SL-WPSR - Wireless Passive SAW Sensor Reader: SL-WPSR is a complete RF/IF transceiver system designed specifically for wireless Surface Acoustic Wave (SAW) based sensing applications. SAW sensors are specifically designed to measure one of physical quantities such as temperature, pressure, torque, acceleration, corrosion, chemical, biological etc…. The SL-WPSR device provides a passive, powerless and low cost solution to monitor the temperature in harsh indoor or outdoor environments. The device operates by sending an appropriate radio request signal, receiving the sensor response, and evaluating the received signal. The radio request signal is transmitted from the transmitter section of the SL-WPSR to the SAW sensor (downlink). The particular sensor responds with a radio signal (uplink) that is received by the receiver section of the SL-WPSR system. The received signal is adequately processed and measured physical quantity is calculated.
The frequency range of the device is from 300 MHz to 3.8GHz covering the three ISM bands - 433MHz, 915MHz and 2.45GHz. The board has separate receiver and transmitter signal paths but can be configured to share a single antenna. The receiver has a quadrature demodulator with RF and baseband automatic gain control amplifiers, low noise synthesizer and two 12 bit 40MHz ADC. The transmitter has a quadrature modulator, also low noise synthesizer and two 12 bit 40MHz DAC and power amplifiers. The hardware allows flexible control on RF front end, clock sources, analog input gain, output power, offset correction and base band sampling through communication busses. The device supports USB.

The circuitry is designed for very harsh environment to measure real and imaginary part of impedance ranging from 0Ω to 100kΩ within adjustable frequency bandwidth. The device uses narrow-band direct conversion architecture. Highly accurate measurements of the amplitude and phase dependent DC voltage is obtained using a Gilbert Cell, a high resolution analog-digital converter (ADC), and precise phase control of the LO DDS. The calculations of impedances are performed in 32 bit microcontroller. High accuracy temperature measurement circuitry is implemented using PT1000. The proposed circuitry is used to measure impedance of MCF (Monolitic Crystal Filter) sensor for viscosity.

Specifications: Signal Generator: 0-10MHz in-phase (I) and quadrature (Q); Frequency resolution: 0.01Hz; Impedance range: 0-500kΩ +/-1%; Applications: Viscosity sensor, general sensor application where a MCF is used as sensor element, crystal filter characterization, crystal parameters measurements, general impedance measurements, etc… 

The circuitry is designed for very harsh environment to measure very high resistance and low capacitance of a liquid. The device is based on a relaxation oscillator circuit with a very low noise integrator measuring very low current and two high speed comparators. Additional logic gate is used to create signal with pulses proportional to measured resistance and capacitance. Conductivity and dielectric constant of liquid placed between the sensor electrodes is calculated in a processor. In addition the circuitry uses direct sampling of the integrator voltage and calculates resistance and capacitance more accurately. High accuracy temperature measurement circuitry is implemented using PT1000 as liquid conductivity typically greatly varies with temperature.

Specifications: conductivity range: 1000pS/m to 5000000pS/m; dielectric constant range: 0-40 ε0. Applications: Oil conditioning, liquid characterization, etc…