A Broad-Band Microwave Noise Source

A STANDARD noise source, such as a hot resistance or a temperature ^ limited diode, has been used advantageously for making measurements of the noise figure of radio receivers in the short-wave and the ultra-short wave region. The use of such a tool eliminates the possible errors which are practically inescapable when using the large amounts of attenuation which are needed for the determination of the ratio of power levels encountered in measuring noise figures with a standard signal generator. For example, the power from a standard signal generator might be measurable and known accurately at a level of 40 db below a watt, whereas the noise power available from a resistance might be 141 db below one watt. 1 It is difficult to ascertain accurately power ratios of this magnitude, 1010. Another advantage of using a standard noise source arises from the fact that ordinarily the bandwidth of the receiver need not be considered, thereby eliminating a time consuming measurement. This assumes, of course, that the bandwidth of the noise source is much greater than that of the amplifier under test. In the microwave region it is possible to match a resistive element to the waveguide over a wide enough band, but ordinary resistive materials will not stand the high temperatures (5000 degrees or more) needed to measure the noise figures encountered in practice. The noise diode is capable of furnishing adequate noise power, but one with wide bandwidth has yet to be developed. A good, stable, broadband microwave noise generator is needed.