Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
Since the microprocessor is scanning U3, the front panel is deactivated. Therefore, switching
front panel controls does not cause a functional change by the unit. Once in the remote mode
U3 handles all bus protocol. If the external controller requires status information, U3 asks the
microprocessor for the instruments status. The processor informs U3 of its current status which
U3 outputs to the external controller. An addressing scheme allows multiple instrument control
on the same IEEE-488 bus. Each instrument has its own unique address. U3 decodes the
IEEE addresses sent by the common bus until it sees its unique adddress, which tells it the
information to follow is for that unit. U3 fetches its unique address from the I/O portion of the
dual-purpose EPROM-I/O chip, U2.
RF AMPLIFIER SECTION
The RF Amplifier PC Board Assembly (A3), functionally depicted in Figure 3-6 contains two
circuits that are instrumental in closed loop leveling in the RF ON mode. These circuits are a
thermistor temperature controller and a bridge-sense high-gain differential amplifier. RF
amplifier board circuitry also provides RF switching according to RF ON/OFF signals from
the microprocessor board and bridge offset sensing that drives the front panel bridge balance
meter.
Figure 3-6 RF Amplifier Functional Diagram
Thermistor Mount Proportional Temperature Controller
The thermistor mount temperature controller senses the imbalance of a thermal bridge and
drives the mount heater in proportion to the imbalance signal to compensate for temperature
variations. This board also controls two indicators. The first of these two indicators is
temperature meter M2, which is located in the center of the front panel (Figure 3-1). The
green band on this meter indicates the proper operating temperature range. The second is
the light-emitting diode DS1, located internally on the RF Amplifier board, that indicates a near
steady-state temperature condition. Figure 3-7 presents a simplified schematic of the termistor
mount temperature controller circuitry.
Figure 3-7 Temperature Control Circuitry
3-11
