Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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2-56 Order Analysis
SR785 Dynamic Signal Analyzer
The combination of Maximum Order and Delta Order also determine the length and
resolution of the Order time records. The resolution of the time record, in revolutions, is
given by: Resolution = 1/(5.12
•
Effective Max Order), where Effective Max Order is
equal to Delta Order • 400, ( if Max Order/Delta Order > 200), Delta Order • 200, ( if
100 < Max Order/Delta Order <= 200), Delta Order • 100, ( if 50 < Max Order/Delta
Order <= 100), or Delta Order • 100, (If MaxOrder/Delta Order <=50).
The length of the time record, in revolutions, is 1/(Delta Order).
The combination of these parameters also determines the Capture Sampling Rate. While
in other measurement groups the Capture Sampling Rate is determined explicitly by the
user with the sampling rate softkey. In the order measurement group, the sampling rate is
given by: Sampling Rate = 5.12
•
Effective Max Order, where the sampling rate is then
rounded up to the nearest integer submultiple of 256 kHz.
Order Measurements
Linear Spectrum
The Linear Spectrum in the order measurement group is similar to the FFT measurment
in the FFT measurement group except that the x-axis is calibrated in orders instead of
absolute frequency. The Linear Spectrum includes both magnitude and phase
information. Phase in the Linear Spectrum is measured relative to the tachometer pulse
unless an external trigger is used in which case the phase is measured relative to the
trigger.
Power Spectrum
The Linear Spectrum in the order measurement group is similar to the Power Spectrum
measurment in the FFT measurement group except that the x-axis is calibrated in orders
instead of absolute frequency. The power spectrum gives the power present in each
spectral order, no phase information is included.
Time Record
The time record is not similar to the time record measurement in the FFT measurement
groups. Time Records in the fft group are measurements of the input signal at fixed time
intervals. In the Order measurement group, time records are measurements of the input
signal at fixed angular positions around the shaft. Since the analyzer only samples the
input at fixed time intervals, the order time record must be calculated by combining the
fixed interval sampling with information from the tachometer about where the shaft
position was relative to the sampled data.
The x-axis of the order time record is calibrated in revolutions, the resolution and
number of resolutions in the time record are determined from the Maximum Order and
Delta Order softkeys.
Windowed Time Record
The windowed time record shows the Time Record multiplied by the Window function.
Remember that in the order measurement group you should use the uniform window if
the expected harmonics are integer multiples (orders) of the rotating machine speed.