V. Wireless Sensing Efforts

Figure 11: Illustration of wireless sensor system.

Wireless sensors have been implemented in various ways. Two main categories of the passive variety are continuous and pulsed interrogation. Pulsed interrogation involves switching the interrogation single on and off. The signal would interact with the sensor and be reflected back. When this signal reaches the receiver, the interrogating signal will be turned off. The received signal would be that of the original signal, but it will have undergone amplitude and phase variations. These variations will contain the information to be monitored. Demonstrations have used FM signals to interrogate the sensor in this pulsed manner, by mixing the interrogating signal with the received signal and using the output(containing information of the phase shift) to quantify the parameter to be sensed. Varadan has used an additional antenna on the sensor to serve as reference to eliminate the interference caused by the differences in sensor location from the interrogator. Another approach is to use two SAW resonators, with one used as a reference. The output signal is the difference in delays of the two, and this helps to greatly reduce the sensitivities to the RF channel84. These methods are limited to high Q sensing devices. Devices must be able to store energy for relatively long periods of time, thus decreasing the switching speeds required for transmitter/receiver system. SAW are capable of this kind of interrogation74. Spectral domain sensors are not suited for pulsed interrogation in a cost efficient manner. The required...