START-STOP RECEIVERS 227 



of the bias parallelogram of a receiver sufltering from internal negative 

 characteristic distortion. 



Some CoNSirs nations In\'Olved in the Measurement and Adjustment 

 OF Start-Stop Receivers 



Because of the effects of characteristic distortion, it cannot be assumed 

 that the ultimate tolerance of a receiver is equal to the sum of the displace- 

 ment of the received test signals and one-half the remaining orientation 

 range, especially if the latter is large. To attain accurate results, the 

 ultimate tolerance must be measured with the orientation adjusted to the 

 center of tolerance. 



For the same reason (the curvature of the "parallelogram" caused by- 

 internal characteristic distortion) measurements of internal distortion on a 

 receiver which is, itself, to be used to measure distortion should be made with 

 displacements of approximately the same magnitude as the distortions 

 which the receiver is to measure. In a receiver which is to be used to 

 measure small distortions, we are interested in the properties of the linear 

 portion of the parallelograms. Hence we measure the receiver's internal 

 bias and skew using small amounts of displacement in the measuring signals. 

 The internal fortuitous distortion may generally be neglected, since it does 

 not affect the shape, but only the size, of the distortion-vs-margin char- 

 acteristic. 



On the other hand, in a receiver which is to be used for receiving signals 

 we are interested not so much in the shape of the characteristic as in the 

 ultimate tolerance to telegraph distortion at an optimum setting of the 

 orientation mechanism. For this reason, a receiver destined for service use 

 is best tested with signals containing fairly large displacements. Internal 

 fortuitous distortion is deleterious in such a receiver, since it decreases the 

 tolerance to displacement of all kinds. Skew, depending upon its sign, 

 affects the tolerance to either space-to-mark or mark-to-space displacements. 



It should be realized that the removal of skew does not necessarily improve 

 a service receiver. In the case of bias or characteristic distortion the 

 introduction of distortion of a given sign will remove internal distortion of 

 the opposite sign, and thus improve the performace of the receiver. But 

 since skew is the difference between two fortuitous distortion effects, it may 

 be removed either by reducing the larger or increasing the smaller effect. 

 The former procedure will increase the receiver's total tolerance to distor- 

 tion, whereas the latter will reduce it. 



In practice bias tolerance is generally considered to be more desirable than 

 "end distortion" tolerance. The reason for this is that most transmission 

 circuits suffer from some bias (of unpredictable sign and amount) which uses 

 up some of the receiver's bias tolerance but none of its "end distortion" 



