556 BELL SYSTEM TECHNICAL JOURNAL 



speech under present operating procedures. The extensive use of telephone 

 channels for nontelephone purposes is thus curtailed. 



VI. Overload Distortion and Noise Threshold 



In designing a microwave system for a large number of channels the power 

 required to override noise may exceed the power capacity of available ampli- 

 fiers. Also, the bandwidth may exceed the limit imposed by microwave 

 transmission phenomena or circuit techniques. In either case, the remedy 

 is to divide the channels into several groups of fewer channels and trans- 

 mit the groups in adjacent narrower bands spaced by the proper factor U, 

 and separable with filters for individual amplification, reshaping or regenera- 

 tion. The power requirement falls ofif linearly with bandwidth. The filter 

 problem for AM pulse transmission is considered in Section \TI. The total 

 frequency occupancy is no greater for this division since the same percentage 

 "guard band" is involved if, in both cases, the neighboring, foreign signals 

 are of the same kind as the wanted signals. In case the neighboring signals 

 are of a different kind, the multiple band arrangement is in fact likely to 

 represent a smaller occupancy because the occupancy is in general more 

 sharply defined when made up of several narrower bands. 



When considering a multiple group arrangement, it may be economical 

 to provide for a substantial amount of common amplification prior to separa- 

 tion into the several bands which receive individual treatment. The non- 

 linearity of the common amplifier then sets a limit to the common amplifica- 

 tion. Experiments bearing on this overload limit were made with the PCM 

 equipment described by Meacham and Peterson.^- Two- and eight-fre- 

 quency groups were employed and the amplifier load was increased until 

 the effects of distortion began to appear. The distortion was measured in 

 terms of the maximum amount of CW interference which, when added to 

 the amplifier output, resulted in no audible effect in the PCM channels. 

 The right-hand part of Fig. 24 plots the results. For eight bands (six of 

 which were not pulsed but were left on as unmodulated carriers) it is seen 

 that the margin provided against CW interference begins to shrink rapidly 

 when the single group load is 20 db below the output at which 1 db compres- 

 sion occurs. The margin is completely used up (the channels begin to show 

 noise) when the load is 13 db higher. The left-hand part of Fig. 24 plots 

 the manner in which the low level limitation (noise) was found to appear. 

 Margin against CW interference shows a reduction for a pulse-to-noise ratio 

 of 28 db and is completely used up at a ratio of 18 db. 



The overload occurred in the 4000-mc power amplifier associated with the 

 repeater, and the noise originated in the receiver. 



In non-reshaping amplitude-modulated systems, the effect of compression 



" Loc. cit. 



