The output noise of a receiving system contains components contributed not only by the termination at the input of the receiving system but also by the receiving system itself. Furthermore, the output signal-to-noise ratio1 of the system will depend not only on the output noise but also on the nature of the signal that is impressed upon the input of the receiver. Hence, any meaningful evaluation of the noise performance of a receiver when used in a particular system must include considerations of the sources that contribute to the output noise, the bandwidth and gain of the receiving system in all of its responses, the nature of the signal and the efficacy of the output utilization circuit. It is evident that no single number can describe completely how well a given receiver will perform in all kinds of systems. What, then, are the pertinent attributes of a receiver, and how are they measured and quoted? From the viewpoint of the designer of the receiver, the attributes must be readily measurable. From the viewpoint of the designer of the system, the numbers quoted by the receiver designer must be such that the output signal to-noise-ratio (SNR) under operating conditions can be calculated.