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Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
U1, a high-gain amplifier with excellent offset drift characteristics, senses the imbalance of a
thermal bridge located in the temperature-stabilized feedthrough thermistor mount via pins 2
and 3 of board connector J5. This thermal bridge, which is composed of two types of resistance
with differing temperature coefficients, balances at approximately 60 C. U1/U2 interaction
provides a'vary ing response to the thermal bridge imbalance signal according to the
relationship between the voltage differential inputs. U1/U2 amplifies an imbalance signal from
a cold thermal bridge that forces the series pass transistor Q3 to pass a current proportional to
the imbalance signal. This current acts to drive the mount heater to restore thermal bridge
balance. As the thermal bridge nears the steady-state condition, Q3 causes DS1 to illuminate.
If the mount is cold, DS1 does not illuminate since the Darlington pair configuring made up by
U2-2 and Q4 is not a conducting state. U1/U2 responds to an imbalance signal from an
overheated mount by turning off Q3 so that it does not pass current to drive the heater or DS1.
Transistor Q2 and resistor R16 combine to provide circuit protection by limiting current in the
event of an output short circuit.
Bridge Offset Sensing Circuit
The bridge offset sensing circuit drives the 1805B front panel bridge balance meter by
providing a total circuit gain sufficient to realize a 200
Ω
per division sensitivity in the front
panel meter reading. To achieve this, instrumentation amplifier U3 senses the bridge
differential and sources the amplified difference to a transconductance amplifier made up of
amplifier U4, U5B and associated components. The transconductance amplifier sources the
meter with a current proportional to the amplified difference seen from amplifier U3. The
high-gain differential amplifier senses the Wheatstone bridge imbalance. This circuit gives the
1805B the required gain to achieve the high sensitivity to the level ofdc bias substituted for
RF levels. Three operational amplifiers make up this circuit. U6 senses the bridge differential
and provides high gain, U7 conditions signals, and U8 drives the RF generator AM Modulation
signal (via board connector J4 and rear panel connector J2 ). Relay K1 forces the output signal
to the maximum attenuation level assuring that the RF power always starts from a low power
state when the RF power is first switched on. This prevents the thermistor mount from seeing
high power transients at switch points.
RF Switching Circ
uitry
The board also derives the switching signals required by most RF switches. The
microprocessor board generates an RF ON/OFF signal that enables or disables Relay K1. This
relay in turn opens or closes a 5-volt loop that drives a switch in the RF generator.
DC SUBSTITUTION SECTION
The dc Substitution PC Board Assembly, A2 contains precision dc circuits which give the 1805B
its primary accuracy. These circuits have two functions: dc substitution and error detection.
Figure 3-8 depicts a simplified schematic of the dc substitution circuit. It is comprised of three
legs of a resistive Wheatstone bridge, whose forth leg is contained in a feedthrough thermistor
mount. This fourth leg containing thermistor beads links to the 1805B via board connector Jl.
Relay K12 allows this circuit board to operate in two states. If Relay K12 is not energized, the
RF OFF functions operate; if K12 is energized, the RF ON functions operate. Switching of
precise resistive sections across the Wheatstone bridge, according to the desired dc substitution
level, cause a change in the bridge currents. This change in current is called the dc substitution.
Without application of RF power (RF OFF) to the system thermistor mount, dc substitution
provides the current through the bridge sufficient to bias this thermistor mount at 30 mW. The
200 ohm equivalent resistance of the TEGAM Model 1109 Feedthrough Mount, which makes
up one leg of the Wheatstone bridge, dictates the 1805B's 30 mW operating point. Relay K12
in this state allows the resistive section consisting of R21 and R22 to shunt the bridge circuit.
This causes the correct current to flow for 30 mW bias. Fine and coarse adjustments to the
bridge current front panel potentiometers correct small errors in the thermistor bias operating
point.
3-12