O
PERATOR
’
S
M
ANUAL
WRXi-OM-E Rev B
141
Power Density Spectrum
The power density spectrum (V
2
/Hz) is the power spectrum divided by the equivalent
noise bandwidth of the filter, in hertz. The power density spectrum is displayed on the dBm scale, with 0 dBm
corresponding to (V
ref
2
/Hz).
Sampling Frequency
The time-domain records are acquired at sampling frequencies dependent on the selected
time base. Before the FFT computation, the time-domain record may be decimated. If the selected maximum
number of points is lower than the source number of points, the effective sampling frequency is reduced. The
effective sampling frequency equals twice the Nyquist frequency.
Scallop Loss
This is loss associated with the picket fence effect.
Window Functions
All available window functions belong to the sum of cosines family with one to three non-zero
cosine terms:
where:
M = 3
is the maximum number of terms,
a
m
are the coefficients of the terms,
N
is the number of points of
the decimated source waveform, and
k
is the time index.
The table of Coefficients of Window Functions lists the coefficients
a
m
.
The window functions seen in the time
domain are symmetric around the point k = N/2.
Coefficients of Window Functions
Window Type
a
0
a
1
a
2
Rectangular
1.0 0.0 0.0
von Hann
0.5 -0.5 0.0
Hamming
0.54 -0.46 0.0
Flat Top
0.281 -0.521 0.198
Blackman-Harris
0.423 -0.497 0.079
FFT Setup
Setting Up an FFT
1. In the menu bar touch
Math
, then
Math Setup...
in the drop-down menu.
2. Touch a Math function trace button:
F1
through
Fx
The number of math traces available depends on the
software options loaded on your oscilloscope. See Specifications.; a pop-up menu appears. Select
FFT
from the menu.
3. Touch
the
Single
or
Dual
(function of a function)
button if the FFT is to be of the result of
another math operation.
4. Touch
inside
the
Source1
field and select a channel, memory, or math trace on which to perform the FFT.
5. Touch
inside
the
Operator1
field: Select
FFT
from the pop-up menu if you selected
Single
function. Select
another math function if you selected
Dual
function. Then touch inside the
Operator2
field and select
FFT
from the pop-up menu.
6. In the right-hand dialog, touch the
FFT
tab.
