Transformer-Based Multisection Quadrature Coupler with 1.5 Octave Bandwidth Using GaAs-Based Integrated Passive Device Technology

This paper presents a 3-dB quadrature coupler with 1.5 octave bandwidth using integrated passive device (IPD) on gallium arsenide (GaAs) substrate. By using the multisection transformers with different coupling coefficients, wider fractional bandwidth of the 3-dB quadrature coupler can be achieved compared with previously reported techniques. The design of the transformer in each section requires maximization of the in-band quality factor to keep the number of curve intersections for
$S_{21}$
and
$S_{31}$
. For validation, a 3-dB transformer-based quadrature coupler at about 1–3 GHz was designed and implemented. The proposed 3-dB quadrature coupler achieved a fractional bandwidth of 98%, which is more than twice the fractional bandwidth of an integrated coupler with at least two curve intersections for
$S_{21}$
and
$S_{31}$
. Moreover, the coupler with low phase error at two-side curve-intersection frequencies for
$S_{21}$
and
$S_{31}$
can effectively separate modulation spurs in pulse-modulation systems while maintaining similar measured performance compared to the integrated quadrature couplers for sub-6GHz multiband applications.