SRS Labs QMS 100 Series, Руководство пользователя

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Programming the RGA ECU 3-11
Technical Reference
Programming the Detector
Positive ions that successfully pass through the quadrupole filter are focused towards a detector that
measures the ion currents directly (Faraday Cup, FC) or, using an optional electron multiplier (CDEM),
measures an ion signal proportional to the ion current.
Use the Detection Control commands to choose the detector type (FC or CDEM), query the CDEM
option, recalibrate the electrometer’s I-V response and set the electrometer’s averaging and bandwidth.
Use the CA command to zero the ion detector at any time.
Important : Faraday Cup detection is the default setting when the RGA ECU is turned on.
Use the MO? command to determine whether an electron multiplier option (Opt02) is available in the
RGA ECU being programmed. The CDEM is available if the response is 1<LF><CR>.
The HV command activates/deactivates the CDEM detector. Its parameter value represents the bias
voltage applied across the multiplier, and determines the gain of the amplifier. The null parameter, HV0,
deactivates the CDEM and reconnects the FC detector to the electrometer.
Note: The ElecMult LED reflects the status of the CDEM at all times.
A temperature compensated logarithmic picoammeter measures the ion currents collected by the FC or
CDEM. The microprocessor automatically configures the electrometer and connects its input to the
correct signal based on the type of detector being used. The sign of the electron multiplier currents is
reversed before the current value is sent out over RS232 so that the computer does not need to do any
sign flipping on the currents received when the CDEM is activated. The accuracy of the ion current
measurements is insured by an internal calibration procedure that calibrates the output of the
electrometer against input current over its entire operating range. The I-V response can be recalibrated
by the user at any time with the CL command.
The bandwidth and detection limit of the electrometer are programmed with the NF (Noise Floor)
command. A decrease in the Noise-Floor setting results in longer measurement times with cleaner
baselines and lower detection limits. Histogram scans, analog scans, single-mass measurements and total
pressure measurements share the same NF setting. The RGA automatically adjusts the scanning rate
and averaging based on the NF setting selected.
Important: The zero of the ion detector is automatically readjusted at the beginning of each analog and
histogram scan so that the baseline is always centered around zero. The zero can also be readjusted at
any time with the command CA. In both zeroing procedures, the output of the electrometer is measured
in the absence of input ion current and stored, as a current value, in the RGA’s memory. The current
value (called offset correction factor) is then automatically used by the firmware to offset-correct all ion
currents measured (i.e. including those from single mass and total pressure measurements) under the
same detector settings. The detector settings are the electrometer’s noise floor parameter value and the
type of detector (i.e. FC or CDEM) in use at the time the zeroing is performed. Offset correction
factors for all the possible combinations of detector settings can be generated and accumulated in the
RGA’s memory. However, all offset correction factors are cleared after a recalibration (CL) of the
electrometer is performed, and when the unit is turned off.