A Tunable Interferometer for High Sensitivity and Resolution Humidity Sensing

Here, a six-port network, consisting of a power splitter/combiner, two hybrid couplers, and a composite right-/left-handed (CRLH) transmission-lines (TLs) voltage-controlled phase shifter (CRLH-TLs-VC PS), is loaded with the reference and sensing re-entrant cavity resonators (RECRs) in substrate-integrated waveguide (SIW) configuration to implement a tunable microwave interferometer for high sensitivity and resolution humidity sensing. The humidity-sensitive (HS) mate- rial poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is deposited only in the sensing SIW RECR to form a phase difference between the reference and sensing RECRs. Then, the phase difference is loaded into the transmission branch of the six-port network and can be easily compensated to 180° by a positive phase provided by the CRLH-TLs-VC PS. In addition, the interferometer implemented in differential configuration not only avoids the use of an additional attenuator but also can suppress the temperature effect. A series of LabVIEW-based real-time measurement experiments are performed using a vector network analyzer (VNA) to demonstrate the good humidity response characteristics, such as the high sensitivity of 678.85 kHz/% RH, the high resolution for the discriminable minimum RH change in the range from 0.37% to 5.5%, and the low temperature coefficient of frequency (TCF) of −10.699 ppm/°C.