FFT and RBW
3- 178
WCA230A & WCA280A User Manual
FFT Window
The phase of the waveform processed by FFT analysis is assumed to start at zero
and end at zero, so the waveform data is an exact multiple of one cycle. If the
phase at the start and end of the waveform are equal, there is no unnatural
discontinuity in the signal waveform, and the frequency and the amplitude can be
accurately calculated.
If the waveform data is not an exact multiple of one cycle, the amplitudes at the
start and end of the waveform will be different. There will be a discontinuity of
the waveform between the beginning and end, and a high-frequency transient
phenomenon occurs. When this happens, inaccurate frequency information is
recorded in the frequency domain.
If a window function is applied to the waveform, the start and end amplitudes
will be closer, and the discontinuity is reduced. The frequency components that
are calculated from the signal by FFT also become more accurate. Choose the
FFT window according to your objective: accurate measurement of the frequency
or accurate measurement of the amplitude of the frequency components.
In FFT windows, the frequency resolution is inversely proportional to the
amplitude accuracy. Select a proper window according to the measurement items
and characteristics of the signal source. Table 3--26 shows characteristics and
usages of major windows.
Table 3- 26: Characteristics and usage of FFT windows
FFT window
Characteristics
Usage
Rectangular
H
Suitable for frequency measurement but not
suitable for amplitude measurement.
H
Same result as measurement without a window.
H
Transient phenomenon or burst; signal levels
before and after the event are roughly same.
H
Sinusoidal wave with little amplitude variation and
stable frequency.
H
Wide bandwidth irregular noise; spectrum
changes slowly.
Hamming, Hanning
H
Suitable for frequency measurement.
H
Inferior in accuracy of amplitude to the rectangu-
lar window.
H
Frequency resolution of Hamming is slightly
superior to that of Hanning.
H
Sinusoidal wave.
H
Repeating narrow bandwidth irregular noise.
H
Transient phenomenon or burst; signal levels
before and after the event are remarkably
different.
Blackman-Harris
H
Suitable for amplitude measurement but not
suitable for frequency measurement.
H
Signal that has one-frequency ruling when a high
order harmonic is detected.
Window Characteristics
Summary of Contents for WCA200A
Page 6: ......
Page 8: ......
Page 29: ...Getting Started...
Page 30: ......
Page 57: ...Operating Basics...
Page 58: ......
Page 86: ...Functional Overview 2 28 WCA230A WCA280A User Manual...
Page 125: ...Reference...
Page 126: ......
Page 208: ...Modulation Analysis Demod Mode 3 82 WCA230A WCA280A User Manual...
Page 272: ...Setting Timing Parameters 3 146 WCA230A WCA280A User Manual...
Page 346: ...Display Line 3 220 WCA230A WCA280A User Manual...
Page 358: ...Marker Operation and Peak Search 3 232 WCA230A WCA280A User Manual...
Page 388: ...File Format 3 262 WCA230A WCA280A User Manual...
Page 392: ...Screen Copy 3 266 WCA230A WCA280A User Manual...
Page 393: ...Appendices...
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Page 430: ...Appendix C Digital Demodulation Symbol Mapping C 4 WCA230A WCA280A User Manual...
Page 437: ...Glossary...
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Page 446: ...Glossary Glossary 8 WCA230A WCA280A User Manual...
Page 447: ...Index...
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