Chapter 8 MATH and Reference
83
Each window is alternatively used between frequency resolution and amplitude accuracy, and the appropriate
window may be selected according to the characteristics of the following windows.
⚫ Rectangular window
This is the best window type for resolution frequencies that are very close to the same value, but this
type is the least effective at accurately measuring the amplitude of these frequencies. It is the best type
of measuring the spectrum of non-repetitive signals and measuring the frequency component close to
DC.
Use the “Rectangular” window to measure transients or bursts of signal levels before or after almost
the same event. Moreover, this window can be used to measure equal-amplitude sine waves with very
close frequencies and wideband random noises with relatively slow spectral variations.
⚫ Hamming window
This is the best window type for resolution frequencies that are very close to the same value, and the
amplitude accuracy is slightly better than the “Rectangular” window. The Hamming type has a slightly
higher frequency resolution than the Hanning type.
Use Hamming to measure sinusoidal, periodic, and narrowband random noises. This window is used
for measuring transients or bursts of signal levels before or after events with significant differences.
⚫ Hanning window
This is the best window type for measuring amplitude accuracy but less effective for resolving
frequencies.
Use Hanning to measure sinusoidal, periodic, and narrowband random noises. This window is used for
measuring transients or bursts of signal levels before or after events with significant differences.
⚫ Blackman-Harris window
This is the best window type for measuring frequency amplitude, but worst for measuring the resolution
frequency.
Use the Blackman-Harris measurement to find the main single-signal frequency waveform for higher
harmonics.
Since the oscilloscope performs FFT transform on the finite-length time record, the FFT algorithm
assumes that YT waveform is continuously repeated. Thus, when the period is integral, the amplitudes
of YT waveform at the beginning and at the end are the same, and waveform will not interrupt. However,
if the period of YT waveform is not integral, the waveform amplitudes at the beginning and at the end
are different, resulting in high-frequency transient interruption at the junction. In the frequency domain,
this effect is called leakage. Therefore, to avoid leakage, the original waveform is multiplied by a
window function, forcing the values at the beginning and at the end to be zero.
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