Elekta Neuromag NM21283A, Руководство пользователя

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6 Principles of SQUID operation
30 NM21283A-A
adjusted: with large negative gate voltage the conductivity is low, and the
effect of the amplifier noise cancellation circuit remains small. When the
gate voltage is made less negative, the compensating effect increases and
finally cancels the amplifier noise. If the gate voltage is further increased
the feedback coupling becomes too large, leading to instability and
increased noise.
Since the current vs. flux characteristics are periodic with respect to flux,
the sign of the derivative
may be either positive or negative.
Therefore, the amplifier noise cancellation circuit only works where the
derivative has a proper sign, say, on the falling slope. (The actual sign of
the slope where amplifier noise cancellation is effective depends on how
the feedback coil is arranged.) On the opposite slope, the noise is
increased rather than decreased. The proper tuning of the gate voltage is
discussed in more detail in section 6.3.
To linearize the periodic current vs. flux characteristics, a so-called flux-
locked loop is employed. The output of the SQUID is integrated and fed
back to the feedback coil. When the feedback loop switch (see Figure 6.4)
is closed, a current proportional to the time integral of the SQUID output
voltage is fed to the feedback coil. As a result, the point of operation along
the current vs. flux characteristics changes and the corresponding current
through the SQUID changes. A stable point is only reached if the input of
the integrating amplifier reaches zero. Therefore, an offset is added to the
current vs. flux characteristics to make the curve pass through zero line.
The system then “locks” to the point of operation where the apparent out-
put of the SQUID (containing the additional offset) is zero; any change in
the input flux from the gradiometer causes a change in the feedback cur-
rent that exactly compensates the input flux change. Thus, the feedback
Figure 6.4 SQUID electronics
V
Bias
V
Gate
V
Out
V
Excitation
V
Offset
SQUID
Pickup
coil
Feedback coil
Amplifier
Amplifier/Integrator
Loop
switch
Buffer
-
-
+
+
+
PI
Digital part Analog part
A
A
D
D
V
∂ ( ) Φ ∂ ( ) ⁄