Data Streaming Commands - Stanford Research Systems SR865A Operation Manual

140 Programming

Data Streaming Commands

Overview
See Appendix D for more detailed information about data streaming.
Data Streaming
The SR865A can stream data points continuously in real time over its ethernet interface.
This stream can be X values, (XY) or (Rθ) values, or (XYRθ) values. Streaming more
values uses more ethernet bandwidth. Data points are streamed as either floating point
values (4 bytes per data point) or integers (2 bytes per data point). Floating point
streaming uses twice the bandwidth.
Data is sent over the ethernet interface in UDP packets containing 132, 260, 516 or 1028
payload bytes. Missed packets are NOT resent. It is important that the receiving computer
be able to keep up with the data rate to prevent data loss and interruption.
Streaming Rate
The streaming rate sets how often data points are sampled and added to the stream. All
parameters are sampled at the same rate (and at the same times).The maximum allowed
streaming rate is determined by the time constant of the SR865A, or the signal frequency
(when the Sync filter is active). Shorter time constants allow faster streaming rates, up to
a limit of 1.25 MHz. The actual streaming rate can be set to the maximum allowed rate
divided by factors of 2.
Starting and Stopping
The STREAM ON/OFF
streaming parameters only take effect when streaming is turned back on. So if you want
to change e.g. the kind data being streamed, you need to turn streaming off, change the
data that is sent, and then turn streaming back on.
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 an integer multiple of the sample rate.
Generally, choose a streaming 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 streaming rate as much as possible.
STREAMCH(?) { X | XY | RT | XYRT | i }
The STREAMCH i
and θ (i=2) or X, Y, R and θ (i=3).
Example
STREAMCH 2
STREAMCH RT
SR865A DSP Lock-in Amplifier
command turns streaming on and off. NOTE: Changes to
command configures the data streaming to X (i=0), X and Y (i=1), R
Capture R and θ
Chapter 4