Stanford Research Systems RGA100, Руководство по эксплуатации и программированию

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6-18 Programming the RGA Head
SRS Residual Gas Analyzer
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Before a new scan starts the RGA checks its internal memory to make sure that no
data from any previous scan is pending to be transmitted. If data is still pending, the
RGA must finish transmitting it before the new scan can start. This process may
result in a delay from the time the scan trigger is received to the time it actually starts.
Using the SC1 command and waiting until the whole scan data stream is transmitted
back to the host computer will minimize the problems that are associated to this
feature.
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Perform a complete Peak Tuning procedure on the RGA Head if the peaks in the
spectrum do not appear at their correct mass values (Consult the Peak Tuning section
in the RGA Tuning chapter).
Setting up Histogram Scans
A Histogram (Bar Mode) Scan consists of a succession of individual single mass
measurements over a pre-specified mass range. A single value is used to represent the
peak heights at each integer mass within the range. Histogram scanning is one of the most
commonly used modes of operation for the RGA. Its two main advantages are a faster
scan rate than analog scans, and a reduced amount of data being exchanged during the
scan.
Histogram scans are triggered with the HS command. The scan parameter can be set for
single, multiple and continuous scanning operation. The mass range for the scan is set in
advance with the commands MI (Initial Mass) and MF (Final Mass). A current value is
transmitted for each integer mass value between MI and MF for a total of (MF-MI +1)
measurements (See HP command). For maximum data throughput, ion currents are
represented as integers in units of 10
-16
Amps, and transmitted directly in hex format
(four byte integers, 2’’s complement format, Least Significant Byte first).
The detector settings (i.e. detector type and electrometer’’s noise-floor setting) to be used
during the scans must be selected in advance with the NF and HV commands. The zero of
the RGA’’s ion detector is automatically readjusted at the beginning of each histogram
scan so that the baseline is centered around zero. The offset correction factor calculated
and stored in memory, will then be automatically used to correct all currents measured
(i.e. including those from single mass and total pressure measurements) under the same
detector settings.
The internal scan parameters used by the firmware to step the RF during the scan, are
checked and corrected at the beginning of the scan to make sure that the correct RF levels
(i.e. as specified by the last Peak Tuning procedure) are programmed on the RF rods as a
function of mass.
Important:
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The detector’’s zero and the internal scan parameters are checked and corrected at the
beginning of each scan resulting in a slight delay before the scan actually starts.
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The measurements are performed with the detector that is active at the time the scan
is triggered.