VACUUM TUBES'FOR^RADAR t.P. AMPLIFIERS 



397 



show that the j)erformaiice of the 6AK5 near the upper end of its present 

 useful frequency range is probably limited to a considerable degree by the 

 lead inductances rather than by transit time effects in the structure. 



^ 1400 



O 



2 



Z 1200 



u 



Z 600 



400 



160 180 200 220 240 260 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 9 — Average input conductance vs. frequency for six 6AK5 tubes — courtesy of 

 Naval Research Laboratory. 



4. Noise 



Although it may be possible to employ enough stages of IF ampUfication 

 to provide the necessary gain and band-width we may still have a relatively 

 insensitive receiver for weak signals. This comes about because there are 

 inherent electrical disturbances in vacuum tubes and passiv^e networks which 

 give rise to random voltages. Since these disturbances may be of the order 

 of the strength of the signal, they must be kept to a minimum in order to 

 maintain a high signal-to-noise ratio. In a receiving s}'stem in whfch no 

 RF ampUfication is used ahead of the first detector, the signal-to-noise ratio 

 is limited to a large extent by the noisiness of the first detector and the first 



