Specifications

2-16
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 a wider impedance range
than the network analysis (reflection coefficient measurement) instruments can offer. This section
discusses the brief operating theory of the RF I-V method using a simplified block diagram as shown
in Figure 2-14.
Figure 2-14. Simplified block diagram for RF I-V method
The signal source section generates an RF test signal applied to the unknown device and typically
has a variable frequency range from 1 MHz to 3 GHz. Generally, a frequency synthesizer is used
to meet frequency accuracy, resolution, and sweep function needs. The amplitude of signal source
output 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 instruments. The V/I input multiplexer alternately selects the Edut and Etr signals so that
the two vector voltages are measured with an 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 the
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.