Stanford Research Systems SR780, Operating Manual And Programming Reference

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2-50 User Math Functions
SR780 Network Signal Analyzer
Operands
Operands are arrays of complex values (real and imaginary). They may be measurement
results (such as FFT(1), Time(1) or Oct(1)), a Trace, or a Constant. An array which is real
simply has zero for its imaginary parts. The array length of an Operand is determined by
the measurement length (number of FFT lines, length of time record, number of octaves,
etc.) or Trace length (length of the data which is stored in the Trace). Constants assume
the length of the user function.
Operands which are measurement results, such as FFT(1), Oct(1) or Spec(1) are exactly
the same as the normal measurements. 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.
Thus, vector averaging can be applied to these operands.
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.
FFTa(1) is the FFT of the Ch1 input using the window for Display A, regardless of which
display is measuring the function. FFTb(2) is the FFT of the Ch2 input using the window
for Display B. This allows a function to be defined which uses both input channels, each
with a different window. Force-exponential transfer function for example.
FFTu(1) and FFTu(2) are un-windowed FFT’s of the Ch1 and Ch2 inputs.
Octave and Swept Sine measurements are always averaged measurements. Use the
[Average] menu to set the averaging parameters.
Auto_Corr(1), Auto_Corr(2), X_Corr, <F2>/<F1> and Coherence are all averaged FFT
measurements, regardless of whether averaging is On or Off in the [Average] menu. The
Averaging Mode is determined by the definition of the measurement. The Averaging
Type (linear or expo) and the Number of Averages are set in the [Average] menu.
FFT, Time, X_Spec and FFT2/FFT1 are NOT averaged measurements. Use the Avg() or
VecAvg() operators to perform explicit averaging on these operands.
Trace operands are simply the static 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 constants which can be defined.
These constants 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