Design of Relaxation Oscillator Based Ultra-wideband SFQ Amplifier for Chip to Chip Interconnection

In Rapid Signal Flux Quantum (RSFQ) logic circuits, on-chip interconnects and multichip module implementations for nearby distances have already been established. However, the flexible interconnection of two distant chips is still not achieved reliably due to impedance mismatching and attenuation. In this work, we propose a circuit that allows the usage of Passive Transmission Lines (PTLs) to transfer single-flux-quantum (SFQ) pulses between two distant chips which are separated by a distance greater than 10 cm by using 50 Omega transmission lines. For this purpose, we design an SFQ amplifier circuit to deal with impedance mismatch and attenuation problems. The circuit consists of two main parts: a relaxation oscillator (RO) circuit and an impedance transformer. The RO circuit utilizes relaxation oscillations occur in the underdamped Josephson junctions. The impedance matching circuit is an 8-section Chebyshev quarter-wave transformer and it eliminates impedance mismatching problem between the amplifier circuit and PTL. We performed circuit simulations and obtained voltage amplitude of about 600 mu V at the output of the circuit. The transformer has a broadband impedance matching with a fractional bandwidth (ratio of the bandwidth of a device to its central frequency) of 1.4 and a maximum Voltage Standing Wave Ratio (VSWR, the maximum voltage divided by minimum voltage on the transmission line) of 1.5.