Type A Inductance Bridge

The instrument manufacturer who makes resistors, capacitors, and inductors of high accuracy and low residual impedance has both the need and the opportunity to build the impedance bridges required for high-accuracy measurements of these components. The General Radio Company has for many years been building, and continually improving in accuracy and stability, the Type 510 Decade-Resistance Units1 which are used in the Type 1432 Decade Resistors. The materials and winding

»Ivan G. Easton, "The New Type 1432 Decade Resistors," General Radio Experimenter, June, 1951, Vol. XXVI, No. 1.

methods used in these units produce not only the excellent stability and low temperature coefficient required for a significant accuracy of ±0.05%, but also the small residual reactances which permit use of the resistors over a wide frequency range. Fixed standards of capacitance have also undergone a continuous process of improvement, and the current silvered-mica Type 1 409 Standard Capacitors2 are accurate to ±0.05%, with a long-term stability of better than 0.01%.

2Ivan Ci. Easton and P. K. McElroy, "New Silvered-Miea Standard Capacitors," General Radio Experimenter, July, 1957, Vol. 32, No. 2.

Figure 1. Panel view of the Type 1632-A Inductance Bridge.

Figure 1. Panel view of the Type 1632-A Inductance Bridge.

Another recent addition to the General Radio line of precision components is a series of highly accurate and stable standard inductors.3 The Type 1482 Standard Inductors, being toroidally wound on ceramic cores and shielded from external thermal, mechanical, atmospheric, and electrical disturbances, have the high stability and reliability4 that, make possible their calibration with the closest tolerance (±0.03%) that the National Bureau of Standards will place on its certification of absolute inductance. For General Radio to certify the inductance of these inductors with the same accuracy limit of ±0.03%, the inductors must be intercompared with a set of NBS-calibrated inductors with a precision of higher order than the tolerance. This need for the measurement of inductance over the 100-yuh to 10-h range of the Type 1482 Standard Inductors with a resolution and precision of 0.003% or better led to the construction for our standardizing laboratory of a new inductance bridge, which made use of our wide range of precision resistors and standard capacitors to measure the inductors. With improvements that make it more convenient to use and more generally useful, this new, wide-range, high-precision bridge is now being produced as the Type 1632-A Inductance Bridge.

For inductance standardization as well as for general inductance measurement, this bridge approaches the ideal, not only in inductance range and accuracy, but also in speed and convenience of operation. Two important features are digital in-line readout and the inclusion of operational data and circuit schematic on the panel.

3Horatio W. T.amson. "A New Series of Standard Inductors," General Radio Experimenter, November, 1952, Vol. XXVII, No. 6.

4Horatio W. Lanisnn, "Standard Inductors, a Stability Record," General Radio Experimenter, May, 1957, Vol. 31, No. 12.

An Owen Bridge

This bridge uses the Owen bridge circuit in which the inductance balance is made with precision decades of resistance, thus achieving a high degree of resolution and a high accuracy at moderate cost. The other bridge circuits commonly used for inductance measurement, the Maxwell and the Hay, require variable capacitors for the inductance balance, and capacitors of the desired range and accuracy are very expensive. In the Owen bridge shown in Figure 2, the fixed arms consist of the standard capacitor Ca and the resistor R B. The unknown Zx is balanced by the decade resistor RN and decade capacitor CN, and at balance the unknown is related to the standards by the equations:

The decades can lie switched to either a series or a parallel connection, so that the bridge can be made direct reading in either series or parallel components of the unknown inductor. When the resistance and capacitance decades are connected in series, the equivalent series inductance, Lxs, and series conductance, Gxs, of the unknown Zx are proportional to Rjv and to CN', when the standards are connected in parallel, RN and CN determine the equivalent parallel inductance, LXP, and parallel conductance, GXP. The equivalent resistance in either configuration is the reciprocal of the conductance, Rx - 1 /Gx. Over a wide range of Q, the unknown can be measured in terms of whichever components are most convenient. Even when the Q is very high or very low, a balance for one of the equivalents can usually be obtained. When the Q is very low, the series components can usually be measured; when the Q is very high, the parallel components can be measured.

The variable standard resistance RN consists of six Type 510 Decade Resistance Units in 10-kilohm to 0.1-ohm steps, with a calibration accuracy of ±0.05% in all except the two lower decades. Since the measured inductance is proportional to this resistance, these six decades give the bridge a resolution of inductance up to six significant figures. The range of inductance covered by these six decades can be changed readily by a change in the CARB product which relates Lx to RN. Through an eight-position range switch a choice can be made among three CA capacitors and six RB resistors to cover a range of full-scale inductance from 1,111 henrys to 111 microhenrys. The minimum inductance indication is, thus, seven decades below 111 microhenrys or 0.0001 microhenry.

The accuracy of the inductance indication is, of course, limited by the accuracy and stability of all of the components entering into Equation (1). The resistance RB is made up of the same stable and accurate wire-wound resistors as are used in the decades of RN, and the residual inductance or capacitance is compensated. Equally stable silvered-mica standard capacitor units are used for CA. These components make possible a direct-reading bridge accuracy of 0.1 % over wide ranges of inductance, Q, and frequency.

Full use can be made of the potential resolution and accuracy in inductance reading only if the resistive component of the unknown can be balanced both with comparable precision and without appreciable interaction between balances as a result of residual impedances. In the Owen bridge such resistance balance requires a variable capacitor of wide range having low losses and both loss and capacitance independent of frequency. The Type 1632-A Inductance Bridge is possible only because General Radio now produces high-quality polystyrene capacitors,5 used in the Type 1419-A Decade Capacitor, which have stable and constant capacitance and low dissipation factor, even at the frequencies below 100 c which are often used in inductance measurements. CN consists of four decades of these polystyrene capacitors, in 0.1- to 0.0001-juf steps, followed by a continuously variable 130-pf* air capacitor. These decades are calibrated to an accuracy of ±1%; better accuracy in the measurement of G is rarely needed and is usually prohibited by errors caused by bridge residuals. When a capacitance greater then 1 ¡jS is required to balance a very high conductance, an external capacitor can be connected at panel jacks to parallel the CN decades.

'"New Decade Capacitors with Polystyrene Dielectric," General Radio Experimenter, July, 1956, Vol. 31, No, 2.

Figure 2. Elementary schematic of the Owen bridge circuits used in the Type 1 632-A Inductance Bridge.

Figure 2. Elementary schematic of the Owen bridge circuits used in the Type 1 632-A Inductance Bridge.

Maxwell Inductance Bridge

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