We have measured the noise temperature of heterodyne receivers employing Nb/AlOx/Nb tunnel junction mixers at frequencies ranging from 70% to 93% of the gap frequency of niobium (∼700 GHz). The sensitivity of the receiver is decreased by the overlap of then=1 andn=2 photon steps of opposite sign. At bias voltages where these photon steps overlap, there is an increase in receiver noise up to 50%. Theoretical calculations using the Tucker theory agree well with the observed mixer performance. This overlap already affects the receiver operation for best performance at frequencies well below 700 GHz. ©1995 American Institute of Physics.