SRS Labs QMS 100 Series, User Manual

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Description of Schematics B-15
Technical Reference
For a high power de-gas, a DPDT relay is used to by-pass the bias regulators, connecting the grid and
repeller directly to the un-reg250Vdc and -150Vdc supplies. During de-gas, the filament
emission current is set to 20mA, which will provide about 8W of power to heat the grid, in addition to
15W of filament heater power.
Schematic name: QMSE_B3
Power Supplies
The unit is operated from +24VDC, and requires up to 2.0A. (The current draw will be the highest
when the unit is scanning at the high end of the mass range, or during de-gas.) The unit is protected
against power supply reversal by D610. A 5A circuit board mounted fuse limits damage in the case of a
severe circuit failure. The DC input is filtered by C615, L601 and C616, which also reduce conducted
emissions from the unit.
A DC-DC inverter is operated from the +24V supply to provide -140V for the repeller,
±
250V for the
mass filter bias sources, and -20V for the negative linear voltage regulators. The primaries to the inverter
are driven by FETs. A switching power supply regulator, U601, provides 12V square wave drive
voltages to the FETs which are sync’d to a sub-multiple of the master clock. Overcurrent protection is
provided by U601 on a cycle by cycle basis. U601 does not regulate the outputs of the inverter, rather,
its COMP pin is tied high to provide the maximum duty cycle.
The first of three secondaries is full wave rectified by D611-D614. This output is used only to bias the
repeller, which allows the filament emission current to be monitored and controlled with high precision.
The next secondary has a pair of half-wave doublers (C635, C636, and D615-D618) to provide
±
250V. The third secondary has a full wave rectifier (D619-D622) to provide -20VDC, which is
filtered by C622,C623 and L603.
All three secondaries are balanced so that no DC current is drawn in order to avoid saturation in the un-
gapped transformer core of T601.
The positive power supplies (+15 for the op-amps, +12 for the RS232 and MOSFET drivers,
+5_CLEAN for the signal multiplexers and A/D converter, and +5 for the logic circuits) are derived via
linear three terminal regulators (U602-605) from the +24V supply.
The negative supplies (-15 for the op-amps, -12 for the RS232, and -5 for the A/D and analog
multiplexers) are provided by the linear regulators U606-608 from the -20V generated by the main
inverter.
Schematic name: QMSE_V1
Signal Conditioning
The output from the mass filter is a very small ion current which is proportional to the pressure of the
selected mass. The lowest pressure which may be measured by the instrument is proportional to the