Stanford Research Systems RGA100, Operating Manual And Programming Reference

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4-10 Mass Filter Power supply
SRS Residual Gas Analyzer
Mass Filter Power Supply
All the necessary electronics required to power up the quadrupole mass filter during mass
measurements are built into the ECU box. The RF/DC levels for each mass are set and
regulated from the ECU, under microprocessor control, and based on internal calibration
parameters permanently stored in non-volatile memory.
The difference between the three RGA models (RGA100, RGA200 and RGA300) is
given by the maximum supply voltage available to the rods. For example, the maximum
RF amplitude delivered to the probe in the RGA300 is 1284 V (2568 Vpp) and
corresponds to ions with mass of 300 amu (8.56 Vpp/amu). The corresponding DC
potential is a maximum of about 430 VDC for masses of 300 amu (+215 Vdc on one rod
pair, and -215 Vdc on the other).
The frequency of operation of the mass filter is the same for all models and is fixed at
2.7648 MHz by an internal clock.
The rod structures in the mass filter are primarily capacitive, so to reduce the circuit drive
requirements, the rod reactance is resonated with an inductance. The advantages of this
resonant drive circuit is the primary reason for operation at a fixed frequency.
Important : The RGA Cover Nipple (i.e. stainless steel tube) that encloses the quadrupole
filter assembly contributes to the overall capacitance of the rod assembly, and should not
be removed from the probe unless it is replaced with a similar grounded metal shield
inside the vacuum system. Failure to replace the cover will push the driving circuit out of
resonance, and will limit the operating mass range of the filter. Consult the Hardware
Modifications section in the RGA Probe chapter of this manual for information on the
modification of this important probe component.
The internal calibration parameters used by the microprocessor to set the RF/DC levels
during scans and measurements, are pre-programmed at the factory as part of the test and
calibration procedures performed on the instrument before shipping. However, the
parameters can be readjusted easily and whenever necessary using the Peak Tuning
procedures described in the RGA Tuning chapter.