Biomedical Engineering Reference
In-Depth Information
the implementation of UWB transmitters are discussed. Methods of eradicating the
complications introduced by the use of UWB receivers in the power-stringent UWB-
based WBAN sensor nodes are discussed herein. The hardware implementation of
UWB sensor nodes is described in detail in the latter part of the chapter. Many
IC-based implementations of the UWB transmitters can be found in the literature.
While this chapter analyzes some of the design techniques used in these IC circuits,
we mainly pay our attention toward the full implementation of the UWB sensor
nodes, which include the control and interfacing circuits. Because of the factors,
such as high power consumption and increased hardware complexity, the use of
a UWB receiver in WBAN sensor nodes is disadvantageous. Hence, this chapter
mainly focuses on the implementations of UWB sensor node designs that implement
UWB transmitter only and eliminate or minimize the use of a receiver.
UWB Transmitter Design Techniques
The UWB transmitter lies at the core of a UWB-based sensor node. UWB transmitters
are straightforward in design compared to UWB receivers. Unlike in the case of the
NB transmitters, the radio frequency (RF) portion of the UWB transmitters does
not dictate the overall power consumption. Hence, care has to be taken in order to
minimize the power consumption of the rest of the transmitter circuitry. This section
will analyze some common transmitter design techniques that are available in the
literature.
A UWB transmitter design starts with a narrow UWB pulse generator. The earlier
versions of the UWB pulse generators used step recovery diodes (SRD) in order to
generate the pulses and Schottky diodes for pulse shaping. In this technique, the SRD
creates a voltage step function with a very short rise time [
22
,
23
]. A delayed version
of this step function is also created by making the step function to propagate through
a transmission line. The original step function is combined with the delayed version
of itself in order to make a narrow UWB pulse. There are several drawbacks in this
method of pulse generation which prevents it from using in WBAN applications. The
length of the transmission line that is used in order to obtain the delayed version of
the pulse is quite large; hence, it results in a large form factor in the circuit design.
The pulse generation method is very sensitive to the reflections that may occur in the
wave propagation paths; hence, the operation of the circuit can be largely affected
even by a small fabrication fault. The amplitudes of the pulses that can be generated
by this method are limited to few hundreds of millivolts [
22
]. Hence, it requires
extensive amplification before transmitting through a wireless link. However, this
method provides the basis for most of the modern UWB pulse generation techniques:
the combining of a waveform and its delayed version in order to generate narrow
pulses.
UWB pulse generators can be categorized into three major categories, namely,
(1) baseband pulse generators, (2) up-conversion pulse generators, and (3) direct
frequency pulse generators. These three pulse generation techniques are further
described in the following subsections.
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