Biomedical Engineering Reference
In-Depth Information
(a)
400
200
0
-200
-400
(b)
80
60
40
20
-0
-20
-40
-60
2.400
2.405
Time (µs)
Fig. 5.9
Transmit pulse for a 100 ps rise time b 250 ps rise time
5.2.2.3 Optimization of PRF for UWB Transmit Spectrum
As depicted in Fig.
5.10
, PRF affects the number of spectral lines and their
amplitudes that lie within a certain bandwidth. A higher PRF system tends to create
lesser number of spectral lines that are higher in amplitude, while a lower PRF
results in spectral lines that are closer to each other with lower amplitudes. As a
result, the peak transmit power for higher PRF systems will be comparatively higher
than that of lower PRF systems. These spectral lines can be characterised by the PSD
of the IR-UWB signal, which consists of a continuous spectrum and a line spectrum
[
9
]. The continuous spectrum is determined by pulse width and rise time of the IR-
UWB pulse stream while the line spectrum corresponds to the PRF. The amplitude
of the line spectrum tends to be 10.log (PRF/1 MHz) higher than the continuous
spectrum, for a resolution bandwidth of 1 MHz selected according to the FCC
regulations [
9
]. They appear at 1/T Hz apart from each other, where 1/T = PRF.
These spectral lines create overshoots above the FCC spectral mask. Even after
averaging over a 1 MHz bandwidth, they result in higher average powers, which
will violate the FCC regulations. For an application that requires a high PRF, these
spectral lines can be reduced by using a modulation scheme that makes the trans-
mitted signal equiprobable [
9
]. Similarly, using a duty cycled IR-UWB data
transmission will reduce the spectral overshoots of a high PRF IR-UWB system.
The amplitude of IR-UWB pulses does not determine the spectral shape or
locations of the nulls, rather it determines the peak amplitude of the transmit
spectrum. The amplitude of the IR-UWB pulses can be varied in order to contain
the transmit spectrum within the FCC spectral mask.
