A Broad -Band Microwave Noise Source 

 By W. W. MUMFORD 



Measurements of the microwave noise power available from gaseous discharges, 

 such as in an ordinary fluorescent lamp, show remarkable uniformity and sta- 

 liilily. Such tubes are therefore suitable for a new type of standard noise source. 



Introduction 



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 wdiich 

 are needed for the determmation 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 avail- 

 able from a resistance might be 141 db below one watt.^ It is difficult to 

 ascertain accurately power ratios of this magnitude, 10^°. 



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 furn- 

 ishing adequate noise power, but one with wide bandwidth has yet to be 

 developed. A good, stable, broadband microwave noise generator is needed. 



Another possible source of noise power consists of a gaseous discharge. - 

 Before we examine the data which have led us to conclude that the gaseous 

 discharge is a good, broad-band, stable microwave noise generator and pos- 

 sibly a calculable noise standard, we review our definitions of noise figure 



^ This figure, 141 db below one watt, assumes that the effective bandwidth is 2 mc. 

 The resistance noise power available from a generator at 290° Kelvin is 204 db below one 

 watt per cycle. 



«G. C. Southworth, Journal of the Franklin Inslilule, Vol. 239, ^U, pp. 285-298, 

 April 1945. 



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