Biomedical Engineering Reference
In-Depth Information
identical versions of a square wave signal through two identical buffers, it is
possible to keep C
L
, k
1
and k
2
common to both buffers; hence the output delay
generated by each buffer can be solely controlled by adjusting the V
DD
of each
buffer. The adjustable voltage range of the buffers depends on the input high
voltage specifications of buffers and XOR gate. The condition that should be
satisfied by supply voltages of the buffers in order to generate a narrow pulse
stream can be shown as:
VDDA
b
VDDB
VIH
XOR
;
where b
¼
VIH
Buffer
VDDB
ð
5
:
2
Þ
where VDDA and VDDB are supply voltages of the two buffers, VIH
XOR
is the
input high voltage of the XOR gate, and VIH
Buffer
is the input high voltage of the
buffer.
The square wave input to the two delay lines is generated using a square wave
oscillator. In reality, the output of a square wave oscillator forms a trapezoidal
signal due to the finite output capacitance at the square wave output. As shown in
Fig.
5.4
, this results in a narrow trapezoidal pulse stream at the output of the XOR
gate.
With the correct amount of delay setting in the buffers, it is possible to bring the
rising and falling edges of these trapezoidal waveforms close together, forming an
approximated triangular pulse waveform at the output of the XOR gate. The power
spectrum of the triangular pulses has suppressed side lobes compared to that of the
rectangular pulses [
9
]. Hence, the power loss that might occur by using a square
wave pulse as the base band pulse for UWB RF section can be reduced by using
the triangular shaped pulses generated using this technique. The IR-UWB pulses
generated by this mechanism are then up- converted using a mixer and sent
through a Band Pass Filter (BPF) for the purpose of spectral shaping. This is done
in the IR-UWB RF section, which is discussed in a subsequent section. The
properties of the IR-UWB pulses, such as rise time, pulse width and PRF play an
important role in the output of the IR-UWB pulse generator. It is important to
analyze these properties in order to parameterize the circuit components of the
IR-UWB pulse generator.
5.2.2 Analysis of UWB Pulse Properties
This section analyses the properties of IR-UWB pulses, such as pules width, rise
time and PRF, in order to investigate their effects on transmit spectrum of the
UWB signals.
