MMA Memo #150

SIS Mixer Analysis with Non-Zero Intermediate Frequencies

S.-K. Pan, A.R. Kerr

March 1, 1996

Keywords: SIS mixers, receivers, IF, intermediate frequency, LO, local oscillator, wide bandwidth

Most design and analysis of Superconductor-Insulator-Superconductor (SIS) mixers has been based on Ticker's quantum theory of mixong, always with the assumption of zero intermediate frequency. This paper relaxes the zero IF constraint and explores the performance of SIS mixers at intermediate frequencies which are a significant fraction of the LO frequency.

The complete expression for the elements of the admittance matrix [Y] of a quantum mixer has been given by Tucker [1]. Examination of this expression shows that for a non-zero IF the symmetry of the admittance matrix elements between opposite sidebands, ie, Y(m,n) = Y(-m, -n)*, is broken by the quantization of the IF. Therefore, the conversion gain and noise temperature of opposite sidebands will be different. Also, the output impedence of the mixer is no loner real and the output resistance varies as a function of the IF. In this study a quasi five-frequency approximation to Tucker's theory (ie, sinusoidal LO and five small-signal sidebands with non-zero IF) is used to simulate the performance of SIS mixers at different intermediate frequencies. It is found that the zero-IF approximation is appropriate for IF/LO < 0.1. The SIS mixer itself should be capable of excellent performance for a very wide IF bandwidth (20% to 30% of the LO frequency) and the design of an optimum coupling network between the mixer and IF amplifier should be straightforward.

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Last modified: 09 December, 1999