MMA Memo #264 Local Oscillator Power Requirements for ALMA SIS Mixers

We discuss requirements for the local oscillator (LO) power specifically for Nb-based SIS mixers for the ALMA Project. The scheme of the LO power generation / distribution for ALMA receivers is discussed. LO power for Nb-base SIS mixer Performance of a SIS mixer is a function of DC bias and applied LO power [1]. We make a number of assumptions to simplify our estimation and to calculate the LO power which is required for optimum SIS mixer operation. First, we assume that the tuning circuit resonated out the SIS junction intrinsic capacitance and the junction’s quantum reactance is negligible. We take the DC biasing of Nb-based SIS mixer at the middle of the first quantum step below the gap voltage. Parameter α that characterizes the normalized level of LO amplitude across the SIS junction at the optimum LO power level is set to α=1, and α = eVlo / h f, where e is electron charge, Vlo is the amplitude of LO signal, h is Plank’s constant, and f is the operating frequency. If we consider sinusoidal shape of the LO signal voltage across the junction then LO power at the SIS junction can be calculated as Plo = (Vlo) 2 / 2 Rlo. Here, Rlo is the DC bias dependent real impedance of the tuned SIS junction with the normal state resistance Rn pumped at level α by LO. For Rlo calculations as a function of frequency f we used model SIS junction current-voltage characteristic (IVC) depicted in the Figure 1. 0 1 2 3 4 5 6 0 0.25 0.5 0.75 1