26 BELL SYSTEM TECHNICAL JOURNAL 



Input Impedance Measurement 



One of the factors which effectually limits the performance of a 

 vacuum tube at ultra-high frequencies is the internal grid resistance or 

 active grid loss. Consequently, this factor is of extreme interest in 

 the development of amplifier tubes for use in the ultra-high-frequency 

 range and much of this work has centered around the development of 

 apparatus and technique for rapidly and accurately measuring these 

 input resistances. The method employed has been the simple resist- 

 ance substitution method used by Crawford.* 



An adjustable quarter- wave Lecher frame is provided with suitable 

 means for inducing a radio-frequency voltage across it and a suitable 

 detector for indicating the current flowing at the short-circuited end. 

 A calibration is made by noting the detector indication corresponding 

 to various known resistances connected across the open end of the 

 frame, with the input voltage held constant. The input circuit of the 

 tube under test is then connected to the end of the Lecher frame in 

 place of the calibrating resistors and the detector indications corre- 

 sponding to various voltages and loads applied to the tube are noted. 

 Since the Lecher frame is initially tuned to the operating frequency, and 

 when the tube input circuit is attached the circuit itself is retuned 

 for resonance, it follows that the quantity actually measured is the 

 efi'ective resistance across the tuned circuit, including both the circuit 

 losses and the active grid loss of the tube. It is of course possible to 

 determine the circuit losses separately and to compute the contribution 

 to the total resistance offered by the tube losses, and also to compute 

 the active grid loss existing directly at the grids of the tube, taking into 

 account the impedance transformation existing between the tube grids 

 and the tuned circuit, brought about by the lead inductances. Practi- 

 cally, however, the total effective shunt resistance across the tuned 

 circuit as actually measured is a more significant quantity, as this 

 quantity determines more or less directly the gain which can be ob- 

 tained from a multistage amplifier. It frequently happens that 

 changes in the voltages applied to the tube produce small changes in 

 the reactive component of the input impedance. These may be taken 

 into account by noting the changes in grid circuit tuning required to 

 maintain resonance. These changes are usually so small as to be of 

 only minor interest. 



The Lecher frame used in these measurements is shown in Fig. 14. 

 The plate bridging the frame nearest the open end carries the detector, 

 an RCA type 955 tube set into the plate. The grid of this tube is 



* A. B. Crawford, "Input Impedance of Vacuum Tube Detectors at Ultra-Short 

 Waves" (Abstract), Proc. I.R.E., vol. 22, pp. 684-685, June, 1934. 



