Tektronix 11801C User Manual page 400

Appendix D: Algorithms
D 4
2p
+ e *j
W
R
R
The linear magnitude (FFT
computed as:
(k) + A(k)
FFT
mag
(k) + acrtan
FFT
phase
where
A(k) = real part of X(k)
B(k) = imaginary part of X(k)
The magnitude and phase for negative frequencies are discarded and linear
interpolation is used to expand the positive frequencies to fill the entire
record length.
The magnitude of the frequency spectrum in decibels is given as:
(k) + 20log(FFT
FFT
magdB
where the 0 dB point is defined as the sine wave of 0.316 V peak
(0.224 V ), which gives 1.0 mW into 50 W.
RMS
FFT Windowing Functions
The selected FFT windowing function is applied to the time domain wave
form before the FFT is computed. The FFT windowing functions are as
follows:
H Rectangular
R * 1
|
x(n) + 1
n + 0
H Triangular
R
2
(2 * 2 n
x(n) +
|
n + 0
H Hanning
R * 1
0.5(1 * cos ( 2p n
x(n) +
|
n + 0
H Hamming
R * 1
0.54 * 0.46(1 * cos(2p n
x(n) +
|
n + 0
) and the phase (FFT
mag
) B(k)
2 2
B(k)
A(k)
(k))
mag
R * 1
|
(2 * 2 n
) , )
R
R R
n +
2
) )
R
))
R
) of the FFT are
phase
Appendices