PRJEQUENCr SHIFT TELEGRAPHY 291 



Level Variation^ 



Extreme and rapid variations of received level exist in H.F. radio trans- 

 mission. It is upon the ability to accommodate these level variations that 

 the merits of an H.F. radio telegraph system must largely be judged. FS 

 telegraph shows its outstanding advantage in this respect. 



In an AM system in order to obtain zero-bias signals and optimum signal- 

 to-noise conditions the operating point must be near the half amplitude point 

 on the demodulated wave. This means that complete failure will result for 

 a drop in level of 6 db unless some compensating arrangement is provided. 

 The slope of the bias-versus-level characteristic depends upon the slope of 

 the demodulated wave which in turn depends on the bandwidth of the sys- 

 tem and upon the degree of low-pass filtering. The bias-versus-level char- 

 acteristics of some AM systems are shown in Fig. 28. Where the level 

 variations are relatively slow compared to the signaling speed, automatic 

 gain control circuits can be used to maintain a nearly constant level into the 

 demodulator. However, w^here large rapid level changes occur, as in the 

 H.F. range, it is seen that a narrow band AM system would fail completely 

 regardless of the amount of transmitted power. For printer operation 

 over an AM system in the H.F. range a fairly wide band and Uttle low-pass 

 filtering should be used, so as to keep the wave shape of the signals as 

 square as possible and thus obtain a fairly flat bias-versus-level charac- 

 teristic. By adjusting the operating point low on the demodulated wave, 

 approaching the spacing noise level, the greatest possible range of acceptable 

 rapid level change will be obtained. The slower level change components 

 may be handled by the usual automatic gain-control circuits. This w4ll 

 cause the bias of the signals to average somewhat marking but the peak 

 distortions will be kept to a minimum. 



In an FS system no bias is produced so long as both the marking and spac- 

 ing frequencies are affected alike, with their received levels remaining equal. 

 Such non-selective fading conditions cause no distortion even when they 

 occur at quite rapid rates. If a balanced type of discriminator is used, 

 amplitude limiting is not essential to obtaining this immunity from non- 

 selective variations in attenuation. It is only when the mark and space 

 levels are different that bias results. In Fig. 32 are shown bias versus mark- 

 to-space level ratio characteristics both with and without a limiter. More 

 bias exists when there is no limiter because the amplitude of the demodulated 

 wave is directly affected and consequently the low-pass filtering also becomes 

 a factor. With a limiter the amplitude of the demodulated wave is held 

 constant and the amount of low-pass filtering has no effect on bias. Some 

 bias is still produced, however, due to the differently shaped frequency 

 transients in the passband of the receiving system when a level change occurs 



