A Harmonic RF Phase-Shifter Based Wireless pH Sensor

This paper describes the design and development of a phase shifter based wireless pH sensor tag. It consists of a receive and transmit antennas, a phase shifter and a harmonic generator, and a pH electrode. When the sensor tag is interrogated with an RF signal it changes the phase of the received signal and transmits back at both the fundamental (f₀) and second harmonic (2f₀) frequencies. The phase shift is governed by the voltage produced by the pH electrodes in direct contact with the sample. The phase shift at (f₀) and (2f₀) is analyzed using a phase detector to sense the pH changes of the sample. Better sensitivity is obtained when the harmonic frequency is used for sensing. In addition, the use of second harmonic improves the signal to noise ratio (SNR) as the undesired clutter is filtered out. The design and implementation of the harmonic phase shifter is validated with simulation and measurement results.