322 BELL SYSTEM TECHNICAL JOURNAL 



of the wave. If it is concave downward the peak value of the wave 

 decreases. 



In the case of a two-frequency input v = P cos pt -\- Q cos qt the second- 

 order products are again unaffected. The third-order products will be: 



^'^ " 3 (M^ ^ 



There are situations in the design of detectors where the /,±2p current may- 

 be disturbing. For example, in a wave analyzer modulation stage when 

 measuring the second harmonic Pi cos {2p)l the desired second-order product 

 is Iq-iip)- This is, however, of the same frequency as the third-order 

 product Iq-2p generated by the intermodulations of the strong fundamental 

 Pi cos pt and the carrier Q cos ql. The level of the wanted product with 

 respect to the unwanted one is given by 



20 log ^^> = 201og^ + 20 log ^^ +20^-6. 



If there are two interfering inputs R cos rt and W cos wt they may, together 

 with the carrier Q cos qt, form an objectionable product iri,w±q of the same 

 frequency as the wanted product ip-q. The level of this disturbing product 

 with respect to the wanted product is then given by 



201og^ = 20 log ^^ -f 20 log ^' + 20 log ^- 12. 



When two input frequencies are present in the input, the effective trans- 

 conductance of the tube becomes 



The amplifier gain depends on the level of all of the components of the input. 



Fourth Order Modulation 



1 d^g 4 

 If the fourth term of equation (3) is absent, but the fifth term ^aTz^ 



is present, fourth-order modulation will occur. The g-e characteristic will 

 be a cubic with an inflection point at Ee . 



