. The Bell System technical journal . n good instantaneous stability, but high enoughnot to introduce difficulties in designing the filter following the modu-lator. The capacitance of the air condenser is about 800 ^u/xf. Fromthe circuit design standpoint a larger capacitance would be desirable,but for the stability required the overall size of the condenser sets anupper limit to the capacitance. The frequency and air condensercapacitance values determine the value of the fixed condenser at 1000mmf and the coil inductance at 50 microhenries. The fixed oscillatoris similar to the variable oscil
. The Bell System technical journal . n good instantaneous stability, but high enoughnot to introduce difficulties in designing the filter following the modu-lator. The capacitance of the air condenser is about 800 ^u/xf. Fromthe circuit design standpoint a larger capacitance would be desirable,but for the stability required the overall size of the condenser sets anupper limit to the capacitance. The frequency and air condensercapacitance values determine the value of the fixed condenser at 1000mmf and the coil inductance at 50 microhenries. The fixed oscillatoris similar to the variable oscillator except for the omission of the vari-able air condenser. HIGH ACCURACY HETERODYNEOSCILLATORS 417 The frequency setting is recorded on a 300-inch film scale such asdescribed above. This gives a spread of two inches per the 50-cycle divisions marked directly the mechanism can bereadily set to an accuracy better than 25 cycles. The visibility of thescale is greatly enhanced by a pilot lamp placed in back of the scale. Fig. 4—Front view of the oscillator. window with an intervening opal glass. A crank on the front of thepanel is used to set the oscillator, the range being covered in 47 revolu-tions. When changing the frequency setting even at a moderate ratethe speed with which the film moves prevents the operator from observ-ing the frequency setting. To make the adjustment more convenient,a coarse scale is recorded on a dial which can be read easily to one 418 BELL SYSTEM TECHNICAL JOURNAL kilocycle while the mechanism is in motion. It can be seen under thehood in the center of the panel. Below the coarse frequency dial is seen a small dial connected to avariable condenser which permits the operator to vary the frequency ofthe oscillator up to ± 50 cycles from the frequency to which it is setand to read the frequency change with an accuracy of about 3 feature is found to be useful in locating peaks of frequency char-acteristics of sharply resona
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