Biomedical Engineering Reference
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pulse generator using IC technology. A switched-voltage control ring oscillator ap-
proach is used in order to generate the UWB pulses. The data is modulated using the
pulse stream generated in this pulse generation approach. The data can be fed into
the circuit using a field programmable gate array (FPGA). This work also presents
the implementation of a UWB antenna on the same printed circuit board (PCB).
However, this sensor node is not fully integrated for independent operation as the
data and the control signals have to be generated using an external FPGA. A UWB
transmitter developed using off-the-shelf components is presented in [
35
]. In this
method, narrow square pulses are generated in the baseband domain using a series
of comparators. The RF pulses are generated by mixing these narrow pulses with
a high-frequency signal generated using a phase locked loop. The power consump-
tion of this circuit is 660 mW; hence, it is not suitable for power-stringent UWB
applications.
This section presents a feasible implementation for the design of UWB sensor
nodes that can be used in wearable WBAN applications. Three different sensor node
types are discussed herein, namely, a voltage-controlled oscillator (VCO)- based
UWB sensor, an amplifier-based UWB sensor, and a VCO-based UWB sensor with
integrated NB receiver. These sensor implementations are equipped with interface
circuitry in order to monitor both analog and digital signals. A microcontroller oper-
ates as the central controlling unit in all three designs. The sensor nodes are intended
to operate in the frequency range of 3.5-4.5 GHz. This frequency band is chosen
in order to avoid possible interference that can be generated due to the operation of
other equipment in RF bands such as 5-GHz WLAN.
IR-UWB sensor nodes can be divided into three major components: the pulse
generator, the RF circuit, and the controller. The function of the pulse generator is
to generate narrow square pulses which can be modulated with the data bits. The
suggested pulse generator in this section is implemented using buffers as the delay
element when creating narrow pulses. The RF circuit is responsible for pulse-shaping
and amplification. Two design concepts for the RF section of the sensor node are
studied in detail herein. Finally, this section describes the overall operation of the
sensor node, which includes the operation of a microcontroller that is used to carry
out the control operations of the sensor nodes such as sleep mode operation and
setting the modulation scheme.
Design of the UWB Pulse Generator
The IR-UWB pulse generator is a common design block in all the three designs. The
basic operating principle of the narrow pulse generator is based on the operation of
the delay elements. The work presented in [
36
] shows that the delay between the
input and the output of a buffer depends on the applied supply voltage of the buffer.
This relationship is given by (1),
1
k
1
+
,
C
L
2
V
DD
1
k
2
t
p
=
(1)
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