626 



BELL SYSTEM TECHNICAL JOURNAL 



butes very little to the ordinary microphonic level reading. Special 

 methods must therefore be used in order to make measurements of 

 sputter noise which are independent of microphonic noise. One 

 method which has been found to be effective and convenient, is that of 

 frequency discrimination. If the audio frequency components of the 

 total noise are cut out, then microphonic noise is completely eliminated. 

 Sputter noise, however, due to its discontinuous character has, theo- 

 retically, an infinite frequency spectrum, and, practically, one which 

 extends at least into the broadcast band of radio frequencies. 



In the microphonic noise test set, sputter noise measurement is 

 provided for by switching in a high-pass filter cutting off sharply at 

 16,000 cycles. Greater sensitivity is also provided by additional 

 stages of amplification to permit the measurement of the lower levels 

 found to be characteristic of sputter noise in this frequency range. A 

 schematic diagram of the microphonic and sputter noise test set is 

 shown in Fig. 7. The weighted amplifier and the calibrating oscilla- 



oo 



MICROPHONIC NOISE 



SPUTTER NOISE AMPLIFIER- 



Fig. 7— Microphonic and sputter noise amplifier schematic diagram. 



tors (one for microphonic noise and one for sputter noise) have been 

 omitted for the sake of clearness. 



Reduction of Microphonic Noise 



The reduction of microphonic noise from the view-point of the 

 vacuum tube designer is chiefly a matter of mechanical design and 

 manufacturing technique. The quietest construction is obviously one 

 which has the stiffest electrodes and supporting members, the shortest 

 distances between points of support, and the highest damping of 

 mechanical vibration. The extent to which these features can be 

 incorporated in a practical tube design, however, is limited by the 

 requirements for favorable electrical characteristics. A low filament 

 current, for example, requires that the filament be small in diameter, 

 which renders it more susceptible to vibration than a heavier filament ; 

 or if a tube has an indirectly heated cathode, it is a problem to support 



