LIMITS TO AMPLIFICATION 93 



level is of the order of 20 db above the tube noise measured with 

 d-c heating. 



Noise Front Mbration^-- •'•'• •'"• ■"' 



Mechanical ^■ibration changes the relative positions of the tube 

 elements and hence causes disturbing noise. This is especially ob- 

 jectionable at audio frequency, although a radio frequency carrier 

 may become modulated sufficiently to produce noise. 



The remedy, used in the so-called "low microphonic" tube designs, 

 is to stiffen the construction of the tube elements and to apply damping 

 to their vibration, as well as to cushion the tube by a suitable mounting 

 and to shield it from sound waves. The indirectly heated cathode is 

 superior to the filamentary cathode in regard to noise from vibration. 



Noise From Poor Insulation ^'^' ^^- ^^ 

 Noise arises from resistance changes at contacts and across thin 

 films of conducting material deposited on insulating supports in the 

 vacuum tube. Leaky capacitors may produce a similar noise. 



Noise From Faulty Resistances 

 Many resistors in which the resistance element is a thin film are 

 sources of noise. If no current flows in them only thermal noise is 

 generated, but when direct current passes through them more noise is 

 produced. The noise voltage is roughly proportional to the direct 

 current. These resistors must be chosen carefully for circuit branches 

 where current flows. 



SiGNAL-TO-NOISE RaTIO ^' ^^' ^^' ^- 



In so far as noise is concerned, the merit of a transmission system is 

 dependent not only on the amount of noise present, but also on the 

 strength of the signal, so that a determination of the ratio of the signal 

 level to the noise level is necessary. Fortunately, this ratio has a 

 reference value for any given transmission system determined uniquely 

 by the ratio of the signal to thermal noise in the input circuit. The 

 ratio of the signal level to noise level is here the greatest the ratio 

 ever can attain, because noise that originates at subsequent points in 

 the amplifier contributes to the noise level without increasing the 

 signal. 



This fact provides a basis for the rating of amplifiers, the thermal 

 noise of the input circuit being used as a comparison signal. For 

 example, the noise output of an amplifying system may be 0.3 mw 

 which falls to 0.2 mw when the input circuit is short-circuited. The 

 thermal noise from the input circuit is then the difference between 



