Application Note :: Fundamentals of RF and Microwave Noise Figure Measurements.
Modern receiving systems must often process very weak signals, but the noise added by the system components tends to obscure those very weak signals. Sensitivity, bit error ratio (BER) and noise figure are system parameters that characterize the ability to process low-level signals. Of these parameters, noise figure is unique in that it is suitable not only for characterizing the entire system but also the system components such as the pre-amplifier, mixer, and IF amplifier that make up the system. By controlling the noise figure and gain of system components, the designer directly controls the noise figure of the overall system. Once the noise figure is known, system sensitivity can be easily estimated from system bandwidth. Noise figure is often the key parameter that differentiates one system from another, one amplifier from an other, and one transistor from another. Such widespread application of noise figure specifications implies that highly repeatable and accurate measurements between suppliers and their customers are very important.
The reason for measuring noise properties of networks is to minimize the problem of noise generated in receiving systems. One approach to overcome noise is to make the weak signal stronger. This can be accomplished by raising the signal power transmitted in the direction of the receiver, or by increasing the amount of power the receiving antenna intercepts, for example, by increasing the aperture of the receiving antenna. Raising antenna gain, which usually means a larger antenna, and raising the transmitter power, are eventually limited by government regulations, engineering considerations, or economics. The other approach is to minimize the noise generated within receiver components.
Noise measurements are key to assuring that the added noise is minimal. Once noise joins the signals, receiver components can no longer distinguish noise in the signal frequency band from legitimate signal fluctuations. The signal and noise get processed together. Subsequent raising of the signal level with gain, for example, will raise the noise level an equal amount.
This application note is part of a series about noise measurement.
Much of what is discussed is either material that is common to most noise figure measurements or background material. It should prove useful as a primer on noise figure measurements. The need for highly repeatable, accurate and meaningful measurements of noise without the complexity of manual measurements and calculations has lead to the development of noise figure measurement instruments with simple user interfaces. Using these instruments does not require an extensive background in noise theory. A little noise background may prove helpful, however, in building confidence and understanding a more complete picture of noise in RF and microwave systems.
Other literature to consider for additional information on noise figure measurements is indicated throughout this note. Numbers appearing throughout this document in square brackets [ ] correspond to the same numerical listing in the References section. Related Keysight Technologies, Inc. literature and web resources appear later in this application note