Agilent Technologies 4284A Service Manual page 74

The actual auto balance bridge circuit is composed of the A2 modulator and A3 range resis-
tor boards. T h e auto balance bridge consists of a null detector, a vector generator, and a
feedback resistor (range resistor).
The null detector consists of a current-to-voltage (I-V) converter, a phase detector, and an
integrator. A n 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. A phase
detector and an integrator covert the error voltage into a dc voltage proportinal to the or-
thogonal vector components.
The vector generator consists of two amplitude modulators, a 90° phase shifter, and a sum-
ming amplifier. T h e 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 signal from the vector generator shifts 180°. The 0 °
phase component of the error voltage is fed into the input of the modulator and this 0 °
phase component modulated output of the modulator is ded directly into the input of the
summing amplifier. T h e 90 ° phase component modulated output of the modulator is fed
into a 90° phase shifter before being fed into the summing amplifier. T h e output voltage of
the summing amplifier is fed back through the range resistor to cancel the error current,
thereby keeping the bridge balanced.
DUT
HIGH
V
LOW
Range Resistor
I-V CONVERTER M
>•
NULL DETECTOR
Figure 3-9. Auto Balance Bridge
3-29
EDUT
ERR
O D E M
A
900
V
900
Shifter
VECTOR
GENERATOR
CO
— •
Ci)
O c t
CA)
0 C r ,
CD
(-)
CD