A novel size reduction of a MMIC I/Q direct down converter in uniplanar technology at 20 GHz using CPW and ACPS stubs loading for an EHF satcom terminal phased array size

This paper proposes a novel compact uniplanar 20 GHz/baseband I/Q direct downconverter GaAs MMIC. It uses previously developed ACPS and CPW structures for capacitive and inductive loading techniques to drastically reduce its size. The proposed I/Q down converter uses direct analog extraction of the baseband in-phase and quadrature (I and Q) signals from the 20 GHz RF signal. Two pairs of antiparallel diodes achieve frequency translation by mixing the RF signal with the fundamental local oscillator (LO) pump while novel ACPS reduced size magic T, provide high local oscillator (LO)-to-RF isolation and good spurious suppression. The circuit was fabricated using OMMIC´s ED02AH foundry process and contains two pairs of 0.18 mum PHEMT based diodes, each having two 15-mum cathode fingers. This 3.15 times 4.1 mm² chip uses a previously developed, reduced size Wilkinson divider as an RF divider and a previously developed, reduced size 90deg coupler. The I/Q down converter conversion gain was measured to be -15.5dB plusmn1 dB over the RF frequency range of 20.2 to 21.2 GHz with the IF fixed at 1 MHz. The amplitude imbalance between the I&Q channels was under 0.8dB and the quadrature phase error was less than 4degover the frequency range. The circuit also does not require any dc bias. Compared to the conventional microstrip I/Q direct down converter a 40% reduction in circuit area was achieved with better performance.