Auto Balance Bridge - Agilent Technologies 4284A Service Manual

The actual auto balance bridge circuit is composed of the A2 modulator and A3 range resis­
tor boards . The auto balance bridge consists of a null detector, a vector generator, and a
feedback resistor (range resistor).
The null detector consists of a cu rrent-to-voltage (I-V) converter, a phase detector, and an
integrator. An I-V converter is used to detect difference between the device under test and
the feedback resistor, and outputs this difference as a proportional error voltage.
detector and an integrator covert the error voltage i nto a dc voltage p roportinal to the or­
thogonal vector components.
The vector generator consists of two amplitude modulators, a 90 0 phase shifter, and a sum­
ming amplifier. The reference signal from the test oscillator is amplitude modulated using
the null detector's dc error voltage output. When the error voltage from the null detector
goes negative, the phase of the output sig nal from the vector generator shifts 1 80 0 • The 0 0
phase component of the error voltage is fed into the input of the modulator and this 0 0
phase component modulated output of the modulator is ded directly i nto the inp ut of the
summing amplifier. The 90 0 phase component modulated output of the modulator is fed
into a 90 0 phase shifter before being fed into the summing amplifier. The o utput voltage of
the summing amplifier is fed back through the range resistor to cancel the error current,
thereby keeping the bridge balanced.
H IGH
Range Resistor
LOW
I-V CONVERTER
NULL DETECTOR
Figure 3-9 . Auto Balance Bridge
3-29
EDUT
ERR
MODEM
VECTOR
GENERATOR
A phase