Elekta Neuromag NM21283A, Руководство пользователя

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Principles of SQUID operation 6
NM21283A-A 31
current is a replica of the input flux. Since the SQUID with its periodic
response is now operated as a null detector, the final output signal, a volt-
age directly proportional to the feedback current, is linear.
For tuning purposes, it is necessary to feed in external flux to see the cur-
rent vs. flux characteristics. Therefore an external flux signal can be fed
through the feedback coil.
In Vectorview the flux-locked loop is implemented using digital signal
processors which allow accurate operation and plenty of flexibility. All
loop characteristics, operating modes, and filters are digital and easy to
control. Signal processors reside physically in the electronics cabinet by
the shielded room. The analog part consists of preamplifiers and circuits
that generate the bias, offset, and gate voltages. These are on top of the
dewar. All the signals between the two parts are converted from analog
regime to digital and vice versa on the signal processor boards.
The operation of both analog and digital parts are controlled by the real-
time computers of the system. The program controlling the analog part is
called Janitor, and the program controlling the signal processors and data
stream to acquisition workstation is called Collector. Normally these pro-
grams are not used directly, but through either the acquisition program or
the tuning program.
6.3 Optimal point of operation
The optimal point of operation of the SQUID is the combination of bias,
offset and gate settings that gives the minimum noise. The noise depends
on all those parameters.
Normally, the voltage bias that gives the maximum modulation depth also
gives minimum noise. The current vs. flux characteristics should be above
superconducting state everywhere (no visible plateaus). For stable opera-
tion, the adjustment should not be just on the edge but rather slightly
above. In some rare cases it may happen that minimum noise is achieved
at a higher bias; this is usually the case if the current vs. flux characteris-
tics is not smooth. Then it may be necessary to adjust the bias higher.
Note that the shape of the current vs. flux characteristics is also affected
by the gate voltage setting. When in doubt, always set the gate voltage to
value 150 to decouple the amplifier noise cancellation before trying to
find a bias setting at which the current vs. flux characteristics looks
smooth.
Warning: Do not adjust gate setting to values below 150 unless the opti-
mum noise is achieved with a lower value. Some FET transistors may
alter their characteristics with low bias voltages. This manifests as a shift
in the optimal gate setting which relaxes in a couple of hours.