A precision dual bridge for the standardization of admittance at very high frequencies

Abstract

The need is stated for a more accurate means of admittance or impedance measurement in order that a general improvement can be made in the accuracy of measurement of other electrical quantities at radio frequencies. A description is given of a precision twin-T dual bridge having a range of admittance measurement associated with coaxial systems between 3 Mc/s and 300 Mc/s. The inaccuracy of measurement does not exceed 0.2% on either component of a complex admittance at a frequency of 200 Mc/s. The calibration of the bridge is based on a range of coaxial susceptance standards whose parameters are calculated from length and time measurements. The dual feature of the bridge enables the true r.f. conductance of its internal standard resistor to be established in terms of these standards; the uncertainty of this determination is not greater than 0.1% for frequencies up to 250 Mc/s. The importance to be attached to the design of coaxial terminations for measuring instruments and standards at these frequencies is emphasized. A practical design is detailed for a coaxial terminal for precision admittance and impedance measuring instruments, and mention is made of a proposed universal coaxial connector system for precision radio-frequency measuring instruments of all types.