3-9 Figure 3-6.—Typical vector-bridge configuration (phase). CONSTANT-CURRENT, IMPEDANCE-MEASURING TECHNIQUE This technique employs an oscillator circuit and a vtvm, as shown in figure 3-7. Figure 3-7.—Constant-current, impedance-measuring method. A large value of resistance, R, is selected so that I_{C} is virtually independent of the range of ZXto be measured. Thus, I_{C}ZXrepresents the value of voltage measured by the vtvm. If R is chosen so that the voltage drop across Z_{X} corresponds to a full-scale reading on the vtvm, a direct reading impedance meter is realized. For example, assume that the audio oscillator open-circuit voltage is 10 volts (rms) and that the full-scale reading of the vtvm is 0.05 volt. If you want to measure Z_{X} values ranging up to a maximum of 5,000 ohms, you should use a 1-megohm resistor for R. This will result in a full-scale, 0.05-volt deflection. An oscillator that does not produce harmonics should be used. IMPEDANCE-ANGLE METER Like vector bridges, impedance-angle meters determine an unknown impedance in terms of magnitude and phase angle. However, a non-bridge technique is used. The simplified circuit of a commercial instrument is shown in figure 3-8. With switches S1 and S2 at the BAL position, the variable