336 BELL SYSTEM TECHNICAL JOURNAL 



Fl is seen to be dependent upon the strength of the high frequency 

 noise as well as upon that of the interfering carrier. It is also to be 

 noted that the flutter will be more serious with the quadratic than with 

 the linear detector by a factor 4/k = 4E/N, which is usually large. 



This derivation of Fq and Fl on the basis of a single frequency 

 noise component serves to indicate important differences between the 

 two types of detector and to show how the flutter changes with the 

 noise level and with the ratio of the incoming carrier amplitudes. In 

 any practical case the noise field would consist of numerous frequency 

 components, but it is reasonable to expect that the proportionalities 

 expressed in (4) and (5) would still hold. However, the absolute 

 values of N and K at which the flutter becomes detectable must be 

 determined experimentally and may be expected to depend upon the 

 width of the received frequency band. 



In the foregoing derivations it has been assumed that there is no 

 automatic volune control in the receiving set. A brief examination 

 of the effect of such a device will now be made. 



Action of an Automatic Volume Control 



The comparative freedom from flutter effects which has been noted 

 in the case of the linear detector may be regarded as due to the fact that 

 the audio-frequency output of such a detector is independent of carrier 

 amplitude over a wide range. If automatic volume control is used in 

 the receiving set, the amplitude of the carrier wave will be maintained 

 practically constant at the input terminals of the detector. If the 

 effective carrier amplitude impressed upon the antenna undergoes a 

 periodic fluctuation, due to very low frequency heterodyning between 

 the two stations, the gain of the radiofrequency amplifier will undergo 

 cyclic variations, so as to keep the carrier constant at the detector. 

 Obviously this will cause a fluctuation in the amplitude of the side- 

 bands, be they due to noise or program. 



From this it is evident that, on the one hand, flutter effects in the 

 presence of a noise background will usually be of minor importance if a 

 good linear rectifier is employed in conjunction with a manual volume 

 control; while, on the other hand, these effects may become extremely 

 objectionable if automatic volume control is used. Because of the 

 prevalent use of AVC in modern radio receivers the low flutter char- 

 acteristics of the linear detector cannot be generally employed to 

 reduce flutter interference on shared channels. 



In the case of the square law detector, the output is proportional 

 to the product of the amplitudes of the carrier and side frequencies. 

 At first glance it might seem that the use of automatic volume control 



