Abstract

A novel combination of a packaged GaAs MMIC with a microstrip-coupled slot high performance 180° hybrid constitutes a downconverter, which provides the very low intermodulation level required by some TV applications. The packaged chip was mounted on a substrate together with a high performance and seldom-used MIC 180° hybrid. No adjustable filters are required due to the broadband behavior of the whole downconverter. This feature is required for cost reduction. A double-balanced mixer and the LO balun have been integrated in the same chip. A properly biased cold FET was used in the mixer to achieve low distortion frequency mixing. An active phase splitter was chosen as the LO balun in order to get small phase errors over a wide frequency band. The MIC 180° hybrid, which makes use of microstrip lines, slot lines and coupled-slot lines, was used as the RF balun. The topology used for this hybrid makes it very useful for balanced mixers, since there is no crossing of MIC transmission lines. On the other hand, there are no active elements, so no distortion is added to the downconverter. The hybrid was designed with an intensive use of a quasy-planar electromagnetic simulator. MMIC package modeling is also discussed in the paper. The MMIC was fabricated using the F20 process of the GEC Marconi foundry. Conversion losses of 8.5 dB can be obtained for the whole downconverter with a LO drive of 5.5 dBm. A 3rd order intercept point of 21 dBm of output power and a spectral purity in the complete RF frequency band of more than 70 dB were achieved.