Tektronix WCA200A, User Manual

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FFT and RBW
WCA230A & WCA280A User Manual
3- 177
FFT Points
The number of FFT points is normally set automatically by the RBW setting.
The value is basically 1024. It can be set in powers of 2 from 64 to 8192. This
value is the number of points in one physical frame in the time and frequency
domains. If the number of points is reduced, the frame period is shortened and
more spectrum variations can be observed in the spectrogram view. If the number
of the points is increased, S/N ratio and frequency resolution are improved.
NOTE
. Setting the number of FFT points greater than the limit (8192) causes the
noise floor to go down and occasionally causes spurious signals to appear (see
Figure 3--126). It cannot be determined whether the spurious signal comes from
the input signal or the instrument.
The number of FFT points is normally limited to 8192 internally to avoid
displaying internally-generated spurious signals. However, you can remove this
restriction and set the number of FFT points up to 65536 using the following
steps:
1. Display the measurement signal on the screen.
2.
Press the
RBW/FFT
key on the front panel.
3. Press the RBW/FFT side key to select FFT .
4.
Press the
Extended Res.
side key to select
On
.
5. Press the FFT Points side key and select the value with the general purpose
knob. The range extends up to 65536 in powers of 2.
The waveform after FFT is displayed with the selected number of points.
Normal noise floor
Spurious signal
Increasing the number of FFT points
causes the noise floor to go down and
occasionally display spurious signals.
Figure 3- 126: Generating spurious signals by increasing the number of FFT points
Limit on the FFT Points