416 BELL SYSTEM TECHNICAL JOURNAL 



tor. It gives a scale length of 300 inches, the equivalent of a 5-foot 

 dial and can duplicate settings to better than one part in 10,000. 



One detail of construction of such long scales deserves mention. 

 Commercial worm driven air condensers carry on the worm shaft a 

 drum or a dial on which fractions of a revolution of the worm shaft are 

 recorded, while the number of revolutions is recorded on a main dial 

 fixed on the rotor shaft. The effective scale length is then equal to the 

 total displacement of the periphery of the small dial or drum. Using 

 such a construction the oscillator has to be set by consulting a calibra- 

 tion chart where the position of the main dial and of the worm shaft is 

 recorded against the oscillator frequency. Thus one of the most 

 valuable properties of the short scale audio frequency oscillator, its 

 direct reading, is lost. 



To remedy this situation a special scale mechanism has been de- 

 veloped for carrier frequency oscillators which combines great scale 

 length with good spread and compactness. It consists of a long motion 

 picture film strip engaged by a two inch film sprocket mounted in place 

 of the conventional drum on the worm shaft. The rotation of the 

 shaft determines the displacement of the film against an index which 

 reads the frequency directly in kilocycles. The loose ends of the 

 strip are wound up on two spools interconnected by a spring mechan- 

 ism which takes up the slack. The whole mechanism is confined in a 

 space about 4" by 5" by 5" accommodating a scale length up to 450 

 inches with a scale spread corresponding to A/ = .05". 



Specific Application 



An example of application of these methods in the design of a 

 heterodyne oscillator is furnished by an oscillator built for use in con- 

 nection with the installation and maintenance of broad band transmis- 

 sion systems. It is shown on Fig. 4. 



It has a frequency range of from 1 to 150 kc. Its variable frequency 

 oscillator covers a range of from 500 to 650 kc. This was chosen as 

 low as possible to obtain good instantaneous stability, but high enough 

 not to introduce difficulties in designing the filter following the modu- 

 lator. The capacitance of the air condenser is about 800 ^u/xf. From 

 the circuit design standpoint a larger capacitance would be desirable, 

 but for the stability required the overall size of the condenser sets an 

 upper limit to the capacitance. The frequency and air condenser 

 capacitance values determine the value of the fixed condenser at 1000 

 mmf and the coil inductance at 50 microhenries. The fixed oscillator 

 is similar to the variable oscillator except for the omission of the vari- 

 able air condenser. 



