National Physical Laboratory

Coaxial AC Bridge Systems

NPL has developed a number of Coaxial AC Bridge Systems, over the last 30 years, suitable for national standards laboratories for precision impedance and AC voltage ratio measurements at the highest accuracy levels.

Some of the key Coaxial AC Bridge Systems are configured to accurately relate the value of a 1 kΩ resistance standard to the 10 pF primary capacitance standard in terms of the quantum Hall resistance. More generally, a number of different bridge systems are used to enable measurement of capacitance standards in the range 1 pF to 1 mF (decade values) between 100 Hz and 10 MHz. The following is a list of some of the key precision bridge systems in the Low-Frequency Electromagnetics Laboratory:

  • Four terminal-pair 100:1 equal power resistance bridge
  • Four terminal-pair quadrature bridge
  • Four terminal-pair 10:1 capacitance bridge
  • AC bridge systems for measurement of AC voltage ratio

The following additional equipment is also required for bridge measurement systems:

  • Sources and detectors
  • Resistance standards
  • Capacitance standards
  • Reference standards for AC voltage ratio (inductive voltage dividers)

Specifications

Four terminal-pair 100:1 equal power resistance bridge

The 100:1 equal power bridge enables accurate comparison of a 1 kΩ resistance standard with a 100 kΩ resistance standard at a frequency of 1.592 kHz. The bridge system is designed to ensure that both resistance standards only dissipate 1 mW of power during measurements.  The 1 kΩ resistance standard is a Gibbings calculable quadrifilar which differs by less than 1´10-9 from its DC value at a frequency of 1.592 kHz.

Four terminal-pair quadrature bridge

This system accurately relates the value of two 100 kΩ resistance standards with two 1 nF capacitance standards at a frequency of 1.592 kHz. Since the quadrature bridge is highly frequency sensitive, the frequency of the source must be stable (or known) to better than 1´10-9 and any contributions from the harmonics of the fundamental frequency component filtered to avoid intermodulation distortion.

Four terminal-pair 10:1 capacitance bridge

This bridge system enables accurate scaling of the value of capacitance standards at a frequency of 1.592 kHz. The combined quadrature and 10:1 capacitance bridge measurements enable the value of the 1 nF capacitance standards to be determined. This value can then be scaled up to 1mF or down to 1 pF using the 10:1 bridge system.

AC voltage ratio bridge systems

The NPL AC voltage ratio bridge systems operate over the frequency range 40 Hz to 120 kHz and are used to calibrate the first 3 decades and 11-section of the customer inductive voltage dividers.

Capacitance and Resistance Standards

The table below shows the capacitance and resistance standards required for the first three bridge systems listed above.

Quantity Description
1 1 kΩ quadrifilar resistance standard, with a 25 Ω PRT
2 100 kΩ resistance standards, with a 25 Ω PRT
2 NPO dielectric 1 nF capacitance standards, with a 25 Ω PRT
2 Fused-silica 100 pF, 10 pF and 1 pF capacitance standards, with a 25 Ω PRT

For further information please contact:

To book a calibration:
Customer Services
E-mail: em-services@npl.co.uk
Tel: +44 20 8943 7166

For technical enquiries: Janet Belliss

Last Updated: 30 Mar 2012
Created: 17 Oct 2007

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