Novel coupled-line conductor-backed coplanar and microstrip directional couplers for PCB and LTCC applications

Novel coupled-line conductor-backed coplanar and microstrip directional couplers, convenient for manufacturing in standard printed circuit board (PCB) or low-temperature co-fired ceramic (LTCC) technology, are proposed. The coplanar coupler, consisting of a layered structure with four strips and covering a -10 to -2.6 dB coupling coefficient, is theoretically ideally impedance matched and perfectly isolated at all frequencies under the assumption of validity of quasi-static approximation and two-mode propagation. The microstrip coupler, a modification of the coplanar one, can be ideally compensated only in an area of strong coupling coefficients. The novel couplers are not sensitive to lateral misalignment of conductive layers, and not sensitive to thickness and dielectric permittivity tolerances of applied dielectric substrates. Preliminary experimental results for 2.7-, 3-, and 8.34-dB couplers, designed both in PCB, as well as LTCC technology are presented.