The present invention relates generally to the field of frequency converters and more particularly to high frequency converters employing semiconductor diodes. Such frequency converters are usually employed in superheterodyne microwave receivers for which the signal frequency is so high that amplification at the signal frequency is technically or economically not feasible. The frequency converter changes the signal frequency to an intermediate frequency which carries the same information as the signal but is at a much lower frequency at which amplification is technically and economically feasible.
Waveguide balanced mixers as described above have been in use for more than thirty years. These mixers generally are implemented such that the radio frequency (RF) signal to be down converted is mixed with a local oscillator (LO) signal by means of semiconductor beam lead diodes. Typically these circuits include a 180.degree. hybrid junction formed by joining two transmission lines so as to provide good isolation between the LO and the RF ports. A balanced waveguide mixer has been disclosed in the International Edition of Microwave Journal, Aug. 19, 1979, Volume 22, Number 8, pages 66-68, by Paul J. Meier in which printed circuit techniques are utilized in conjunction with a metal waveguide housing that is used for support and isolation. In that device the circuit is printed on a dielectric substrate and embedded inside the waveguide in the E-plane. More particularly, this prior art mixer has a double ridge waveguide to slotline transition to transform the RF signal from the TE.sub.10 waveguide mode to the E-field slotline mode and it uses an LO waveguide to microstrip transition which terminates in a coplanar transmission line. In this prior art device the beam-lead diodes are mounted in a quarter-wave section of the coplanar line which is excited in the balanced mode from the RF port and the unbalanced mode from the microstrip LO port. The IF is returned to ground by high impedance wires bounded to the rear surface metalization via holes drilled through the board. The IF is extracted from the diode mount through a high impedance microstrip line which serves as the LO matching transformer. This prior art mixer contains a diplexer to separate the LO and IF signals. The junction between the transmission lines and the LO-IF diplexer design determines the useable bandwidth of the mixer. In particular it is important that the balanced RF slotline and the unbalanced LO transmission line be properly terminated at the diode junction. In this prior art mixer disclosed by Meier and in many other mixer designs the LO line is converted to a coplanar line and the RF slotline is divided and continued into the two coplanar slots. Thus, for some length, the RF and LO signals coexist on the same transmission line. The length of the coplanar line may also limit the IF bandwidth separating the LO-IF junction from the LO-RF junction. In general, the previously used mixer designs have limitations in operating RF and IF bandwidth caused by the poor termination of the transmission lines and by not having the LO-IF diplexer as an integral part of the LO-RF junction.
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