Elekta Neuromag NM21283A, User Manual

Page: 53 / 85

Basic measurements 8
NM21283A-A 47
The I-Phi measurement results can be displayed using following view
modes:
I-Phi curve
This mode shows the curve of the sensor SQUID when a single
channel is selected. The overview mode (all channels) shows a bar
diagram where the top and bottom of the bar show the maximum
and minimum values of the
curve. This view is useful to check
that the offset settings of all channels are proper. Each bar should
cross the zero line so that about one third of the bar is below the
line.
Modulation (abs)
This mode shows the curves of the sensors same way as the I-
Phi mode, except that in the overview display all bars have been
shifted to start from the zero line, so that they display the modula-
tion depth clearly.
Modulation
This mode is similar to “Modulation (abs)” except that the modula-
tion depths have been normalized using the nominal modulation
depth at the nominal operating point. For setting / resetting the nom-
inal operating point, see “Operating point parameters” on page 53.
In this view all bars should have height of exactly 1.0. Any change
in modulation depth is easily noticeable. This mode is useful for
checking the presence of flux trapped SQUIDs and shifts from the
nominal operating point.
8.6 Noise measurement
Another basic operation is to measure the white noise level of the sensors.
This measurement gives a reasonably good and robust estimate of the
noise level with a quick measurement. The measurement band is in high
frequencies, so that normal external disturbances do not affect it much. As
a further refinement, line frequency disturbances are filtered away from
the data.
This mode is a tool that can be used to check quickly the quality of the
data on all channels. It is also a building block used in all noise optimiza-
tion jobs. In addition to white noise level, this job measures also the dc
I-
Φ
I-
Φ
I-
Φ