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This article establishes a lower bound for the noise figure of two networks in cascade. This is illustrated by means of a passive low-level frequency converter which utilizes a back diode as a nonlinear conductance. Other investigators have analyzed the properties of low-level frequency converters. However, the greatest lower bound of a combination of the converter and intermediate frequency amplifier system was not considered previously and is the subject of this paper. Knowledge of this lower bound has practical significance because if a measured value of noise figure is less than this lower bound, it follows that the measurement is in error.
For illustrative purposes a lower bound is established on one of the highest quality systems available, employing a special General Electric diode as a frequency converter for the first network, and a 60 MHz IF amplifier for the second network. This resulted in a noise figure lower bound of 3.86 db for the overall system.
This paper shows how this lower bound was obtained. Also the controversial right angle diode characteristic is found to be undesirable from an impedance viewpoint.