Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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Display Setup Menu 4-29
SR785 Dynamic Signal Analyzer
Freq. Response = <FFT1* • FFT2>/<FFT1* • FFT1>
Peak Hold Average
Freq. Response = MAX(FFT2)/
√
<FFT1*• FFT1>
Both the RMS averaged and Vector averaged frequency response contains both
magnitude and phase information. The phase is the relative phase (at each frequency)
between the two channels. The RMS averaged frequency response is computed by taking
the ration of the cross spectrum to the input power spectrum, a technique called the tri-
spectral average.
Coherence
The coherence function is a two channel measurement defined as
Coherence = Mag
2
( RMSAvg( CrossSpec ) / ( Pwr1 • Pwr2 )
Averaging is always On and the Averaging Modes are defined by the measurement
above. The Type and Number Of Averages are still selected in the [Average]menu.
Coherence is a unitless real quantity and varies from 1.0 (perfect coherence) to 0.0 (no
coherence). Coherence measures the percentage of power in the response channel (2)
which is caused by (phase coherent with) power in the reference or input channel (1).
Ideally, a coherence of 1.0 means that the corresponding Frequency Response is
completely legitimate. All of the response power came from power at the input. If there
is noise or other signals generated from within the device under test (which is not related
to the input signal), it will result in a coherence of less than 1.0.
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. FFT measurements can use this stored data as input data by choosing Playback as
the Input Source in the [Input] menu.
The capture buffer often stores many time records. 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.