Two Oscillators Of The Auto Balance Bridge - Agilent Technologies 4284A Service Manual

The auto barance bridge circuitly used in the 4284A ensures that the vector voltage across
the feedback resistor will be an accurate proportional representation of the c urrent through
the device under test. The basic principle of the auto-balance-bridge circuit is explained in
terms of the two oscillator model illustrated in Figure 3-8.
Test S i g n a l
IV
So urce
Figure 3-8. Two Oscillators of The Auto Balance Bridge
Cu rrents i x and i r can be balanced by controlling the output of the valiable amplitude phase
oscillator as follows . Firstly, the null detector detects the difference ( i d ) between the current
trough the DUT ( i x ) and the current throug h the range resistor ( i r ) . The u nbalance current
( i d ) which has the information required to balance the bridge, is output to the feedback
control ci rcuit for the variable amplitude phase osci llator. After the variable amplitude phase
oscillator (e r ) is adjusted, the i d is equal to zero (0). So finally the i x is equal to the i f ' and
the im pedance of the DUT is calculated as follows.
, ' , Ix =
EDUT
H I GH
LOW
Z x
Nu l l
Fs
EDUT
ERR
=
. . Z x
R r
Z x
3-28
lr
-
ERR
R r
Va r i a b l e
- - - - -�
Amp l i tude­
Phase
Osci l l a t o r
=
F r Fs
EDUT
= R r x
ERR