Biomedical Engineering Reference
In-Depth Information
(a)
400
200
0
-200
-400
(b)
400
200
0
-200
-400
(c)
400
200
0
-200
-400
2.400
2.405
Time (µs)
Fig. 5.7
Transmit pulse for a 1 ns pulse width b 0.5 ns pulse width c 2 ns pulse width
5.2.2.2 Effect of Rise Time on UWB Transmit Spectrum
Transmit spectrums for IR-UWB pulse trains with 100 and 250 ps rise times are
shown in Fig.
5.8
. A pulse width of 0.5 ns and a PRF of 10 MHz are used in all
pulse streams for the purpose of comparison between signals. By comparison of
the output power spectrums shown in Fig.
5.8
, it can be seen that the position of
nulls within the transmit spectrum depends on the rise time of IR-UWB pulses. In
fact, it can be observed that the occurrence of a null in the transmit spectrum of
UWB signals depends on both rise time and pulse width, while the bandwidth of a
single sinc component of the spectrum depends only on the pulse width. Occur-
rence of a null at the center frequency causes the time domain amplitude to be
lower, causing weaker transmitted signal strength as shown in Fig.
5.9
.Itis
preferable for the peak of the transmit spectrum to be aligned with the center
frequency of the intended band of interest in order to obtain the maximum signal
amplitude. The condition that should be satisfied in order to obtain a spectral peak
at the intended center frequency can be derived as:
f
c
:
t
r
þ
t
w
ð
Þ
0
:
5c
ð
5
:
5
Þ
where f
c
is the centre frequency and c is an odd integer. In an up-conversion
IR-UWB transmitter, such as the one discussed in this chapter, the base band
portion of the UWB pulse spectrum (i.e. the low frequency portion) is selected
using a filter and is then up-converted to the frequency range of interest using a
