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User’s Manual
Data Span
Usually the entire simulated time span is used for the fourier analysis. To specify a
smaller time interval click Specify and enter the start and end times.
Note that if you specify a fundamental frequency, the time may be modified so that a
whole number of cycles is used. This will occur whether or not you explicitly specify
an interval.
Window
A window function is applied to the time domain signal to minimise spectral leakage
(See above).
The choice of window is a compromise. The trade off is between the bandwidth of the
main spectral component or lobe and the amplitude of the side-lobes. The rectangular
window - which is in effect no window - has the narrowest main lobe but substantial
side-lobes. The Blackman window has the widest main lobe and the smallest side
lobes. Hanning and Hamming are something in between and have similar main lobe
widths but the side lobes differ in the way they fall away further from the main lobe.
Hamming starts smaller but doesn't decay whereas Hanning while starting off larger
than Hamming, decays as the frequency moves away from the central lobe.
Despite the great deal of research that has been completed on window functions, for
many applications the difference between Hanning, Hamming and Blackman is not
important and usually Hanning is a good compromise.
There are situations where a rectangular window can give significantly superior
results. This requires that the fundamental frequency is specified and also that the
simulated signal is consistent over a large number of cycles. The rectangular window,
however, usually gives considerably poorer results and must be used with caution.
Probing Busses
It is possible to probe a bus in which case a plot representing all the signals on the bus
will be created. Usually this will be a numeric display of the digital bus data, but it is
also possible to display the data as an analog waveform. Buses may contain either
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