Chapter 7
473
Language Reference
FFT Fast Fourier Transform
The FFT algorithm assumes that the sampled signal is periodic with an
integral number of periods within the time-record length (that is, the
sweep time of the analyzer). Given this assumption, the transform
computed is that of a time waveform of infinite duration, formed of
concatenated time records. In actual measurements, the number of
periods of the sampled signal within the time record may not be
integral. In this case, there is a step discontinuity at the intersections of
the concatenated time records in the assumed time waveform of infinite
duration. This step discontinuity causes measurement errors, both
amplitude uncertainty (where the signal level appears to vary with
small changes in frequency) and frequency resolution (due to filter
shape factor and sidelobes). Windows are weighing functions that are
applied to the input data to force the ends of that data smoothly to zero,
thus reducing the step discontinuity and reducing measurement errors.
Three types of windows are available, using the TWNDOW command.
FLATTOP
provides optimum amplitude accuracy.
HANNING
provides an amplitude accuracy and frequency
resolution compromise, which is useful for general
purpose measurements, as well as noise
measurements.
UNIFORM
provides equal weighing of the time record for
measuring transients.
Some important parameters of the three available windows are shown
in the table below. In the bandwidth entries, multiply the entry by
one-divided-by-sweeptime (1/sweeptime).
FLATTOP
HANNING
UNIFORM
Noise equivalent bandwidth
3.63
×
1.5
×
1
×
3 dB bandwidth
3.60
×
1.48
×
1
×
Sidelobe height
<
−
90 dB
−
32 dB
−
13 dB
Amplitude uncertainty
0.10 dB
1.42 dB
3.92 dB
Shape factor (60 dB BW/3 dB BW)
2.6
9.1
>300
Summary of Contents for 8560E
Page 21: ...21 1 Quick Start Guide ...
Page 48: ...48 Chapter1 Quick Start Guide Manuals Available Separately ...
Page 49: ...49 2 Making Measurements ...
Page 164: ...164 Chapter2 Making Measurements Example 13 Making Pulsed RF Measurements ...
Page 165: ...165 3 Softkey Menus ...
Page 182: ...182 Chapter3 Softkey Menus Menu Trees ...
Page 183: ...183 4 Key Function Descriptions ...
Page 228: ...228 Chapter4 Key Function Descriptions Key Descriptions Figure 4 3 CRT Alignment Pattern ...
Page 289: ...289 5 Programming ...
Page 323: ...Chapter 5 323 Programming Math Functions Figure 5 10 Display Units ...
Page 339: ...339 6 Programming Command Cross Reference ...
Page 369: ...369 7 Language Reference ...
Page 654: ...654 Chapter7 Language Reference VTL Video Trigger Level ...
Page 655: ...655 8 Options and Accessories ...
Page 665: ...665 9 If You Have a Problem ...
Page 674: ...674 Chapter9 If You Have a Problem Power Requirements Figure 9 2 AC Power Cables Available ...