User Manual

System Components
Power Amplifier Control Functions
The Power Amplifier Control Functions on the Power Amplifier Control
Board are accomplished by the 87C552 master processor located on
this board in conjunction with a variety of support circuitry. All of the
major power amplifier parameters such as forward and reflected
power, heat sink temperature, input drive RF. power, drive stage
voltage, and the master power supply voltage are monitored in this
board by the processor. The 87C552 has a built in A/D converter for
these analog signals and they are sampled after suitable scaling in the
operational amplifier stages of U6 through U8. The gain of most of
these operational amplifiers is controlled by a potentiometer located in
one of the feedback stages as can be observed on the schematic
located in Appendix. Each of these adjustment points can also be
located in Figures 4 and 5. All of these adjustments are factory preset
and should not require field adjustment. However, for convenience,
the following is a summary of these potentiometers and their function.
R24
Driver Voltage Meter Calibration Adjust for proper reading on meter with external voltmeter.
R33
Input RF Threshold Adjustment Sets the input RF level at which the fan circuit is activated.
R43
Forward Output Power Calibration Sets the gain between the forward power sensor and the
input to the microprocessor. Setting depends on frequency
and power options and is factory preset. This input is used
to control the VSWR protection circuit.
R18
Input DC Voltage Meter Calibration This control is used in conjunction with a voltmeter to set
the input DC voltage reading on the display meter.
R19
Output RF Power Level Adjustment This control is used to adjust a control voltage to the pre-
amp stage that ultimately controls the level of the output
forward RF power
The microprocessor, U1, on the Power Amplifier Control Board also provides the
monitoring of the VSWR using the forward and reflected power levels and will
automatically shut the amplifier down if the VSWR exceeds the threshold level set in
the software.
In addition, the microprocessor also continuously monitors the temperature of the
heat sink near the high power final stage and will shut the amplifier down if this
temperature reaches 70 degrees centigrade. Should this event occur, the fan will
continue to run and should the temperature drop to 50 degrees centigrade, the
processor will automatically re-enable the amplifier for full power.
Version 3 3-7 07/18/01