Biomedical Engineering Reference
In-Depth Information
Fig. 15 Comparison of the time domain plot for Tx-Amp and Tx-VCO UWB pulses
Control and Interface Circuitry
The microcontroller acts as the central controlling module of the sensor node (see
Fig. 8 and Fig. 12 ). The microcontroller can control the power to the baseband and
RF portions of the circuits. Hence, it can shut down the power-consuming RF and
baseband part during an intermittent data transmission situation. The microcontroller
also acts as the mediator for analog and digital data inputs. It performs the analog to
digital conversion, determines the transmission format and the modulation scheme,
and sets the data rate. The UWB pulse rate is independent of the data rate and is
arranged by the pulse generator. The generated UWB pulses are multiplied with the
binary data bits created by the microcontroller.
The role of the interface electronics is to provide low noise amplification for phys-
iological signals such as electrocardiogram (ECG), electroencephalogram (EEG). A
typical ECG/EEG signal has amplitude of less than 500
V with frequency less than
100 Hz. An instrumentation amplifier (INA321) and an active LPF (LTC6081) is
used in this circuit to provide a measurable input to ADC of the microcontroller. The
input signals are amplified by 60 dB, with a cutoff frequency at 100 Hz.
Both UWB sensor node designs are powered by a 3.7-V battery. The analog,
digital, and pulse generation circuits operate with a supply of 3.3 V while the RF
components use a 3-V supply. A National Semiconductor LP5996 linear regulator is
selected to power the entire circuit.
μ
Comparison of the Sensor Node Designs
The baseband approach used in the amplifier-based sensor node has advantages
over the VCO method in terms of circuit simplicity and the avoidance of the local
oscillator leakage. Although the oscillator-based approach is less affected by change
of pulse width, it does not require an additional amplifier and has higher flexibility
to operate in different frequency bands. The measured time domain plot for the
generated UWB pulses of both these sensor types is presented in Fig. 15 . Both
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