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Trace and Scan Menu
The A, B and C parameters may be set to the quantities X, Y, R, θ,
Xnoise, Ynoise, Rnoise, Aux Inputs 1-4 (on the rear panel), Frequency,
and unity. C may also be any quantity squared. X, Y, R and θ are the
normal lock-in output quantities. Frequency is useful when the reference
is unknown or changing (as in an external sweep). The rear panel aux
inputs can digitize a slowly varying signal with16 bits of resolution to
monitor external parameters which affect the lock-in measurement. The
noise values are calculated by the SR850 and represent the noise of the
signal in the equivalent noise bandwidth of the low pass filter time con-
stant. See the SR850 Basics section for more about noise
measurements.
Normal measurements rarely require a trace other than simply X, Y, R or
θ. Ratio normalization to an external voltage requires a trace defined as
R/Aux Input 1 for example. Noise or frequency may be displayed by
defining a trace as Xnoise or Frequency.
Store/Do Not Store Pressing this key selects whether the trace selected above will be stored
in the data buffer or not. If store is chosen, when the scan begins, trace
data will be stored in the data buffer at the sample rate. The data buffer
can record up to 64000 data points in memory. This buffer can be 64000
points of a single trace, 32000 points of two traces, or 16000 points of all
four traces.
Data is stored only while data acquisition is in progress. The [START/
CONT] key starts a data scan. The Run or Run 1 scan indicators are
turned on in the status bar at the bottom of the screen.
Only traces that are stored may be displayed in strip chart form.
See the SR850 Operation section for more about trace scans.
Sample Rate This key selects the Sample Rate as the active field. Use the knob to
adjust the sample rate. The sample rate can be varied from 62.5 mHz (1
point every 16 sec.) up to 512 Hz. Increasing the sample rate past
512 Hz will select Triggered samples. In this mode, a sample is recorded
within 2 ms of a rising edge trigger on the rear panel Trigger input. Trig-
gers which occur faster than 512 Hz are ignored. When the sample rate
is set to Triggered, the chart displays assume a sample rate of 1 per
second for scaling purposes. Simply read the graphs as triggers per divi-
sion instead of seconds per division.
The sample rate sets how often points are added to the storage buffers.
All stored traces are sampled at the same rate (and at the same times).
Swept parameters (internal oscillator frequency or Aux Outputs) are syn-
chronized with the data acquisition. Swept parameters change once per
sample (even if no traces are being stored). The parameter is updated
after the data points for each sample are taken. The settling time for
each point is thus 1/(sample rate).