The electronics system performs a variety of tasks to convert changes in the state of the SAW sensors into meaningful pressure, torque or temperature signals for the vehicle controller. It must wirelessly stimulate the SAW elements, read back their resonant signals, determine their frequencies, and then calculate the torque, pressure or temperature value using stored calibration information.

Dependent on the sensing application, up to five individual SAW resonators are interrogated to complete a single measurement of the output parameter. The sensor is a narrowband device typically containing two or three resonators designed with nominal frequency peaks occupying the 433.05 to 434.70 MHz ISM band. Spacing between peaks must be sufficiently large to prevent any crossover of individual frequency peaks as they shift in response to the external condition under measurement. Otherwise the system could lose track of the relationship between each resonator and its corresponding fre-quency resulting in ambiguity in the calculation of the final parameter. Regardless of mechanical packaging, all resonators in a given application are connected in parallel so the interrogation system sees the equivalent of a single one-port resonator with a number of resonant peaks.

A DSP controller that is connected to an RF ASIC directs wireless interrogation of a SAW resonator. The process begins with transmission of a narrowband RF burst signal with a frequency close to the resonant peak of one of the SAW resonators. The wireless interface between interrogation electronics and passive sensor varies dependent on the application. Planar micro-strip couplers are typically used for torque applications to allow uninterrupted linkage between the stationary and rotating components. Dipole antennas have been used for TPMS applications. Transmission power levels during this phase of interrogation typically range from about 0.2 to 3 mW. Following wireless excitation of the SAW resonator, the RF ASIC switches from transmit to receive mode so that it can capture the returning RF signal. The receive path in the ASIC consists of a low-noise amplifier (LNA) followed by a single sideband (SSB) mixer that downconverts the ∼433 MHz signal to a first intermediate frequency (IF) of 11 MHz. The signal is then filtered and amplified before entering an I-Q mixer, which downconverts the signal further to its second IF of 1 MHz.

At this point the SAW signal has been split into separate low-frequency channels in quadrature as I + jQ. These I and Q signals are sent from the ASIC to the DSP where they are simultaneously sampled by an internal analog-to-digital converter (ADC). This interrogation process is repeated a number of times so that multiple I-Q response signals from a SAW element can be combined in a time-synchronized fashion known as coherent accumulation, which improves the signal-to-noise ratio (SNR) of the signal by reducing the effect of random errors and phase noise in the RF ASIC. The next processing step involves the calculation of a discrete Fourier transform (DFT) on the complex signal formed by sampled I-Q data in order to determine the exact frequency of the resonant peak. This approach is more accurate than a DFT computed from a single-channel input. A full spectrum fast Fourier transform (FFT) is not performed because the sensor is narrowband and the peak frequency in the sampled signal will lie within the second IF ± the maximum frequency shift of the resonator when subjected to its full range of measurement. A coarsely spaced set of spectral lines is first calculated to determine the rough location of the frequency peak. A finer spacing of spectral lines is then computed as the basis of a final inter-polation to determine the exact resonant frequency.

The process of interrogation and determination of resonant frequency is sequentially performed for each SAW resonator in the transducer resulting in a set of frequencies that are the basis for calculating differential shifts proportional to the sensed parameter. These frequency differences provide input to a set of model-based equations that the DSP uses to determine final measurement values of torque, temperature or pressure for transmission to the vehicle controller or for other higher-level processing.