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Chapter 4
Programming
135
below). This can be useful in situations when an external event determines the end of
when data should be captured (i.e., post-triggering).
When data capture is stopped with the CAPTURESTOP command or by hardware
trigger, zeros are written to the remaining portion of the current 2 kbyte block until it is
filled. Data is thus always acquired in 2 kbyte blocks, and the point wherecapture is
stopped is where the data becomes zero. The actual number of bytes of real data captured
(excluding the zero fill) is reported by the CAPTUREBYTES? query.
Capture Modes
Data capture is started with the CAPTURESTART command. OneShot or Continuous
acquisition is specified, as well as the capture Mode. There are three possible Modes:
Immediate start, externally triggered start or stop, and one-sample-per-trigger mode.
Immediate start mode begins capturing data immediately. Capture stops when the buffer
is filled (OneShot) or when the CAPTURESTOP command is received.
In many situations, it is important to synchronize the beginning or end of data acquisition
to an external event. The SR865A supports this with the rear-panel TRIG IN input BNC.
To enable a hardware trigger to start (but not stop) capture, configure with
CAPTURESTART set for OneShot and external trigger. To enable a trigger to stop
capture (i.e. post-trigger), configure with CAPTURESTART for Continuous and external
trigger. In both cases, a falling edge at the BNC input triggers the start or stop of capture.
The TRIG IN BNC has a 10 kΩ internal pull-up resistor to +3.3 V, giving users flexibility
around interfacing.
In one-sample-per-trigger mode mode, a new sample is captured every falling edge of the
external trigger. The data capture rate is determined by the rate of external triggers, and
need not be periodic. The external trigger rate should not exceed the maximum internal
data rate of the SR865A reported by the CAPTURERATEMAX? query. One-sample-per-
trigger capture is stopped when the buffer is filled (OneShot) or when the
CAPTURESTOP command is received.
Once capture has stopped, data in the buffer can be read over the remote interfaces. The
CAPTURESTAT?
query returns the state of the capture. The CAPTUREBYTES? query
returns the number of data bytes held in the capture buffer (excluding zero fill). These
commands may also be used while capture is active to monitor the progress of the
capture.
Aliasing Effects
In any sampled data stream, it is possible to sample a high frequency signal such that it
will appear to be a much lower frequency. This is called aliasing.
Aliasing occurs whenever the signal being sampled contains signals at frequencies
greater than 1/2 the sample rate. The effect is most noticeable when trying to sample an
output frequency at or near an integer multiple of the sample rate.
Generally, choose a capture rate higher than the time constant bandwidth. The lock-in
time constant and filter slope should be chosen to attenuate outputs at frequencies higher
than 1/2 the capture rate as much as possible.
SR865A DSP Lock-in Amplifier
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