NEGATIVE IMPEDANCE TELEPHONE REPEATERS 1091 



necessary, the critical elements are stable passive networks and changes 

 in the sensitivity of the measuring device, which includes variations in 

 oscillator output and sensitivity of the detector circuit, will not afTe(;t 

 the accuracy of measurement. 



An impedance reference is established, as shown in Fig. 27(a), when 

 the transmission through the hybrid network is measured with con- 

 nection to the resistance standard. A second transmission measurement 

 is made through the hybrid coil network with the standard impedance 

 connected instead of the unknown. After replacing the unknown with 

 the resistance standard, the transmission through the hybrid coil is 

 adjusted by varjdng the resistance standard to obtain the same value 

 of loss, as measured with the unknown impedance. When the two trans- 

 missions are equal, the resistance in ohms, as read on the resistance 

 standard, will be the same as the magnitude of the unknown impedance. 

 With the magnitude of the resistance standard and unknown impedance 

 the same, the phase angle of the unknown is readily determined by com- 

 paring the transmission through the hybrid coil for two polings of one 

 of the hybrid windings. The first pohng, Fig. 27(c), which is used as a 

 reference, provides a measure of the transmission through the hybrid 

 network when the current in the unknown branch and the current in 

 the resistance branch are added vectorially in the detector winding. The 

 reverse poling of one of the coil windings. Fig. 27(d), provides a measure 

 of the transmission through the hybrid when the currents in the re- 

 sistance and unknown branches are subtracted vectorially in the de- 

 tector circuit. In addition to being a method of measuring the phase 

 angle of an impedance, this comparison of two measurements with 

 different poling is a return loss measurement of an unknown against a 

 known impedance of equal magnitude. A curve of phase versus return 

 loss may be used for convenient interpretation of the return loss measure- 

 ment in terms of phase angle. After the phase angle has been ascertained 

 it is necessary to determine the sense of the angle, that is, whether the 

 unknown impedance contains positive or negative reactance. A reference 

 condition is established, Fig. 27(e), by shunting a small value of capaci- 

 tance first across the unknown impedance and then across the resistance 

 standard and noting the change in transmission through the hybrid 

 network. An increase in the transmission, in going from the reference 

 to measure conditions, indicates a positive reactance and a decrease in 

 transmission indicates a negative phase angle. The reference condition 

 is established to minimize indicational errors at small phase angles of 

 the unknown impedance. 



Two additional pieces of information are revealed in the return loss 



