Monolithic GaAs Passive Lowpass 3 dB In-Phase Splitter/Combiner

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  • Author: Rick Campbell, Cascade Microtech and Terry White
The design of an integrated low-pass broadband passive in-phase split-ter/combiner is described in this article. The circuit is implemented on a stan-dard GaAs process with three thick metal layers, high Q capacitors, high Q inductors, nichrome resistors and tightly coupled transformers. The splitter topology consists of a coupled transmission line structure and a 50 ohm to 25 ohm low-pass impedance transforming net-work. Measurements are presented for an example circuit with 50 percent bandwidth centered at 2 GHz. Across the design band-width the insertion loss in excess of the 3 dB power division varies from 1.1 to 1.5 dB, out-put port isolation is greater than 22 dB, and return loss at all ports is greater than 20 dB.The 2 GHz splitter has a monotonic lowpass response, with 28 dB attenuation at 4 GHz. The absolute amplitude difference between the two output ports is less than 0.1 dB with a variation of a few hundredths of a dB across the passband. The absolute phase difference between the output ports is 0.4 degree, and varies less than 0.1 degree between 1.5 and 2.5 GHz. Die area is smaller than 1 mm2.