Agilent Technologies 4284A Service Manual page 73

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 current through
the device under test. T h e basic principle of the auto-balance-bridge circuit is explained in
terms of the two oscillator model illustrated in Figure 3-8.
Te s t S i g n a l
Source
Figure 3-8. Two Oscillators of The Auto Balance Bridge
Currents ix and ir can be balanced by controlling the output of the valiable amplitude phase
oscillator as follows. Firstly, the null detector detects the difference (id) between the current
trough the DUT (ix) and the current through the range resistor (ir). T h e unbalance current
(id) which has the information required to balance the bridge, is output to the feedback
control circuit for the variable amplitude phase oscillator. After the variable amplitude phase
oscillator (er) is adjusted, the id is equal to zero (0). S o finally the ix is equal to the ir, and
the impedance of the DUT is calculated as follows.
EDUT E R R E
Zx R r E
3-28
i r
ERR
V a r i a b l e
A m p l i t u d e -
Phase
O s c i l l a t o r
Fr = F s
D U T
Z x = R r x
R R