New method for determining time constant of resistors.

We report a new method for determining the time constant of ac resistors with values around 10 kΩ using a digital impedance bridge for the comparison of two nominally equal resistors. This method involves adding a probing capacitor in parallel to one of the resistors to induce a quadratic frequency dependence in the real component of the admittance ratio between the two resistors. The magnitude of this quadratic effect is proportional to the self-capacitance of the unperturbed resistor, enabling us to determine its value and the associated time constant with an estimated standard uncertainty (k = 1) of 0.

Measurement of the frequency dependence of four terminal-pair air capacitors with a vector network analyzer.

Four terminal-pair air capacitors are important transfer standards to calibrate LCR meters up to a frequency of 10 MHz. We report a simple and new method to obtain the frequency dependence of the four terminal-pair capacitance of these standards using a four-channel vector network analyzer (VNA). The frequency dependence of the capacitance of an air capacitor and its uncertainty can be obtained from a single set of measurements without changing connections between the standard and the VNA, as has been the case in previously published work.

Comparison of a 100-pF Capacitor With a 12 906-Ω Resistor Using a Digital Impedance Bridge.

We tested a digital impedance bridge in a hybrid structure for comparison of a capacitor with a resistor where the impedance ratio was measured in two separate parts. The modulus of the impedance ratio was matched arbitrarily close to the input-to-output ratio, in magnitude, of a two-stage inductive voltage divider by adjusting the operating frequency of the bridge; the residual deviation between the two together with the phase factor of the impedance ratio was measured using a custom detection system based on a four-channel 24-bit digitizer. The ratio of the inductive voltage divider was calibrated, , using a conventional four-arm bridge with two known capacitors.

Using a Natural Ratio to Compare DC and AC Resistances.

The simulation and construction of a direct current (DC) and alternating current (AC) resistor, based on a silicon wafer, has been described and demonstrated. By applying the van der Pauw method and the Thompson-Lampard theorem, to within approximations accommodating the conditions of the resistor's construction, a constant resistance ratio, (π/ln2), was derived that is independent of the sample resistivity and thickness. The constant ratio, valued at approximately 20.

Eddy Current Rail Inspection Using AC Bridge Techniques.

AC bridge techniques commonly used for precision impedance measurements have been adapted to develop an eddy current sensor for rail defect detection. By using two detection coils instead of just one as in a conventional sensor, we can balance out the large baseline signals corresponding to a normal rail. We have significantly enhanced the detection sensitivity of the eddy current method by detecting and demodulating the differential signal of the two coils induced by rail defects, using a digital lock-in amplifier algorithm.