Biomedical Engineering Reference
In-Depth Information
4.2.1 Base Band UWB Pulse Generators
In this approach, a base band pulse is generated initially in the form of a rect-
angular pulse [
3
-
6
]. The square base band pulse provides signal with a wide
spectrum. However, initial base band pulse does not comply with the FCC spec-
trum requirements. Hence, a filtering stage is used in order to shape the pulse
spectrum, such that it complies with the FCC spectral mask. This approach is
shown in Fig.
4.1
.
In base band pulse generators, the square pulse and its delayed version is passed
through an XOR gate, forming an edge combining circuit. The narrow square
pulses formed by the XOR output are then filtered using either a passive Band Pass
Filter (BPF) [
3
,
5
] or a Finite Impulse Response (FIR) based filter [
7
]. The square
pulses that form the input to the XOR gate can be obtained either using the input
data waveform itself [
8
,
9
], through a flip-flop arrangement [
5
] or using a separate
clock waveform [
4
].
An example pulse generator that uses a clock as the square wave signal source
is depicted in Fig.
4.2
[
10
]. The use of an 'AND' gate after the 'XOR' gate creates
IR-UWB pulses at every positive edge of the clock signal. The UWB pulses are
then modulated with the data signal using another 'AND' gate.
The base band pulse generation method provides advantages in terms of sim-
plicity in design. It avoids the complexities of directly generating the UWB pulses
that comply with the FCC spectrum requirements. A significant portion of the
power spectrum of the square wave has to be filtered in order to bring the UWB
pulse spectrum into the target frequency range. This results in significant power
loss. The amplitude of the UWB pulse spectrum after the BPF stage is often lower
than the FCC spectral mask. Thus, a power amplification stage may be needed
after the BPF in order to use the maximum allowable spectral amplitude. The use
of a power amplifier further increases power consumption of the UWB transmitter.
4.2.2 Up-Conversion-Based UWB Pulse Generators
The up-conversion method uses a mixer to up convert the frequency of the base
band pulses into the target frequency range. Both rectangular [
11
] and triangular
[
12
] pulses can be used as the base band pulse stream. Up-conversion of the pulses
eliminates the requirement of a base band pulse with a wide spectrum, such as a
square pulse in order to generate the final UWB pulse stream. Hence a triangular
pulse stream is more suitable as the basis of the pulse generation. The power
spectrum of the triangular pulses has suppressed side lobes, compared to that of the
rectangular pulses. Hence the power loss that might occur by using a square wave
pulse as the base band pulse can be reduced. However, it should be noted that
although the triangular pulse generation techniques are easily achievable in CMOS
IC based designs, the rectangular pulse based approach is the most convenient
