Elekta Neuromag NM21283A, Руководство пользователя

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Principles of SQUID operation 6
NM21283A-A 29
If the bias voltage is set to level in Figure 6.3(a), the current through
the SQUID varies as a function of magnetic flux as given by the lowest
curve in Figure 6.3(b). The sensor remains partly in superconducting
state; therefore, there is a plateau in the current vs. flux characteristics. At
a bias voltage of
the sensor is biased just above the superconducting
state, and a maximum modulation depth appears, as seen from the middle
curve of Figure 6.3(b). If the bias voltage is further increased to
, the
modulation depth decreases, finally tending to zero. At large bias voltages
the SQUID looks like a resistor.
6.2 SQUID electronics of Vectorview
Figure 6.4 illustrates the principle of operation of the Vectorview readout
electronics. The signal is coupled from pickup coil to the SQUID via flux
transformer, as discussed in previous section. In addition, there is an aux-
iliary coil, a so-called feedback coil which is used by the readout electron-
ics to couple various signals to the SQUID.
The bias voltage is set with the help of an operational amplifier which is
also employed to amplify the SQUID signal. The operational amplifier
tries to keep the voltage across its input terminals constant by feeding cur-
rent via the feedback resistor back to the input. As a result, the voltage
over the SQUID remains at a preset value of
, and the current through
the feedback resistor which is directly proportional to the operational
amplifier output voltage then is the actual output signal.
Even with the best operational amplifiers the noise of the amplifier
exceeds that of the SQUID. Therefore, Vectorview employs a so-called
amplifier noise cancellation circuit. The basic idea is to couple the SQUID
voltage through a variable conductance, here a field-effect transistor
(FET), and an auxiliary coil back to the SQUID magnetically. This posi-
tive feedback decreases the effect of the preamplifier voltage noise. By
adjusting the gate voltage of the FET the strength of this coupling can be
Figure 6.3 a) Examples of voltage bias points on I-V curve. b) Flux-
current characteristics at the example bias points.
Φ
a
= MI
sh
= B
ext
MA
p
/ (L
p
+ L
s
)
V
B3
V
B3
V
B2
V
B2
V
B1
V
B1
(b) (a)
I
B
V
B
1
V
B
2
V
B
3
V
B