Abstract: A microwave input circuit including a parametric upper sideband down converter for receiving an input signal whose frequency is not much different from the intermediate frequency produced at the output of the down converter. The down converter may be either of the series type or of the parallel type and has its signal input directly connected to a microwave antenna and terminated by the real antenna resistance (R.sub.g) or the real antenna conductance (G.sub.g), respectively, at the input signal frequency (f.sub.s1). The parametric down converter comprises a cascade connection of a parametric down converter stage and a parametric up converter stage with both of said stages being of the series type or of the parallel type. The down converter stage is terminated at its image frequency (f.sub.sp) with a real resistance (R.sub.sp) or a real conductance (G.sub.sp), respectively, and satisfies the following relationship for a series type down converter: ##EQU1## WHERE R.sub.

Abstract: An input circuit for use in a microwave receiving system. The input circuit includes an antenna for receiving the microwave signals and a parametric down converter, which operates in a lower side-band frequency position. The parametric down converter includes a mixing diode, a pump circuit, an input signal circuit and an output circuit. The input signal circuit of the parametric down converter is directly connected to the antenna so that the input signal circuit of the parametric down converter is terminated by the real portion of the antenna impedance. The impedance of the antenna is selected so that for p >> z (where p is the pump frequency and z is the intermediate frequency) the impedances of the antenna at the signal frequency p + z and at the reflective frequency p - z approach the same value.