348 BELL SYSTEM TECHNICAL JOURNAL 



6b. Selectivity Requirements Imposed by the Use of Demodulators. 

 In all demodulators, the range of desired output frequencies should 

 not be included in the input frequency band because the input fre- 

 quencies amplified appear in the output of the demodulator as the 

 first order modulation product. The output band of the demodulator 

 should be at a lower frequency than the input band in order to reduce 

 the number of undesired modulation products in the output and in 

 order to obtain the benefit of greater selectivity from circuits operating 

 at lower frequencies. Of course, if all the required selectivity can be 

 conveniently put before the first demodulator, there is no valid reason 

 why multiple demodulation should be used. 



In all demodulators except the final demodulator of a radio receiver, 

 the band of frequencies allowed to pass into the demodulator should 

 not be greater in width than the absolute value of the lowest desired 

 frequency in the demodulator output. This requirement is apparent 

 when we consider second order modulation products of interference 

 within the band accepted by the demodulator. Suppose we assume 

 the use of double demodulation and choose 30 kilocycles as the lowest 

 desired frequency in the output of the first demodulator. Then if the 

 band impressed upon the demodulator be more than 30 kilocycles in 

 width, two interfering signals within the band might together give a 

 difference frequency of 30 kilocycles producing load in subsequent 

 stages and possibly tone or noise in the output circuit. 



Second order modulation between two signals, one lying within the 

 band accepted by the demodulator and one outside it, may also give 

 rise to interference, due to the difterence frequency falling in the out- 

 put band of the demodulator. Assume that one interfering signal 

 lies within the band accepted by the demodulator and is -f- 60 TU 

 referred to the minimum desired signal at the grid of the first tube. 

 An equal signal at a frequency outside the band and subject to the 

 selectivity provided for meeting the load requirement will be — 20 

 TU referred to the minimum desired signal at the same point. Since 

 the second order output from a demodulator is approximately pro- 

 portional to the product of the grid voltages producing it,^° we may 

 write (in TU) : 



Relative desired signal = (0) + (Beating oscillator voltage). 

 Relative interference = (+ 60) + (- 20) - + 40. 



Tests have shown that an interrupted tone, similar to telegraph 

 interference, which is heard at a frequency of 1,100 cycles in a tele- 



'"J. R. Carson, "A Theoretical Study of the Three-Element Vacuum Tube," 

 Proc. I. R. E., 7, 187; April, 1919. 



