Stanford Research Systems SR785, Operating Manual And Programming Reference

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2-62 User Math Functions
SR785 Dynamic Signal Analyzer
computed using whatever averaging is selected with the <Display Average> softkey.
They are computed from the input data (real time Analog or capture Playback) and use
the frequency and windowing parameters from the menus. Frequency domain
measurements are amplitude calibrated, time records are not. FFT and Time record
measurements are also triggered just like the normal measurements.
Operands which are measurement results which are not enclosed in angle brackets, such
as FFT1 or Time1 represent unaveraged, instantaneous versions of the measurement.
Operands which contain an explicit averaging type, such as Vec<F1> or PeakHold<F2>)
always are averaged according to their indicated type, regardless of the setting of the
<Display Average> softkey.
Octave and Swept Sine measurements are always averaged measurements. Use the
[Average] menu to set the averaging parameters.
FFT(1) and FFT(2) are the FFT of the Ch1 and Ch2 inputs. These operands use the
window chosen in the [Window] menu for the display which is measuring the function.
FFTu(1) and FFTu(2) are un-windowed FFT’s of the Ch1 and Ch2 inputs.
Correlation operands such as RMS<Fu1
•
F1> are used to compte the RMS and Vector
averaged auto and cross correlation measurements. The Fu refers to the un-windowed
FFT. See the descriptionof the Correlation measurements for exact definitions of the
correlation measurements in terms of these operands.
Trace operands are simply the data stored in the Traces. For example, Traces can hold
reference data used for normalization or calibration. There are 5 Traces which can be
stored. These Traces are shared by all 3 Measurement Groups.
Constants are complex constants which are the same for every array point. Constants
such as pi are commonly used in equations. There are 5 user defined constants which are
shared by all 3 Measurement Groups.
X Axis
Operands have an X axis type (frequency or time) as well as an X axis (array) length.
The X axis type of the User Function determines the X axis labeling of its display.
In general, the length of a User Function is determined by the length of the longest
operand array. Operations between operand arrays are performed on a point by point
basis, starting at the beginning of each array. Operands of different lengths (400 point
FFT1 and 800 point FFT1 stored in a Trace) can be combined in an User Function. The
math is performed over the longest operand with the shorter operand array being used in
a circular fashion. This usually leads to meaningless results.
Measurement operands (FFT(1), Time(1), etc.) and Traces have an X axis type, either
frequency or time domain. Arrays of frequency data start at the lowest frequency. Arrays
of time data start at time=0. A frequency domain operand (FFT(1)) can be added to a
time domain operand (Time(1)) point by point even though the result is meaningless. The
X axis type of a User Function is determined by examining its equation from left to right