The proposed circuit can provide equal power division with 0˚, 180˚
phase difference at diverse operating frequencies by simultaneously
tuning six varactors at different voltage levels. The performance of
frequency agile rat-race coupler at different frequencies is verified
using both circuit and finite element method (FEM) based full wave analysis.
It is verified that the proposed device operates at different
frequencies of 3.0 GHz, 2.4 GHz, 1.9 GHz depending upon various reverse bias voltage of 19V(0.3pF),
5V(0.66pF) 1.5V(1.33pF) applied to varactor diodes. Very less amplitude and phase imbalance (±0.1 dB, ±2˚),
reflection coefficient less than -10dB along with isolation more than 25dB are observed between the
output ports at all achieved frequencies within the intended range. The circuit is designed
on commercially available FR-4 substrate and has also compact dimension (0.17λ×0.27λ×0.127λ) at 2.5 GHz,
50% less than conventional configuration. Due to uniqueness of frequency agility, compactness and
low fabrication cost the device may be considered as a suitable candidate for being used in MMICs.
More in http://ieeexplore.ieee.org/document/7509194/