Microwave core-chips are highly integrated MMICs that are in charge of all the beam-shaping functions of a transmit-receive module within a phased array system. Such chips include switches, amplifiers and attenuators, phase shifters, and possibly other elements, each to be controlled by external digital signals. Given the large number of control lines to be integrated in a core-chip, the embedding of a serial to parallel interface is indispensable. Digital design in compound semiconductor technology is still rather challenging due to the absence of complementary devices and the availability of a limited number of metallization layers. Moreover, in large arrays, high chip yield and repeatability are required. This paper discusses and compares challenges and solutions for the key sub-circuits of GaAs serial to parallel converters for core-chip applications, reviewing the pros and cons of the different implementations proposed in the literature
