0.1 Design of an in-vivo radiation measurement scheme using a  (Page 4/7)

Principle of Operation:

• When the input voltage to the terminal Vin is low, the PMOS transistor is turned ON and the NMOS transistor is turned OFF and output is connected to the terminal VDD via PMOS.
• When the input voltage to the terminal Vin is high then the transistor NMOS is turned ON and Vout is pulled to GND potential.
• Consider the terminal Vin is supplied with a saw-tooth waveform. When the voltage slowly rises from 0 to Vdd the PMOS is initially turned ON and when the voltage rises above the threshold voltage of NMOS then NMOS turns ON and PMOS is turned off pulling the voltage at Vout to GND. When the voltage drops from Vdd to 0V NMOS is initially turned ON and when the voltage reached the PMOS threshold voltage NMOS turns OFF and PMOS is turned ON.
• The switching point voltage is defined as the voltage at which V(Vin) = V(Vout). The switching voltage is denoted by Vm.

The threshold voltage of the NMOS decreases and PMOS increases with radiation. The switching voltage Vm changes with an increase in radiation. The change in the switching voltages is tabulated in the table 2.

Table 2

The switching voltage Vm during a transition from 0 to 1 or 0V to 5V at terminal Vout is given by Output (0-1). The switching voltage Vm during a transition from 1 to 0 or 5V to 0V at terminal Vout is given by Output (1-0).

An increase in radiation dose causes the pulse width of the waveform at the output Vout to decrease as shown in the figure 11. The switching voltage of the inverter under radiation is modeled in HSPICE. The green waveform represents the voltage at the output Vout for a normal inverter pair. The yellow waveform represents the voltage signal at the output Vout for a radiated inverter pair. The pulse width of the waveform is reduced due to radiation effect. Thus this signal is sent to a RF chip through a capacitor (100pF) and the waveform at the output gives a measure of the radiation received by the inverter pair.

Fig. 11

6. RF Transmitter and Receiver:

The design of a reliable wireless transmitter and receiver pair is an essential step in being able to carry out the radiation dose assessment. In the proposed detector system, an RF transmitter will be placed along with the miniature bio-compatible radiation sensor and an RF receiver will be located external to the human body along with an electronics package containing the data collection and calibration unit.

As a proof of concept, we chose an off-the-shelf RF transmitter and receiver pair that meets our specifications. We selected TXM-916-ES (RF transmitter), RXM-916-ES (RF receiver) of Linx technologies and ANT-916-CHP-x Chip antennas from Antenna Factor for our experiment. These devices offer the advantages of 1) Ultra compact SMD packaging, 2) FM/FSK modulation for noise immunity, 3) Excellent sensitivity for outstanding range, 4) Wide range analog capability, 5) High data rate (maximum 56kbps) and 6) Cost effectiveness. They also provide useful features such as audio reference, low voltage detect and level adjustment. These modules require no tuning or any other external RF components. They operate in the 900MHz frequency band. Fig 1 illustrates the pin connections of both transmitter and receiver modules.