Biomedical Engineering Reference
In-Depth Information
(a)
PW=0.5ns,PRF=10MHz
-40
-50
-60
-70
-80
(b)
-40
-50
-60
-70
-80
3
4
5
6
Frequency (GHz)
Fig. 5.8
Transmit spectrum for a 100 ps rise time b 250 ps rise time
mixer. For example, the UWB RF section discussed in this chapter filters out the
lower frequency portion within the frequency band of 0-1.4 GHz from the base
band pulse spectrum, and up-converts it by 4 GHz using a mixer. Hence, the
occurrence of nulls in the band of interest can be avoided as long as the bandwidth
of the first sinc component that belongs to the base band portion of the spectrum is
sufficiently large to cover the base band bandwidth of interest (0-1.4 GHz for this
design). In other words, only the pulse width plays a significant role in charac-
terizing the transmit spectrum in an up-conversion transmitter. Although it is
possible to control the pulse width by controlling the supply voltages of the buffer
amplifiers, the rise time of the square pulses depends on the electrical character-
istics of the components, such as the square wave generator and the XOR gate, and
is hard to control. Hence, by using the up-conversion technique as suggested in this
chapter, it is possible to avoid the requirement to control the rise time of the pulses
in order to generate a UWB pulse stream with a spectrum that has no nulls in the
band of interest. The up-conversion technique suggested in this chapter only
depends on the easily controllable pulse width for the latter purpose.
