Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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Display Setup Menu 4-33
SR785 Dynamic Signal Analyzer
Command: MEAS(?) d {, i}
The following Measurements are available in the Octave Analysis Measurement Group:
Octave Spectrum
The octave spectrum 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.
Capture
The capture buffer stores sequential time domain data in memory. See ‘Capture’ in
Chapter 2 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 will
automatically pan as the capture fill and playback progress through the buffer. During
capture fill, if the capture buffer contains more points than can be displayed, points are
skipped. 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 Length 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 1/2.56 times the sample rate
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
distorted in the time record. However, ALL of the spectral information (up to 1/2.56
times the Sample Rate) is preserved by the Nyquist theorem as long as the value of each
sample is accurate.
Amplitude calibration is performed in the frequency domain. Hence, the captured time
data amplitudes are not calibrated.
User Functions
A User Function displays the results of a user defined math function. User Functions
defined within the Octave Analysis Measurement Group may include octave
measurement results. Use the [User Math] menu to define a function.
A User Function may not be selected as the measurement if it uses a Trace which is
empty or which contains data taken in a different Measurement Group.