With Low Residuals

The simple, independent balance conditions, which constitute one of the advantages of the Owen bridge, can be obtained in practice only if the bridge arms are free of residual impedances. In the Type 1632-A Inductance Bridge many of the possible residuals are minimized by the use of capacitors with very small residual resistance and of resistors with very small residual inductance and capacitance. Other residuals are reduced or controlled by the careful internal shielding shown in Figure 3.

The most troublesome residuals are capacitances across the unknown arm and across the standards, RN and CN, when they are in series. These capacitances are minimized by the shield enclosing the CA and RB arms of the bridge. By extension of this shield into the base of the unknown terminals, the capacitance added across the unknown by the bridge is reduced to the 1-pf capacitance between the external terminals. In the "standard" arm another shield encloses the resistance decade RN to reduce the capacitance across the series combination of Rn and Cy to the negligible value of less than a picofarad. The capacitance of this shield to ground appears across CN and is included in the calibration of the conductance decades. This shield capacitance limits in minimum CN to 200 pf, and results in the "add 2"

that appears on the instrument panel below the window of the fourth G dial.

Further special shielding within the transformer used between bridge and detector permits the detector to be grounded without additional contribution to the bridge residuals. The inter-shield capacitances of the transformer, however, add some 100 pf to the CA arm of the bridge, which can be included in the calibration of CA. Since the total C A is only 1000 pf in the lowest or a range of the bridge, the lower stability and higher loss of the transformer capacitance limit somewhat the accuracy of this range. An accuracy of the order of ± 1% can be realized on the a range, and this reduced accuracy is indicated by a red a for this multiplier position. Since the primary use of this a range is the measurement of the very low inductance of leads and of shorted terminals, the reduction in accuracy here is not detrimental.

With this control and reduction of residual impedances, the 0.1% inductance accuracy of the bridge can be maintained over wide ranges of inductance, Q, and frequency. The ranges of measurement are so wide, however, that additional errors arise from the remaining very small residuals at the extremes of the ranges. When the Q of the unknown is very low, these residuals, including the small dissipation factors of the capacitors used as CA and CN, can increase the inductance error by ±.05%/Qx- At the frequencies of 10 kc and higher, the error is also increased by the small, uncompensated reactances of the Rb arm, particularly when RB has its extreme values of 1 ohm and 100,000 ohms at, the very low and high ends of the range.

Figure 3. Bridge schematic showing the internal shielding to minimize residual-impedance errors.

The Type 1G32-A Inductance Bridge is designed not only to make measurements of high resolution and accuracy, but to make such measurements with maximum ease and rapidity. For this purpose the dials of the decades indicating L and G are arranged to show only the pertinent digit of each decade, and these digits are placed for convenient vertical, in-line, digital readout of the six significant figures. The eight-position range switch both indicates the units of L and G and automatically places the decimal point in the line of digits. Operation is further simplified, particularly for the occasional user, by the presentation on the panel of the bridge circuits and balance equations, as well as a table of the limits of maximum input voltage. The bridge can be balanced easily and rapidly to its full resolution because in an Owen bridge with such small residuals the L and G balances are independent and there is no trouble with a sliding balance and false nulls.

Neither generator nor detector is built into this bridge, because the versatility required in them to match that of the bridge can best be built into external units. Adequate generator range and power for most bridge uses can be provided by the Type 1304-B Beat-Frequency Audio Generator or by the combination of the Type 1210-C Unit R-C Oscillator and the Type 120G-B Unit Amplifier. Since these generators are designed for operation into loads of the order of GOO ohms and the bridge input impedance on the lower four ranges is of the order of 1 to 100 ohms, a matching transformer (Type 1G32-P1) with turns ratios of 1:20 and 1 :5 is supplied to raise the bridge input impedance to match the generator output. In order to keep its magnetic iield away from the bridge and the inductor being measured, this transformer is designed to be plugged into the generator instead of being built into the bridge.

A detector of high sensitivity and low noise is required to make use of the full six-figure resolution of this bridge. The requirements can be met by the Type 1231-B Amplifier and the Null Detector with the Type 1231-P5 Adjustable Filter, followed by a null indicator with additional gain, such as a pair of headphones, an oscilloscope, a millivoltmeter, or another Type 1231-15 Null Detector. For measurements requiring only the direct-reading accuracy of =1=0.1%, a single Type 1231-B with filter is usually adequate. The maximum available sensitivity, particularly at low frequencies, has been realized by making the detector transformer impedance as high as possible and by locating the transformer to satisfy best the condition that the bridge is most sensitive when the detector is connected at the junctions of arms having equal impedances. As another aid to sensitivity, a maximum sensitivity switch has been provided to. permit on some ranges a choice of the magnitude of the C a and Rji arms to satisfy best this condition. On the middle (d, e, f) ranges, this switch changes both RB and CA by a factor of ten without changing their product, CARB, and thus does not alter the multiplying factor of the bridge which has been set with the range switch.

The Type 1632-A Inductance Bridge is designed for the precise measurement of either the series or parallel components or two-terminal, grounded inductors at audio frequencies. Full-scale ranges of inductance extend from 1,111 henrys to 111.1 microhenrys. Six-figure resolution and high sensitivity make this bridge particularly suitable for standardization measurements of high accuracy, since standard inductors, such as the Type 1482, can be intercompared to better than 5 parts in a million. Its direct-reading inductance accuracy of ±0.1%, the ease of balance, and in-line readout make it convenient to use. Although designed primarily for use at frequencies of 1000 c and lower, it can be used, with some decrease in accuracy, to at least 10 kc.

The bridge is well suited for the measurement of inductors with ferromagnetic cores, since its sensitivity makes a balance possible with only a small voltage applied to the inductor, and measurements can thus be made in the region of initial permeability. Measurements of inductors with nonlinear characteristics are further facilitated because in this bridge the voltage across the unknown inductor will not change appreciably when the L and G controls are varied to balance the bridge. The bridge is not suitable for the measurement of incremental inductance at high ac or dc excitation, because the dissipation in the precision resistors in the bridge must be limited to the order of one watt. The maximum voltage which can be safely applied to the bridge varies from 1 to 100 volts, depending upon the range being used, and these limits are indicated in a table engraved on the bridge panel. The bridge can beusedfor incremental inductance measurements at levels within these limits, when the desired direct current is supplied to either the generator terminals of the bridge or directly to the unknown. The capacitor CA blocks the direct current from the RN standards, and a capacitor connected in series with the internal detector transformer prevents any flow through that path. Use can also be made of the range and resolution of the bridge in making measurements of mutual inductance and of magnetic core materials.

Acknowledgments

Work on a similar Owen bridge was begun some years ago by R. F. Field. Much of the development of the present Type 1G32-A Bridge is the result of the work of Horatio W. Lamson and of many others of the Engineering Department. After Mr. Lamson's retirement in 1958, the bridge was completed by Ivan G. Easton and John F. Hersh.

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