Direct Reading Fully Automatic Vector Impedance Meters

Two new instruments designed to measure impedance magnitude and phase angle quickly and easily over a broad frequency range

Block Diagram Vector Impedance Meter
Fig. 1. This low-frequency -hp- Model 4800A Vector Impedance Meter makes rapid measurements of components and circuits with direct readout over the frequency range from 5 Hz to 500 kHz, and over the impedance range from 10 ohms full scale to 10 megohms full scale.

Engineers faced with the problem of determining component or circuit impedance over a wide range of frequencies have had to accept tedious balancing adjustments and point plotting associated with most bridge techniques. When the results are finally obtained, they are valid for the components as attached to the terminals of the instrument, that is, detached from their working circuit.

In critical applications where it is necessary to determine impedance at a particular point in a circuit, the engineer resorts to a series of calculations based upon his individual impedance measurements. Added to the tedious measurement process, these calculations still leave an important unknown — the effect of stray capacitance, lead inductance and other interactions within the circuit.

Two new instruments have been designed to read impedance directly over a wide frequency range. Called Vector Impedance Meters, they are extremely valuable as circuit design tools. Both are fully automatic, requiring no nulling or balancing.

Impedance magnitude and phase angle are read out on front panel meters. Continuous plots of impedance versus frequency and phase angle versus frequency may be easily made by merely adjusting the frequency dial

Fig. 2. In the -hp- Model 4800A, the unknown impedance is in the feedback loop of the transresistance (Rt) amplifier. The R,r amplifier is designed to be very broadband, critically damped with extremely critical phase-gain relationships so that unknowns with wide variations in phase angle and amplitude may be placed in the feedback loop without causing oscillation or instability in the amplifier.

DC DIFFERENTIAL AMPLIFIER

ACTIVE DETECTOR

DC DIFFERENTIAL AMPLIFIER

ACTIVE DETECTOR

Vector Impedance Meter Block Diagram

AMPLIFIER

-Current Channel

AMPLIFIER

-Current Channel and setting the range switch. Both instruments together cover the frequency range from 5 Hz to 108 MHz, but their applications differ somewhat.

5 Hz to 500 kHz Vector Impedance Meter

The lower frequency instrument, the -hp- Model 4800A Vector Impedance Meter, Fig. 1, provides continuous coverage from 5 Hz to 500 kHz in five bands and measures impedance from 1 ohm to 10 megohms in seven ranges. It is designed for making passive measurements of components attached to its front-panel terminals.

Impedances in the 1-ohm to 1000-ohm range are measured by passing a predetermined current through the unknown and measuring the voltage across it. For impedances between 1000 ohms and 10 megohms, a predetermined voltage is put across the unknown and the current is measured and read out in ohms on the front panel meter.

Phase angle information is obtained by comparing the relative phase between the voltage and current by means of a phase detector. The basic elements of the instrument are shown in Fig. 2.

Constant-current mode. In the low impedance ranges of the -hp- Model 4800A, the current is held constant across the unknown by means of an automatic level control (ALC), Fig. 3(a). The current sensor is a transresistance (Rt) amplifier which accepts the input current and supplies an output voltage equal to the input current times the effective transresistance. This voltage (proportional to the current in the unknown) is used to reference the AGC amplifier signal level which, in turn, feeds a leveled signal to the current-determining resistor.

The voltage across the unknown is applied to a differential amplifier, then fed to an averaging detector. Output from the detector is read out in ohms on the impedance magnitude meter.

Constant-voltage mode. Above 1,000 ohms it is difficult to maintain a constant current for a decade change in impedance. Therefore a constant voltage is maintained across the unknown, Fig. 3(b).

COVER

Measuring impedance at a point in a circuit using the probe terminal of a new direct-reading RF vector impedance meter. Rapid measurements over a broad frequency range can be made without tedious manipulation or point-by-point plotting.

Block Diagram Vector Impedance Meter
(a)

OSCIttATOf

Block Diagram Vector Impedance Meter

Fig. 3. The low-frequency impedance meter is operated so that current through the unknown is held constant for impedances less than 1,000 ohms (a). Since it is difficult to hold current constant at higher impedances, the instrument is switched to the constant voltage mode (b).

OSCIttATOf

WÊMS&: i„- v CURRENT

CHANNEL

-L

IMPEDANCE

(b)

DETECTOR

Fig. 3. The low-frequency impedance meter is operated so that current through the unknown is held constant for impedances less than 1,000 ohms (a). Since it is difficult to hold current constant at higher impedances, the instrument is switched to the constant voltage mode (b).

Block Diagram Vector Impedance Meter

Fig. 4. Current and voltage channels and switching functions are contained in a plug-in which also contains the measurement terminals.

Fig. 4. Current and voltage channels and switching functions are contained in a plug-in which also contains the measurement terminals.

The AGC amplifier signal is fed to the attenuator which puts the known voltage across the unknown. The voltage is sensed and fed back to the AGC amplifier as in the constant-current mode. The current in the unknown is converted to a proportional voltage, amplified and read out on the impedance magnitude meter.

Measurement Amplifier Plug-in. The voltage and current sensors and switching functions necessary to provide operating signals to the current and voltage channels are contained in a front-panel plug-in (Fig. 4). The current sensor is the RT amplifier mentioned previously. The voltage sensor is a differential amplifier which monitors the voltage across the unknown without loading the measurement terminals. Current and voltage channels in the plug-in are separated by a shield. This plug-in concept permits flexibility in future design. With the present plug-in, neither terminal may be grounded.

Phase Measurement. Phase angle is measured in the same manner in both the voltage and current modes. Signals from the voltage and current channels are compared to obtain the phase angle. The signal from the voltage channel goes to a zero crossing detector whose output turns on one half of a bistable multivibrator when the voltage signal passes through zero in a positive direction.

The current signal goes to an identical zero crossing detector whose output is used to turn off the same half of the multivibrator that the voltage channel turned on. The time that the flip-flop is on is proportional to the phase difference, and thus its output voltage is proportional to the phase difference between the voltage and current. A zero-center phase meter calibrated in degrees reads this voltage as phase angle.

High Frequency Vector Impedance Meter

Designed to use a probe terminal arrangement, a higher frequency instrument, the -hp- Model 4815A RF Vector Impedance Meter, Fig. 5, covers the frequency range from 500 kHz to 108 MHz. It is designed to measure the magnitude and phase angle of the driving point impedance placed across its probe tip to ground. This makes direct-reading in-circuit measurements possible, and the instrument is also designed to measure impedance of active circuits including those having negative real components.

Basically, the block diagram of the RF Vector Impedance Meter resembles that of the lower frequency -hp- Model 4800A with four notable exceptions — a phase lock loop, samplers, probe and grounded measurement capability, Fig. 6. In addition the instrument operates only in the constant-current mode.

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