HIGH ACCURACY HETERODYNE OSCILLATORS 415 



setting, into an electric coordinate, the output frequency. In planning 

 the oscillator design, therefore, it is necessary to give as much attention 

 to the construction of the scale as to the construction of the circuit 

 elements. 



For maximum scale length economy the scale should be so subdivided 

 that a frequency interval equal to the tolerable frequency error AF 

 could be read. The scale interval A/ corresponding to this frequency 

 interval, will vary with the measuring conditions. For well illuminated 

 scales on panel mounted equipment to be read conveniently at arm's 

 length an interval A/ of at least .05" is needed. For portable apparatus, 

 intervals as small as .02" have been used. With the aid of a vernier it 

 can be brought down to .001". Scale spreads such that a frequency 

 interval much smaller than hF can be read are not only uneconomical 

 but are also objectionable because they encourage the use of the 

 instrument beyond its accuracy limits. 



Having chosen A/ and the frequency error AF at all points of the 

 scale, the scale shape / = f{F) can be determined by the approximation 



Jo ^F' 



As an example, in audio frequency applications the most common 

 form of frequency accuracy desired is that having a constant percentage 

 value AF/F — p at the upper part of the scale. At lower frequencies 

 this accuracy is higher than necessary and the requirement is changed 

 to a constant AFo. A smooth shape is obtained by making the transi- 

 tion point Ft at such a frequency that AFo/ Ft — p. The scale shape 

 is then approximately 



and 



*Ft 



f 



'= ' ik""-^." f- ^<^' 



The scale of common type of audio frequency oscillator can be 

 spread over a ten inch dial giving a satisfactory accuracy. 



For carrier applications, where the spread of any voice band is inde- 

 pendent of its position in the frequency range the error function takes 

 the form of a constant and the scale should be linear. Usually the 

 scale lengths involved are much larger than in audio oscillators. To 

 obtain sufficient scale length a precision worm and gear mechanism 

 has to be used to drive the tuning condenser of the heterodyne oscilla- 



