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BELL SYSTEM TECHNICAL JOURNAL 



homodyne method of detection has all the conversion efficiency of the usual 

 double-detection arrangements and, in addition, has the advantage in this 

 particular application of having a very low noise level due to the relatively 

 narrow band required for the audio signals. The 40-decibel level difference 

 mentioned above is accordingly not a serious handicap. 



Other precautions must be observed. The homodyne carrier can be 

 brought in quadrature with the signal for two different phases 180° apart. 

 This is illustrated in Fig. 2. In many applications, where only the variation 



Fig. 3 — Variable phase shifter using a polystyrene vane. 



in phase difference is of importance, this uncertainty of 180° can be ignored. 

 The correct setting of the homodyne carrier phase can, however, be deter- 

 mined very easily. Assume that the input probe is connected to the receiver 

 and that the phase has been adjusted for a balance. Then disconnect the 

 audio frequency drive from one of the crystals in the balanced modulator. 

 The residual carrier will now no longer be suppressed and the error angle 

 A(/> of Fig. 2 will become larger. Whether the homodyne carrier is lagging or 

 leading the signal carrier can be determined by observing whether more or 

 less phase shift, respectively, must be introduced to restore balance. A 

 similar test performed with the output probe will indicate whether or not it 

 is necessary to add 180° to the measured phase difference. If either probe 



