An improved, high frequency, high or low power, broadband quadrature combiner having a significant reduction in weight, and improved electrical performance such as, decreased insertion loss, improved phase linearity, reduced Intermodulation Distortion (IMD) and improved VSWR, as well as improved simplified tuning opportunities. A reduction in the number of transformers used in the prior art quadrature combiner design facilitates weight reduction and independent tuning of a zero and ninety degree phase network. Specific design improvements to the prior art quadrature combiner include: elimination of the input hybrid, scaling of the phase networks to twice its prior art impedance and optimization of values over the frequency range, and the substitution of an output hybrid utilizing a single hybrid transformer in series with an impedance matching transformer.

A 90-degree digital phase shift network includes at least two all-pass digital filters which have a phase difference of 90 degrees therebetween. These digital filters are adapted to receive the same signal. Each of the all-pass digital filters has phase characteristics .theta.(.omega.) given by ##EQU1## where T represents a sampling period, P.sub.i a real number indicative of a filter coefficient, .omega. the angular frequency, m a weight variable, n'=n/2 (n=even number), n'=(n+1)/2 (n=odd number), n an order of a 90-degree digital PSN, m being a positive value other than 1, and satisfying .vertline.2m.omega..vertline./.vertline.P.sub.i .vertline.<1 when .omega.T is sufficiently lower than 1, while satisfying .vertline.-P.sub.i .omega.T.sup.2 .vertline./2m<1 when .omega.T is substantially equal to .pi..

A 90-degree digital phase shift network includes at least two all-pass digital filters which have a phase difference of 90 degrees therebetween. These digital filters are adapted to receive the same signal. Each of the all-pass digital filters has phase characteristics .theta. (.omega.) given by ##EQU1## where T represents a sampling period, P.sub.i a real number indicative of a filter coefficient, .omega. the angular frequency, m a weight variable, n'=n/2 (n=even number), n'=(n+1)/2 (n=odd number), n an order of a 90-degree digital PSN, m being a positive value other than 1, and satisfying .vertline.2m.omega..vertline. / .vertline.P.sub.i .vertline.<1 when .omega. T is sufficiently lower than 1, while satisfying .vertline.-P.sub.i .omega. T.sup.2 .vertline. / 2m<1 when .omega. T is substantially equal to .pi..

In a .pi./2 power divider, particularly a .pi./2 hybrid for the radio-frequency antenna of a nuclear magnetic resonance tomography apparatus, a decoupling circuit having unbalanced-to-ground line transformers is provided. Each branch thereof contains an all pass network consisting unbalanced-to-ground line transformers at the output. A broadband .pi./2 hybrid is obtained particularly with a broadband 3 dB/180.degree. hybrid as the decoupling circuit.

A microwave coupler which receives an input signal and converts it into two output signals is dislosed wherein the output signals exhibit the same phasing, the output signals are isolated, and a predetermined ratio exists between the power level of one output signal and the power level of the other output signal. An equal-split power divider transforms the input signal into intermediate signals which are then isolated and phase shifted. A quadrature hybrid combines the phase shifted signals and produces the output signals.