An 85–115 GHz SIS Mixer Demonstrating Nearly Constant Dynamic Resistance

We report the design and experiment results of 85-115 GHz SIS mixers that demonstrate nearly constant dynamic resistance and uniform conversion gain. The dynamic resistance close to 50 Omega allows stable bias and direct connection of the SIS mixer and the cryogenic low noise amplifier without an IF isolator. The tuning circuit consists of a single junction tuned by an equivalent shunt inductance and an impedance transformer used for impedance matching. Since the impedance transformer and the shunt inductance have opposite frequency-dependent admittances, we were able to eliminate the frequency-dependent components of the tuning circuit and managed to achieve nearly constant embedding impedance in the frequency range. Measurement results showing dynamic resistances ranging from 30 Omega to 50 Omega and conversion gain changing less than 3 dB reasonably agree with the simulations.