Tektronix CSA7154, Руководство пользователя

Страница: 295 / 480

Creating and Using Math Waveforms
CSA7000 Series, TDS7000 Series, & TDS6000 Series Instruments User Manual
3- 217
H
Coherent gain. The gain factor normally associated with different window
functions is correctly scaled into the magnitude spectrum output. Therefore,
the magnitudes in the output spectrum do not change as different windows
are selected.
H
Scallop Loss. This is the magnitude error of an FFT when the frequency of
the observed signal is exactly half way between two frequency samples of
the spectrum when the interpolation ratio due to zero fill of the FFT is one.
The spectral analyzer FFT length is controlled so that zero fill is always in
effect. This essentially eliminates scallop loss because zero fill in the time
domain causes interpolation in the frequency domain. This results in accurate
magnitude measurements for all window functions.
--10
Hanning scallop loss is 1.42 dB
Frequency bins
+2
+1
0
--1
--2
--3
--4
--5
--6
--7
--8
--9
dB
Figure 3- 52: Example of scallop loss for a Hanning window without zero fill