Q. Explain how phase angle measurement are carried out with vector impedance meter.
Sol. Impedance measurements are concerned with both the magnitude (Z) and the phase angle of a component. At frequencies below 100 MHz, measurements. Of voltage and current is uaually sufficient to determine the magnitude of the impedance. The phase difference between the voltage waveform and the current waveform indicates whether the component is inductive or capacitive. If the phase angle can be determined, for example, by using a CRO displaying a Lissajous pattern the reactance can be calculated. If a component is fully specified, its properties should be determined at several different frequencies and many measurements may be required. Especially at the higher frequencies, these measurements may become rather elaborate and time consuming and many steps may be required to obtain the desired information.
The development of such instruments as the vector impedance meter makes impedance measurements possible over a wide frequency range. Sweep-frequency plots of impedance and phase angle versus frequency, providing complete coverage within the frequency band of interest, can also be made.
The vector impedance meter, makes simultaneous measurements of impedance and phase angle over a frequency range of from 400 kHz to 110MHz. The unknown component is simply connected across the input terminals of the instrument, the desired frequency is selected by turning the front panel controls and the two front panel readouts indicate the magnitude of the impedance and the phase angle.
The operation of the vector impedance meter is best understood by referring to the block diagram of a representative instrument curr. Two measurements take place: (1) The magnitude of the impedance is determined by measuring the ent through the unknown component when known voltage is applied across it or by measuring the voltage across the component when a known current is passed through it, (2) the phase angle is found by determin9ing the phase difference between the voltage across the component and the current through the component.
The block diagram of fig. shows that the instrument contains a signal source (Wein bridge oscillator) with two front panel controls to select the frequency range and to continuously adjust the selected frequency. The oscillator output is fed to an AGC amplifier which allows accurate gain adjustments by means of its feedback voltage. This again adjustment is an internal control actuated by the setting of the impedance range switch, t which the AGC amplifier output is connected. The impedance range switch is a precision attenuator network controlling the oscillator output voltage and at the same time determining the manner in which the unknown component will be connected into the circuitry that follows the range switch.
The impedance range switch permits operation of the instrument in two modes: the constant-current mode and the constant-voltage mode. The three lower ranges (X1, X10 and X100) operate in the constant-current mode and the four higher ranges (X1k, X10k, X100 and X1M) operate in the constant-voltage mode.
In the constant-current mode the unknown component is connected across the input of the ac differential amplifier. The current supplied to the unknown depends on the setting of the impedance range switch. This current is held constant by the action of the impedance ranges switch. This current is held constant by the action of the transresistance or R amplifier, which converts the current through the unknown to a voltage output equal to the current times its feedback resistance. The R amplifier is an operational amplifier whose output voltage is proportional to its input current.
The output of the R_{T }amplifier is fed to a detector circuit and compared to a dc reference3 voltage. The resulting control voltage regulates the gain of the AGC amplifier and hence the voltage applied to the impedance range switch. The output of the ac differential amplifier is applied to an amplifier and filter section consisting of high and low-band filters that are changed with the frequency range to restrict the amplifier bandwidth. The output of the bandpass filter is connected, when selected to a detector that drives the Z-magnitude meter. Since the current through the unknown is held constant by the R_{T }amplifier, the Z-magnitude meter, which measures the voltage across the unknown deflects in proportion to the magnitude of the unknown impedance and is calibrated accordingly.
In the constant-voltage mode, the tow inputs to the differential amplifier are switched. The terminal that was connected to the input of the trans-resistance amplifier in the constant-current mode is now grounded. The other input of the differential amplifier that was connected to the voltage terminal of the unknown component is now connected to a point on the Z-magnitude arrange switch which is held at a constant potential. The voltage terminal of the unknown is connected to this same point of constant potential, or depending on the setting of th Z-magnitude range switch to a decimal fraction of this voltage. In any case, the voltage across the unknown is held at a constant level. The current through the unknown is applied to the trans-resistance amplifier which again produces an output voltage proportional to its input current.
The roles of the ac differential amplifier and the trans-resistance amplifier are now reversed. The voltage output of the R_{T }amplifier is applied to the detector and then to the Z-magnitude meter. The output voltage of the differential amplifier controls the gain the AGC amplifier in the same manner that the R_{T }amplifier did in the constant-current mode.
Phase-angle measurements are carried out simultaneously. The outputs of both the voltage channel and the current channel and the current the current channel and the current channels are amplified and each output is connected to a Schmitt trigger circuit. The Schmitt trigger circuits produce positive-going spikes every time the input sine wave goes through a zero crossing. These positive spikes are applied to a binary phase detector circuit. The phase detector consists of a bitable multivibrator, a differential amplifier and an integrating capacitor. The positive-going pulse from the constant-current channel sets the multivibrator and the pulse from the constant-voltage channel resets the multivibrator. The "set" time of the MV is therefore determined by the zero crossings of the voltage and current waveforms. The "set" and "reset" outputs of the MV are applied to the differential amplifier, which applies the difference voltage is directly proportional to the zero-crossing time interval and is applied to the phase-angle meter which then indicates the phase difference, in degrees, between the voltage and current waveforms.
Calibration of the vector impedance meter is usually performed by connecting standard components to the input terminals. These components may be standard resistors or capacitors. An electronic counter is needed to accurately determine the period of the applied test frequency. When the value of the component under test and the frequency of the test signal are both known accurately, the impedance or reactance can be calculated and compared to the indication on the Z-magnitude meter. With a standard resistor connected to the input terminals the phase-angle meter should read 0°.