GaAs digital IC for FM demodulator

A middle-scale GaAs digital IC for a pulse-counting FM demodulator has been designed and fabricated for use in microwave communication systems. The IC is constructed with the normally-on (depletion) MESFET logic with epitaxial active layers. It exhibits a high-speed pulse train with a rise time of 150 ps and with an output voltage of 2.3 V into a 300-Ω load. Output pulsewidth can be altered by the external voltage for tuning the IC to give the highest demodulation sensitivity. By merely implementing a low-pass filter at the output of the chip, the pulse-counting FM demodulator can be realized at high frequencies where the use of Si IC's has never before been possible. We estimate the demodulation sensitivity to be 20 mV/MHz at the microwave IF band.