The use of a radio frequency transformer to increase the sensitivity of a radio frequency admittance bridge

This paper describes a technique by which the sensitivity of a radio-frequency admittance bridge was increased by using an r.f. transformer, in order to measure small changes of admittance. The bridge measured the admittance of the primary circuit with the unknown admittance connected across the secondary, the nominal value of the secondary to primary turns ratio being ¿2. Using this method, measurements at frequencies between 5 and 30 MHz were made of the admittance of a plane capacitor of about 1 pF capacitance, to an accuracy of about 2%, and of changes in this admittance to less than 4%. The system was calibrated by a least-squares method, which also enabled the standard errors in the calibration to be evaluated. The results of measurements made on the plane capacitor in vacuum are presented, and the advantages and limitations of the method are discussed.