A Wireless Passive pH Sensor With Clutter Rejection Scheme

This paper describes the design and development of a passive wireless pH sensor tag. Under wireless interrogation, the sensor tag changes the phase of the interrogation signal according to the pH. The conversion of pH data into phase information was implemented using a pH electrode coupled to a phase shifter. A theoretical study was performed on the varactor diode parameters to design a highly linear and sensitive phase response with pH change. In addition, a dual frequency operation was implemented based on fundamental interrogation frequency at 2 GHz and the sensor tag reply at the second harmonic (4 GHz) to reduce the clutter effect. The harmonic frequency was generated using a double-balanced frequency multiplier with a conversion loss of 12.2 dB at received input power of 0 dBm. The harmonic sensor tag sends back the phase information at 4 GHz, which is down-converted and read using a phase measurement reader. In addition to the clutter rejection, the harmonic implementation also helps with improved sensitivity. Different pH solutions from 4 to 10 were used to characterize the sensor tag. A linear phase response of 2.32 (degree/pH) was obtained experimentally at a standoff distance of 18 cm from the reader. The design and implementation of the passive pH sensor were validated with simulation and measurement results.