Abstract :
A new instrument is discussed for measuring negative conductance of an active element at the microwave region with enough stability and with high precision. The fundamental principle of the instrument is that of the impedance comparator. This instrument has a small rotating shielded loop located just above the center of the Tee branch, the three terminals of which are connected to the active element, to the standard susceptance, and to the signal generator, respectively. The instrument has the following special features: 1) the use of a shielded loop, 2) the use of negative standard susceptance for allowing the normalized conductance of the sample to be positive, 3) no restriction for the value of the negative conductance, and 4) the use of a short-circuit line as the susceptance standard for the dc return circuit of the active element. Analytical considerations are given for errors concerned with the loss in the short circuit, with the position and the shape of the small shielded loop, and with the phase difference between the current on the Tee branch and the magnetic field at the center of the loop. This instrument has proved to have high precision in measuring the admittance of an Esaki diode at 1 GHz. (The residual VSWR is less than 1.03). Therefore, it can be said that this instrument is an excellent aid for measuring negative conductance up to several GHz.
