Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
TERMINATING OPERATION
The following procedures may be used to terminate 1805B operation at any time under either
local or remote control:
a. Place (or ensure that) the RF ON/OFF switch is in the OFF position.
b. Place the POWER switch to either of the following:
• STANDBY position (if more calibrations are to be performed and avoiding the warm-
up period is desired). Leaving it in the STANDBY position allows the temperature of
the mount to remain stabilized. The STANDBY indicator should illuminate; the
TEMPERATURE meter should stay in the green range; and all other indicators
should extinguish.
• OFF position (to terminate all 1805B operation). All indicators should extinguish; the
TEMPERATURE meter should indicate zero (0) volts; and the BRIDGE BALANCE
meter should indicate a zero (0) mid-scale reading.
THEORY OF OPERATION
The following paragraphs contain the theory of operation for the Model 1805B RF
Control Unit. The following paragraphs explains the principles upon which the Model
1805B's design is based and provides a systematic explanation of the RF Control Unit's
internal components and circuitry.
The TEGAM Model 1805B RF Control Unit combines with a temperature-stabilized
feedthrough thermistor mount and an RF generator to form a closed loop RF control
system (Figure 3-5) that achieve precise control ofdc substitution power levels. At the
core of this system are two thermistor beads with negative temperature coefficients
whose resistances total 200 ohms when biased at 30 mW. The temperature-stabilized
feedthrough thermistor mount/power splitter contains these thermistor beads.
These thermistor beads, whose resistance decreases as a function of increasing power,
compromise a leg of self-correcting, de-biased, Wheatstone bridge circuit, which origi
nates in the TEGAM Model 1805B RF Control Unit. When a precise level ofdc bias is
removed from this bridge circuit by manual or bus-controlled switching of the Model
1805B, an unbalance bridge condition occurs. The output of this unbalanced bridge is
directly related to the precise level dc bias removed. After amplification and filtering, the
voltage output of this unbalanced bridge drives a voltage-controlled attenuator. This
component varies the attenuation of RFpower incident upon a feedthrough thermistor
mount in a manner that makes the RF power proportional to the decrease in dc bias
that created the unbalanced condition. Application of the attenuated RF power via one
port of a power splitter restores the total 200 ohm bias to the thermistor beads and thus
bridge balance. The resistance of the thermistor pair is directly related to the sum ofRF
power applied from one output port of the power splitter and the dc power from the
bridge. Since an equal RF power level is present at the second output port of the power
splitter, this port sources a precision level ofRF power. The Model 1805B primarily
consists of four circuit groups consisting of a Motherboard\Power Supply Section, a
Microprocessor Section, a DC Substitution Section, and a RF Amplifier Section.
POWER SUPPLY
MOTHERBOARD SECTION
The input power source is routed through the Power Entry Module Assembly (XF1)
before being applied to the Power Supply\Motherboard Assembly (A6). The Power Entry
Module Assembly allows the user to select between the two operational voltage ranges
that can be accepted by the Power SupplyVMotherboard Assembly (A6). This Power
Supply\Motherboard
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