RigExpert IT-24
A screen shot at the top (density graph)
shows the spectrum of a Wi-Fi access
point operating in the “beacon” mode
at the 1st Wi-Fi channel (the center
frequency is 2412 MHz, the range is 2401
to 2423 MHz). In the “beacon” mode,
data is transferred at low rate (1 or
2 Mbit/s), so the shape of the spectrum
form a Gaussian curve (looks like a bell).
When the data is intensively transmitted
(data rate is 54 Mbit/s), the shape
changes considerably — see the screen
shot in the middle. This is because the
OFDM modulation with 52 independent
carriers is used. The signal levels of all
carriers are the same, so the top of the
picture becomes relatively flat.
The screen shot at the bottom (a
waterfall mode) shows a transition
from the “beacon” transmission (top
of the image) to the intensive data
transmission (bottom of the image). The
wireless access point still transmits at
the 1st Wi-Fi channel.
The data packets of the “beacon” are
usually transmitted with a period of
100 ms. The packets are quite short
(less than 1 ms), so they are displayed
as individual dots inside the frequency
limits of the chosen Wi-Fi channel.
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