SRS Labs QMS 100 Series, Руководство пользователя

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B-12 Description of Schematics
SRS QMS Gas Analyzer
The 18-bit DAC output is also used to set the DC potentials applied to the mass filter. RF_SET is
multiplied by 4 by the differential amplifier U304A, which uses the bottom PCB for its ground reference.
The output of U304A is passed to the bottom PCB via JP301 to control the DC bias sources.
Schematic name: QMSE_B1.
Mass filter RF Supply
The Toroid: The design approach was dominated by the characteristics of the RF transformer. This
iron-powder toroid provides a step-up of 39:1 for the RF. The secondaries have a self-inductance of
about 16 µH so as to resonate at 2.7648 MHz with the capacitive load presented by the rods in the
mass filter together with the parasitic capacitance of the secondaries.
The primary of the transformer is a single turn driven by a square wave voltage at 2.7648MHz through a
series inductor (T401) of 0.5
µ
H. The step-up transformer is driven slightly off resonance, so that its
input has a capacitive reactance, which resonates with the 0.5
µ
H inductor at the drive frequency. This
approach is used to reduce power losses in the drive circuit (the FETs are switched when the current is
nearly zero), and to reduce the distortion in the RF output (as the drive current is very nearly sinusoidal).
Due to variations in the core material used in the step-up transformer, each unit must be tuned by adding
capacitors across the primary. This is done by programming the unit to operate at 100 amu, and by
finding the combination of capacitors (connected between J401 and J403) which minimize the drive
voltage as measured at J400 (which is RF_CT/5). Polypropylene capacitors with a voltage rating of
50V are used to tune the primary reactance for minimum power loss.
The 39 turn secondaries are wound in a very special fashion: the two windings are placed side-by-side
so that the magnetic flux seen by the two windings is very nearly identical. Measurements on these
transformers show a difference in self-inductance between the two secondaries of less than 0.1%. This
is important to assure that the magnitude of the RF for the two secondaries will be very nearly equal.
Silver plated, Teflon coated wire is used for the secondaries to reduce skin effect losses and to minimize
interwinding capacitance, and to reduce the loss tangent in the dielectric.
Primary Drive: The squarewave voltage drive is provided by a pair of IRF510 MOSFETS, Q402 and
Q403. Theses FETs have delay and transition times on the order of 25ns, and are operated as switches.
The gates of the FETs are driven by complimentary square waves at 2.7648MHz with 50% duty cycles
from U401, an SN75372. Each FET is on for about 180ns.
The amplitude of the squarewave drive is controlled by the DC supplied by Q401, a D44VH10 high
speed npn power transistor. The op-amp U400 integrates the error signal from U306 on the top PCB
to maintain the detected RF equal to the signal “RF_SET” from the 18-bit D/A.
The primary current is passed through the FET's 0.25
Ω
source resistor. The voltage across this resistor
is amplified by U402B and may be read by the CPU. Approximately 1.5A is required to generate
2568Vpp required at 300 amu.
At low drive levels, the charge injected via the gate-source and gate-drain capacitances of Q402/403
are a significant source of error. To compensate, the network of R413, R414 and C406 couple a signal