A Capacitance Bridge for High Frequencies
J. S. Mendousse, Philip Goodman, W. G. Cady · Review of Scientific Instruments · 1950
A type of bridge for measurements at frequencies in the megacycle range is described, containing two fixed capacitors and a variable capacitor. The unknown impedance is connected in the fourth arm as a ``load.'' The unbalanced high frequency e.m.f. is rectified by a germanium crystal rectifier and measured as a small d.c. voltage. The bridge is ``balanced'' by varying the variable capacitance until the ratio of output d.c. to input h-f voltage is at a minimum. The impedance of the variable capacitor is then numerically equal to that of the load. In a second and perhaps more important application, the bridge is used for examining vibrational modes and measuring the Q of piezoelectric crystals or transducers. Here the variable capacitor is set at a fixed value equal to the parallel capacitance of the crystal, and the output voltage is observed, or recorded graphically, while the frequency is slowly varied over the resonance range. Some graphical records are shown, illustrating the performance of crystals at various frequencies and under different mechanical loads. The theory of the bridge is given, and sources of error are discussed.