Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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2-42 Octave Analysis
SR785 Dynamic Signal Analyzer
Center Frequency
n
= × ×








1000 2 2
1
24 12
for n = -160 .. 43
Octave Measurements
Octave Analysis is a Measurement Group. Choose the Measurement Group within the
[Display Setup] menu. The available measurements are rms averaged Octave spectrum
(power spectrum), Capture Buffer and User Function.
Octave
This is the basic measurement of octave analysis. It is simply the rms averaged outputs
from the parallel bank of octave filters (also called the power spectrum). The spectrum
gives a stable reading of the rms signal amplitudes and noise levels within each band.
RMS averaging results in a real spectrum and there is no phase information.
The number of Octave Channels limits the measurement choices of both displays.
If 2 Octave Channels are selected, then both inputs may be measured. The Measurements
of both displays are independent. The Highest Band for all Octave Resolutions is
decreased by 2 in this case.
If 1 Octave Channel is selected, then only one input may be analyzed. If both displays are
making an octave measurement, they must both use the same input and frequency range.
Changing the input or frequency range of the active display will change the other display
if necessary. Choosing 1 Octave Channel may change the current measurements so that
both displays use the same input.
Capture
The capture buffer stores sequential time domain data in memory. See ‘Capture Buffer’
for more details. The Capture measurement displays the contents of the capture buffer.
Octave measurements can use this stored data as input data by choosing Playback as the
Input Source in the [Input] menu.
The capture buffer is often very long. To graphically expand a region of the display, use
the Pan and Zoom functions in the [Display Setup] menu. The capture buffer display can
automatically pan with the capture fill or playback progress through the buffer. During
capture fill, if the capture accumulates points faster than they can be displayed, some
points are not shown. This speeds up the display update so that it keeps up with the real
time capture but allows visual aliasing to occur. Once capture is complete, the display is
redrawn showing the envelope of all points, eliminating any visual aliasing effects.
To measure from a region of the buffer, set the Playback Start and Stop in the [Input]
menu.
The capture data is filtered and down-sampled according to the capture sample rate. Only
baseband data (data bandwidth starts at DC) are captured. The capture buffer resembles a
digital oscilloscope display. Signals at frequencies above the sample rate/2.56 have been
filtered out.
The capture buffer is not a continuous representation of the input signal. The data is
sampled and has a time resolution of 1/(sample rate). High frequency signals will appear