. The Bell System technical journal . e resultant carrier is inquadrature with the sidebands, it is desirable that the residual carrier be lowin level compared with the homodyne carrier. The error in phase A0introduced by the residual carrier is shown in the vector diagram of Fig. difference in level of about 40 decibels between the homodyne and residualcarriers will give an error of not more than half a degree in phase. The 102 BELL SYSTEM TECHNICAL JOURNAL homodyne method of detection has all the conversion efficiency of the usualdouble-detection arrangements and, in addition, has the ad
. The Bell System technical journal . e resultant carrier is inquadrature with the sidebands, it is desirable that the residual carrier be lowin level compared with the homodyne carrier. The error in phase A0introduced by the residual carrier is shown in the vector diagram of Fig. difference in level of about 40 decibels between the homodyne and residualcarriers will give an error of not more than half a degree in phase. The 102 BELL SYSTEM TECHNICAL JOURNAL homodyne method of detection has all the conversion efficiency of the usualdouble-detection arrangements and, in addition, has the advantage in thisparticular application of having a very low noise level due to the relativelynarrow band required for the audio signals. The 40-decibel level differencementioned above is accordingly not a serious handicap. Other precautions must be observed. The homodyne carrier can bebrought in quadrature with the signal for two different phases 180° 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 correct setting of the homodyne carrier phase can, however, be deter-mined very easily. Assume that the input probe is connected to the receiverand that the phase has been adjusted for a balance. Then disconnect theaudio frequency drive from one of the crystals in the balanced residual carrier will now no longer be suppressed and the error angleA(/> of Fig. 2 will become larger. Whether the homodyne carrier is lagging orleading the signal carrier can be determined by observing whether more orless phase shift, respectively, must be introduced to restore balance. Asimilar test performed with the output probe will indicate whether or not itis necessary to add 180° to the measured phase difference. If either probe MEASURING MICROWA VE FREQUENCIES 103 test indicates a lead, whereas the other prob
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