Agilent Technologies 4294A Service Manual download pdf (Page 164)

156 8

Theory of Operation

OVERALL OPERATION

OVERALL MEASUREMENT THEORY

The 4294A measures the impedance of the Device Under Test (DUT) by measuring vector

voltage to current ratio. A vector is a value which consists of a magnitude and a phase. The

impedance of the DUT is determined by the vector ratio between the voltage across the

DUT and the current flowing through it.

Figure 8-1 illustrates this principle

Figure 8-1 Voltage Current Ratio Measurement Principle

In Figure 8-1, a DUT is connected in series with a test voltage source (including an output

resistance), voltmeter V, and ammeter A. If the ammeter measures “ix” ampere when the

source voltage is “Edut”, the DUT’s impedance “Zx” is , “Zx=Edut/ix”. The other

parameters, such as L, C, and R, are derived mathematically from the measured impedance

value. The 4294A employs the most accurate measurement technique

(Auto-Balancing-Bridge) to measure the voltage across the DUT and the current through it.

OVERALL OPERATION AND FUNCTIONAL GROUPS

The 4294A consists of five main functional groups: a power supply, a digital control, a

source, a transducer, and vector ratio detector. Each group consists of several major

assemblies, and performs a distinct function in the 4294A. However, all the groups are

interrelated to some extent and affect each other’s performance.

Power Supply: The power supply functional group consists of the A40 preregulator, the

A50 DC-DC Converter, and the A9 Floating Power Supply. It supplies power to the other

assemblies in the analyzer.

Digital Control: The digital control group consists of the A1 CPU, the A30 keyboard, the

A34 I/O connector, the A32 Instrument BASIC interface, the A51 GSP(Graphics System

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Agilent Technologies 4294A Service Manual Page 164