Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
the meter as a low or high indication in microwatts. In operation, COARSE NULL ADJUST
control and FINE NULL ADJUST control are adjusted to achieve a 0 or "null" reading on Ml
prior to application of RF power. After applying RF power (either manually via the RF
ON/OFF switch or automatically via the IEEE-488 control bus) the meter should, after a
momentary deviation, again achieve a null reading to indicate that the Model 1805B has
"locked on" to the RF source and is applying the proper amount ofdc substituted power for
calibration purposes.
RF OFF INDICATOR
The RF OFF indicator (DS1) is a green LED located in the upper right-hand comer of the front
panel. When RF ON/OFF switch is toggled to the right (OFF) position, this indicator
illuminates, indicating that 30 mW dc power is supplied to the mount, and the appropriate RF
control signal is routed to the rear panel. If the 1805B is under remote IEEE-488 bus control,
DS1 illuminates when the functions of RF OFF (0) have been performed by the automatic
system controller.
FINE NULL ADJUST CONTROL
The FINE NULL ADJUST control (Rl) is a continuously-variable, knob-controlled
potentiometer located near the upper right-hand comer of the front panel. This control provides
a fine adjustment of the bridge balance reading on BRIDGE BALANCE meter prior to
application of RF power.
COARSE NULL ADJUST CONTROL
The COARSE NULL ADJUST control (R2) is a continuously-variable, knob-controlled
potentiometer located in the right-hand center portion of the front panel. This control provides
coarse null adjustment of the bridge balance reading on the BRIDGE BALANCE meter prior
to the application of RF power.
ERROR INDICATOR
The ERROR indicator (DS3) is a red LED located near the lower right-hand comer of the front
panel. This indicator is active only when RF power is NOT applied, and when illuminated,
indicates either an open circuit (hookup or thermistor), or failure of the bridge to balance at 30
±1mW.
RF ON/OFF SWITCH
The RF ON/OFF switch (S2) is a two-position toggle switch located near the center of the
front panel. During manual operation, placing this switch in the left (ON) position applies RF
power, activates the 1805B circuitry to subtract the chosen dc substitution power from the
thermistor elements in the mount, deactivates the RF OFF indicator and activates the RF ON
indicator. During remote IEEE-488 bus controlled operation, this switch is left in the OFF
position and the automatic system controller performs its function.
TEMPERATURE METER
The TEMPERATURE meter is located in the lower center of the front panel, and provides a dc
voltage analog reading of temperature and temperature stability within the thermistor chamber.
A green band on the meter scale provides a visual indication of normal operating temperature
limits. Although some mounts may cause the meter to indicate above or below this green band,
this does not affect the normal operation of Model 1805B, if the ERROR is not illuminated and
the needle is stable and does not drift or vibrate erratically.
ADDRESS SWITCH
Remote operation of the 1805B requires settings its IEEE-488 ADDRESS switch for a unique
five-bit address. The ADDRESS switch is located to the left of the IEEE-488 BUS connector
on the rear panel. Refer to Figures 3-2 & 3-3.
3-4
