Gain Expansion and Compression of SIS Mixers

It is a well known fact that SIS mixers exhibit gain compression when the output IF voltage swing becomes non-negligible compared to the width of the photon step. In this paper, we show that under certain circumstances, SIS mixers can also exhibit the inverse phenomenon-gain expansion. This non-linear behavior has been observed experimentally when the output power-voltage curve of an SIS mixer has a double peak feature over the photon step. In this case, if the device is biased at the valley position between the two peaks, the conversion gain of the mixer will increase with input signal power. Consequently, the noise temperature of the mixer deduced from a Y-factor measurement will be artificially reduced. Two different experimental methods have been used to characterize the linearity of SIS mixers. The accuracies of both methods are analysed.