A compact LTCC differential branch-line coupler

In order to meet the requirements of high integration of microwave circuits and electromagnetic crosstalk suppression, a compact differential branch-line coupler is proposed by distributing the coupled lines with certain odd-mode characteristic impedance on different layers of low temperature co-fired ceramic (LTCC). On one hand, the coupled lines have the characteristic of relatively concentrated electric field distribution on the basis of ensuring differential-mode transmission, which can overlap each other in LTCC and thus reduce the size, effectively. On the other hand, compared with single strip line, the coupled lines can reduce the number of ground plane so that the loss can be reduced. For demonstration, an LTCC differential branch coupler prototype is designed, which operates at 1.84 GHz and with the size of 0.07λ_g×0.07λ_g (λ_g is the guided wavelength at the center frequency). The measured results show that the minimum insertion loss of 0.6 dB, the 15 dB return loss fractional bandwidth of 18.2%, 15 dB isolation fractional bandwidth of 18%, and ±10 degree output phase difference fractional bandwidth of 40.0%. The proposed design has the features of compact size, low loss, enhanced bandwidth and easiness to connect with the differential chip, which can be used as a practical microwave device.