Specifications

2-6
Note: The balancing operation that maintains the low terminal potential at zero volts has the
following advantages in measuring the impedance of a DUT:
(1) The input impedance of ammeter (I-V converter) becomes virtually zero and does not
affect measurements.
(2) Distributed capacitance of the test cables does not affect measurements because there is
no potential difference between the inner and outer shielding conductors of (Lp and Lc)
cables. (At high frequencies, the test cables cause measurement errors as described in
Section 4.5.)
(3) Guarding technique can be used to remove stray capacitance effects as described in
Sections 2.4.7 and 3.4.
Block diagram level discussions for the signal source, auto-balancing bridge, and vector ratio detec-
tor are described in Sections 2.3.1 through 2.3.3.
2.3.1. Signal source section
The signal source section generates the test signal applied to the unknown device. The frequency of
the test signal (f
m
) and the output signal level are variable. The generated signal is output at the Hc
terminal via a source resistor, and is applied to the DUT. In addition to generating the test signal
that is fed to the DUT, the reference signals used internally are also generated in this signal source
section. Figure 2-4 shows the signal source section block diagram of the Agilent 4294A precision
impedance analyzer. Frequency synthesizer and frequency conversion techniques are employed to
generate high-resolution test signals (1 mHz minimum resolution), as well as to expand the upper
frequency limit up to 110 MHz.
Figure 2-4. Signal source section block diagram