Biomedical Engineering Reference
In-Depth Information
2 ns
Fig. 5.13
The base band pulse stream generated by the IR-UWB pulse generator
These equations determine the maximum pulse width that the pulse generator
should generate in order to cover a certain signal bandwidth around a particular
center frequency. By solving these equations it can be estimated that a pulse width
of 2 ns will give a transmit spectrum that will cover the intended transmission
frequency range of 3.5-4.5 GHz with the center frequency at 4 GHz. A pulse
width of 2 ns can be achieved in the pulse generator shown in Fig.
5.12
by
choosing the supply voltages of the two buffers to be 3.3 V and 3 V. The supply
voltage of the XOR gate determines the peak amplitude of the IR-UWB pulse
stream. The base band pulse stream generated by the pulse generator at a 40 MHz
PRF is shown in Fig.
5.13
.
5.2.4 IR-UWB RF Section
IR-UWB RF section is responsible for the up-conversion of the base band
IR-UWB pulse stream into the intended frequency range of 3.5-4.5 GHz. Basic
block diagram of the IR-UWB RF section used in this sensor node design is shown
in Fig.
5.14
.
The data generated by the micro-controller is modulated by the IR-UWB pulse
stream using an AND gate before entering the RF portion of the circuit. The base
band pulse stream produced by the UWB pulse generator consists of a power
spectrum with several frequency lobes (sinc components) spread throughout the
UWB bandwidth. Amplitudes of these frequency lobes decrease towards the upper
part of the UWB spectrum as shown in Fig.
5.15
a. UWB RF section employs a
Low Pass Filter (LPF) in order to filter out the 0-1.4 GHz section of the UWB
pulse spectrum. This portion of the spectrum is of the highest power compared to
rest of the spectrum. Filtered spectrum is then shifted using a mixer and a Voltage
Controlled Oscillator (VCO) operating at 4 GHz. A band pass filter is used at the
