Recent Improvements

Since the original model was introduced, several methods of adding to the operating convenience and accuracy have been developed. One of these improvements eliminates the effect of the junction inductance, which causes errors at the higher frequencies; another one makes possible higher accuracy when multipliers larger than unity are used; and a third extends the direct-reading, low-frequency range and also eliminates the need for the long awkward stub. The type number of the improved instrument has been changed to the Type 1602-B Admittance Meter to avoid confusion with previous models.

Junction Inductance

The basic principle behind the method used to eliminate the effect of junction

3To be described in a future issue of the Experimenter.

Figure 3. View of the Admittance Meter as set up for measurement of a resistor, showing the line stretcher and component mount.

Figure 3. View of the Admittance Meter as set up for measurement of a resistor, showing the line stretcher and component mount.

inductance is similar to that used on the Type 1601-A VHF Bridge to compensate for the capacitance of the unknown terminals. However, the applicability of this principle to the Admittance Meter was first realized and suggested by Messrs. G. D. Monteath and P. Knight of the British Broadcasting Corporation. The understanding of the method used to compensate for the junction inductance requires a knowledge of the principle of operation of the Admittance Meter. For convenience it will be briefly reviewed in the following paragraphs.

In the Admittance Meter, the currents flowing in three branch coaxial lines, fed from a common voltage source at a common junction point, are sampled by three independently adjustable loops, which couple to the magnetic field in each line as shown in Figure 4. One of the branch lines is connected to a conductance standard, one to a susceptance standard, and one to the unknown circuit. The outputs of the three loops are connected in parallel, and the coupling of each loop to its respective branch line is adjusted by rotating the loop until a null is obtained. At a null the settings of

Figure 4. Schematic diagram of the Admittance Meter, showing the arrangement of coupling loop.

the loops are direct indications of the magnitudes of the conductance, the susceptance, and a multiplying factor.

The basic principle of operation assumes that the voltage at a point under the center of the coupling loop on each branch line is the same. However, in the practical case, the loops must be located at least a short distance from the actual common junction point and, hence, a short length of line exists between the common junction point and the center of each pickup loop as shown in Figure 4.

Figure 5a. Equivalent circuit of the junction.

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