Measuring Frequency Characteristics of Linear Two-Port Networks Automatically

In implementing a linear two-port device characterizing facility on the computer operated transmission measuring set, several factors had to be considered. 1 First, the advantages of automated measurements could be retained only if the switching required to obtain four independent measurement configurations were done automatically. Second, the implementation must be broadband to take advantage of the 50 Hz to 250 M H z frequency range of the measuring set. F i n a l l y , the measurement method must be inherently capable of utilizing the high accuracy of the measuring set. An implementation was chosen which uses standard insertion and bridging measurements. 2-4 T h i s choice is particularly compatible with the above factors. Insertion and bridging measurements are made with the unknown terminated in a nominal impedance environment, in the present instance 50 ohms. At this impedance level, broadband, solenoid-oper1313 1314 TI-IE BELL SYSTEM TECHNICAL JOURNAL, M A Y - J U N E 1969 ated coaxial switches are available which introduce only minor reflections in 50 ohm transmission circuits. In the computer operated transmission measuring set such switches are extensively used to provide automatic commuting of the unknown among the four independent configurations in which measurements are made. By arranging relay switching so that dc bias can be continuously maintained when measuring active unknowns such as transistors, multiple warm-up periods are eliminated and thermal equilibrium must be reached only once.