MEASURING MICROWAVE FREQUENCIES 



101 



ured phase by allowing for the known phase shift in the line between the 

 probes and the actual terminals of the unknown. 



So much for the general method. Certain precautions are necessary in 

 order to avoid errors in measurement. In practice the carrier is not com- 

 pletely suppressed in the output of the balanced modulator. It may be at a 



CRYSTAL 

 DETECTOR 



Fig. 1 — Schematic circuit for microwave phase measurement. 



SUPPRESSED 



CARRIER Ec 



UPPER \. . LOWER 



SIDEBAND ^ \ / SIDEBAND 



RESULTANT 

 CARRIER 



A4>__ ____-- - 



\ 



REFERENCE 

 CARRIER 



ALTERNATE CONDITION 

 OF BALANCE 



Fig. 2— Vector diagram of balanced condition with the resultant carrier in quadrature 

 with the signal sideband. 



level of the order of 10 to 20 decibels below the sidebands. Since the 

 residual carrier will be added to the homodyne carrier in the detector, and 

 since the null adjustment will be reached when the resultant carrier is in 

 quadrature with the sidebands, it is desirable that the residual carrier be low 

 in level compared with the homodyne carrier. The error in phase A0 

 introduced by the residual carrier is shown in the vector diagram of Fig. 2. 

 A difference in level of about 40 decibels between the homodyne and residual 

 carriers will give an error of not more than half a degree in phase. The 



