Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Description of Schematics 10-11
SRS Residual Gas Analyzer
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
P
H. The step-up transformer is driven
slightly off resonance, so that its input has a capacitive reactance, which resonates with
the 0.5
P
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 square wave voltage drive is provided by a pair of IRF510
MOSFETS, Q402 and Q403. Theses FETs have delay and transition times on the order of
25 ns, and are operated as switches. The gates of the FET’’s are driven by complimentary
square waves at 2.7648MHz with 50% duty cycles from U401, an SN75372. Each FET is
on for about 180 ns.
The amplitude of the square wave 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
:
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 with the correct amplitude and phase into the primary drive
inductor, T401, to cancel the charge injected via the FET gates.