Elekta Neuromag NM21283A, User Manual, Page: 59 / 85

Measuring characteristics 10
NM21283A-A 53
C H A P T E R 1 0
Measuring characteristics
This chapter describes how to measure several characteristic curves of a
SQUID sensor. These measurements are based on the basic measurements
described in Chapter 8. Typically a curve is measured by repeating a mea-
surement with different parameter settings and picking up some particular
aspects of the measurement result.
Measuring the I- curve is regarded as a basic measurements and is
described in “I-Phi measurement” on page 46.
10.1 Operating point parameters
The operating point of a SQUID is defined by the bias, offset, and gate
settings as described in Chapter 6. In addition to these settings, the char-
acteristics of the SQUID at some particular operating point can be
described by a set of parameters like modulation depth etc. Some of these
parameters are needed by the tuning routines, some are useful in checking
the proper operation of the sensor.
The program has two concepts for the operating point: the current operat-
ing point and the nominal operating point. The first one is naturally the
current state of the sensors, the second one is the “correct” operating point
that was either loaded from a state file, or the state which prevailed when a
state file was saved (which ever has occurred last). The current state can
be set as the “nominal” operating point also by selecting
Commands >
Snapshot
from the menus.
One can examine the estimated parameters of each channel using “Chan-
nel info” dialog available through
View > Channel info
menu button. For
each parameter two values are shown; one for the current state (from last
measurement performed, if any) and for the nominal operating point
(from the state last loaded or saved from/to a parameter file).
The parameters shown in the channel info dialog are:
Channel Running channel number of the sensor.
Bias Bias setting.
Offs Offset setting.
Gate Gate setting.
Modulation Modulation depth (volts in input).
Phi0 Flux quantum size (“volts” in excitation line).
Fraction Where I- curve crosses the zero line (0% = lower edge,
100% = higher edge).
dV/dB Voltage change per bias adjustment unit in I- mode.
dV/dO Voltage change per offset adjustment unit in I- mode.
Bias drift (Not used)
Φ
Φ
Φ
Φ