Two‐section asymmetric coupled‐line impedance transforming directional couplers

Abstract

Two-section impedance transforming directional couplers have been proposed. It has been shown that such asymmetric coupled-line directional couplers can provide equal power split between the coupled and transmission ports as well as impedance transformation in the same broad frequency range, as classic two-section coupled-line couplers. The theoretical analysis has been presented and appropriate formulas allowing for calculation the required per-unit-length parameters of broadband two-section impedance transforming couplers have been derived. Moreover, it has been shown that the impedance transformation ratio of the presented coupler can be increased by the appropriate modification of the two-section coupled-line directional coupler in a way that the directly fed and coupled-line transformation ratios are different. The proposed modification allows for the design of broadband impedance transforming directional couplers with increased transformation ratio over the limit fixed by the coupling coefficient of the tightly coupled section and having at the same time acceptable impedance match and isolation between the respective ports. The presented theory has been confirmed by the measurements of a two-section 3 dB impedance transforming directional coupler having bandwidth f u /f l = 3:1 and impedance transformation ratio R = 2.

High‐performance differentially fed coupled‐line directional couplers realised in inhomogeneous medium

“…Passive circuits in microwave electronics are commonly designed by using unbalanced transmission lines, in which the signal is propagating between the line and common ground plane. Therefore, the most fundamental networks such as directional couplers [1][2][3][4][5], phase shifters [2,3], impedance transformers [6], filters [7][8][9] etc., are designed as single-ended structures due to their simplicity of design and realisation. On the other hand, the rapid development of monolithic microwave integrated circuits and high-speed circuits together with shifting their frequency of operation towards the higher frequencies are the reasons for which amplifiers, mixers and other active devices [10][11][12] are designed as balanced circuits and operate under the differential excitation due to the superior interference rejection.…”

Broadband coupled-line directional couplers with high impedance transformation ratio

“…Another issue is the bandwidth of the impedance transforming directional couplers, which is limited by the transforming properties of a single-section quarter-wave transformer [13], where the bandwidth depends on the chosen transformation ratio R and the required return losses. Recently, it has been shown in [15] that the bandwidth of the impedance transforming directional couplers can be increased by application of multi-section quarter-wave transformers, at the expense, however, of the lower transformation ratio. It has been shown in [15] that with a two-section asymmetric 3-dB directional coupler, bandwidth defined as the ratio of higher ( f u ) to lower ( f l ) frequency of the bandwidth, f u / f l = 1.5 can be obtained, together with the transformation ratio R = 1.43.…”

“…Recently, it has been shown in [15] that the bandwidth of the impedance transforming directional couplers can be increased by application of multi-section quarter-wave transformers, at the expense, however, of the lower transformation ratio. It has been shown in [15] that with a two-section asymmetric 3-dB directional coupler, bandwidth defined as the ratio of higher ( f u ) to lower ( f l ) frequency of the bandwidth, f u / f l = 1.5 can be obtained, together with the transformation ratio R = 1.43. Further modification has led to increased R = 2 but at the expense, however, of return loss and isolation degradation.…”

“…In this paper, we propose a novel type of 3-dB impedance transforming directional couplers, which are based on the recently published coupled-line directional couplers composed of coupled-line sections having different electrical lengths [16] and impedance transforming couplers described in [12–15]. The proposed coupler is superior to the previously published impedance transforming directional couplers, since it offers wide operational bandwidth together with very high impedance transformation ratio R .…”

Generalised capacitive‐loading method for miniaturisation of forward‐wave directional coupled‐line couplers with arbitrary strip impedances and coupling levels

“…Directional coupled-line couplers are popular components found in various microwave circuits and systems, such as balanced amplifiers, baluns, beamforming networks, radar systems and among others [1][2][3][4]. The most famous type is the backward-wave coupler, the operation of which relies on the difference between the even-and odd-mode impedances.…”