal!
Dynamic Reserve in the SR865A
Unlike most lock-ins, the SR865A does not have a dynamic reserve setting. As
mentioned above, the real world dynamic reserve is the noise to signal ratio at the input.
The SR865A is designed to achieve the best possible measurement as easily as possible.