 measurements are most commonly used for FM sets. However, they can be used for other modes as well. For AM it is simply a matter of changing the type of modulation. For SSB it is necessary to ensure that the receiver is tuned to exactly the correct frequency SO that the audio tone can be notched out. Once this has been done the measurement can be made in the normal way. Noise factor and noise figure Apart from signal to noise ratio and SINAD specifications, the noise performance can be specified in terms of a noise figure. This measure- ment is more versatile than the signal to noise ratio because it can be used to determine the performance of a piece of equipment whether it is a complete system, a receiver, or a smaller item such as a preamplifier. Essentially this measurement gives an indication of the level of noise which each item introduces. The idea of noise figure and noise factor is built around the fact that in any system there is a certain amount of noise below which it is not possible to go and each element will introduce some noise over and above this. This thermal noise is the limiting factor and it is dependent upon the resistance of the system. A 50 ohm resistor generates a certain level of thermal noise. As an antenna looks like a resistor to the input of the receiver, this too generates noise of its own. As a result of this thermal noise, any signal being picked up by the antenna has a certain signal to noise ratio associated with it. If the signal is very strong the signal to noise ratio will be good, but if it is weak it will be poor. When the signal is passed through an amplifier, receiver or any other piece of electronic equipment the signal to noise ratio will be degraded because the circuit will introduce additional noise. The noise factor is determined by taking the signal to noise ratio at the input and dividing it by the signal to noise ratio at the output. For these calculations the signal to noise ratio must be given as a ratio and not expressed in decibels. As the circuit will always