Tunable Branchline Coupler Using Microfluidic Channels

A branchline coupler is presented that uses 12 microfluidic channels (3 per branch) to tune its center frequency. A lumped-element equivalent circuit model (ECM) is extracted for the fluidic channels and is used to predict the response of the coupler prior to fabrication. The circuit is realized on a 1.524-mm-thick Rogers 4003C substrate with \epsilon _{r} = 3.55 . The microfluidic channels are milled through the ground backplane and are filled with ethyl acetate (\mathrm {C_{4}H_{8}O_{2}}) , for which \epsilon _{r}=6.00 . Experiments with the coupler show that starting with empty (i.e., air-filled) channels and progressively filling them with the fluid, the center frequency of the coupler can be tuned from 2.19 to 1.80 GHz, yielding a tuning range of 19.5%. A comparison between the modeled and measured results shows that the ECM predicted the coupler tuning range with an error below 2.8%.