  The tunneling spectrum of an electron and a hole in a superlattice of NS
junctions is computed using the BTK approach and the transfer matrix method. It
shows sharp resonances at some energies above the superconducting gap. The
sharper the resonance is the more layers the superlattice has. We find for the
first time a mechanism to balance the incident and outgoing currents on the
superlattice by averaging over the phase between the incident electron and the
incident hole. This mechanism is more natural and physical than those in
literatures.
