Agilent Technologies 4294A Specifications download pdf (Page 30)

2-5. Theory of RF I-V measurement method

The RF I-V method featuring Agilent’s RF impedance analyzers and RF LCR meters is an advanced

technique to measure impedance parameters in the high frequency range, beyond the frequency cov-

erage of the auto balancing bridge method. It provides better accuracy and wider impedance range

than the network analysis (reflection coefficient measurement) instruments can offer. This para-

graph discusses the brief operating theory of the RF I-V method using a simplified block diagram as

shown in Figure 2-13.

The signal source section generates an RF test signal applied to the unknown device and has a vari-

able frequency range from 1 MHz to 3 GHz (typical). Generally, a frequency synthesizer is used to

meet frequency accuracy, resolution and sweep function needs. The amplitude of signal source out-

put is adjusted for the desired test level by the output attenuator.

The test head section is configured with a current detection transformer, V/I multiplexer, and test

port. The measurement circuit is matched to the characteristic impedance of 50 Ω to ensure opti-

mum accuracy at high frequencies. The test port also employs a precision coaxial connector of 50 Ω

characteristic impedance. Since the test current flows through the transformer, in series with the

DUT connected to the test port, it can be measured from the voltage across the transformer’s wind-

ing. The V channel signal, Edut, represents the voltage across the DUT and the I channel signal, Etr,

represents the current flowing through the DUT. Because the measurement circuit impedance is

fixed at 50 Ω, all measurements are made in reference to 50 Ω without ranging operation.

The vector ratio detector section has similar circuit configurations as the auto balancing bridge instru-

ments. The V/I input multiplexer alternately selects the Edut and Etr signals so that the two vector

voltages are measured with identical vector ratio detector to avoid tracking errors. The measuring

ratio of the two voltages derives the impedance of the unknown device as Zx = 50 × (Edut/Etr). To

make the vector measurement easier, the mixer circuit down-converts frequency of the Edut and Etr

signals to an IF frequency suitable for the A-D converter’s operating speed. In practice, double or

triple IF conversion is used to obtain spurious-free IF signals. Each vector voltage is converted into

digital data by the A-D converter and is digitally separated into 0° and 90° vector components.

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Agilent Technologies 4294A Specifications Page 30