Section 4
4-17
4.2.4. 6675A Signal Acquisition
This section details the three stage signal acquisition process consisting of a Rough Null
Phase, a Fine Null Phase and a Measurement Phase, and also the derivation of the formula
used to determine the null turn (T
n
).
In the following discussion:
nanovolt detector reading = nV(T,I)
current Ix = I
and turns Nx = T
Rough Null Phase
a) Ix = +0.1 x(I
TEST
) ; Turns = 0 ; Time = (1/4)x(Reversal Rate) ; nV(0,+I)
b) Ix = +0.1 x(I
TEST
) ; Turns = 648 ; Time = (1/4)x(Reversal Rate) ; nV(648,+I)
c) Ix = -0.1 x(I
TEST
) ; Turns = 0 ; Time = (1/4)x(Reversal Rate) ; nV(0,-I)
d) Ix = -0.1 x(I
TEST
) ; Turns = 648 ; Time = (1/4)x(Reversal Rate) ; nV(648, -I)
Therefore :
T
n
Rough
= Int[ 648 x (Rx/Rs)]
and
T=
+
nV +I)- nV( -I)
nV( I)- nV( I)
n
Rough
Int 648
1
1
648 648
00
(, ,
,,
During this measurement phase :
the tracking dac is set to 0
the ratio (Rx/Rs) is shown as a three digit number
if the preset value for Rx is setup, then this Rough Null step is skipped.
Fine Null Phase
This is a regular measurement phase with :
a) Ix =
±(I
TEST
)
b) Tracking dac set to 0
c) Tn = T
n
Rough
d) t = 1
e) the ratio (Rx/Rs) is shown as a six digit number
For the Fine Null measurement phase :
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