THE L3 SYSTEM — QUALITY CONTROL REQUIREMENTS 947 



The general plan has been implemented by imposing on important 

 component elements certain so-called "distribution requirements" which 

 incorporate quality control procedures for assuring a high degree of 

 statistical uniformity in the quality of product delivered for service. 

 The aim of the distribution requirements is to place a continuing limita- 

 tion on the pattern and the spread of measured values (of a final critical 

 characteristic of the product) around their average and to impose close 

 limits on the departures of the average from a desired nominal value. 

 To obtain these ends, close cooperation between the element designer 

 and the production engineer is essential. In fact, compatibility of the 

 specification requirements and the process capability is one of the basic 

 provisions of the general plan. This should be established, if at all possi- 

 ble, in the design stage. 



In some cases where distribution requirements have been applied to 

 a final characteristic of an element, the production engineer has found it 

 advantageous to introduce quality control techniques in some of the 

 earlier manufacturing steps, as for example, on materials, piece-parts, 

 or process operations which are found to have a major effect on end 

 quality.* The character of such controls can rarely be planned in ad- 

 vance, but must be tailor-made to fit the particular process being used. 

 Often too, a major difficulty encountered has been not the process itself 

 but the precision and accuracy of the measuring equipment. The resolv- 

 ing of such problems during the design and the early production stages 

 has been one of the aims of the general plan. 



2.2 SELECTION OF CHARACTERISTICS TO BE CONTROLLED 



The general procedure calls for imposing distribution requirements 

 on not more than one characteristic of any component element. Before 

 assigning limits on an individual component, therefore, it is important 

 that the characteristic selected for control be the key characteristic. 

 This statement seems to be trite, but its importance cannot be over- 

 emphasized. Controlling all characteristics of a component is not only 

 inherently uneconomical but may be found impossible in practice. In 

 the case of an inductor, for example, it should be ascertained which 

 characteristic is important to the circuit designer. It may be the value 

 of inductance, of Q, of temperature coefficient, or of parasitic capacity. 

 In addition, of course, requirements should be specific and apply, for 

 instance, at a given frequency or Avithin a definite temperature range. 



* R. F. Garrett, T. L. Tuffnell and R. A. Waddell, The L3 Coaxial System — 

 Application of Quality Control Requirements in the Manufacture of Compo- 

 nents, see pp. 969-1006 of this issue. 



