Characterization of SIS Receivers Using a Digital Spectrometer

Fourier transform based digital spectrometers, which offer wide instantaneous bandwidth and high spectral resolution, are widely used in SIS receiver systems. In this paper, we demonstrate that a digital spectrometer connected to the output of an SIS receiver can be used to probe the standing wave structure in the optics between the SIS mixer and a terminating input load. It can be shown that this standing wave induces ripples in the output power spectrum of the mixer, the amplitude of which depends on the product of the voltage reflection coefficients of the load and the mixer. This relation can be exploited to derive the input reflection coefficient of the SIS mixer as well as its sideband ratio. Both the theory and experimental details will be presented.