31/01/96
Copyright TEL
2.3.2
Thermal Protection
At excessively high temperatures the output power will automatically reduce to a preset
level, thus preventing the PA from overheating.
An NTC (R37), being part of a voltage divider (R36, R37), senses the internal tempera-
ture of the PA close to the output balance resistor (R69) and applies a voltage to a com-
parator (IC1c, pin 9). The threshold of the comparator is set by RV35, which sets the
shutdown temperature.
The output from the comparator is summed into the power control network by RV39 so
that the power level to which the PA will turn down can be set.
2.3.3
Forward And Reverse Power Alarms
If forward power drops below, or reverse power rises above, presettable limits, alarms
may be triggered.
The output from the 'wire-line' directional coupler is applied to the comparators (IC1a,
pin 4 & IC1d, pin 11), with thresholds adjusted by RV10 and RV6 respectively. When
activated, the comparators trigger the output stages (Q3 & Q4), which are open drain
with 500mA sink capability (providing the internal power dissipation is kept below
500mW). Internal diode protection makes them suitable for driving relays.
To prevent damage to the unit, the maximum externally applied voltage must not
exceed 50V.
Tx enable is applied to the forward power alarm stage to prevent an alarm indicating
when the transmitter is not keyed.
2.3.4
Forward And Reverse Power Metering
The levels of forward and reverse power applied to the comparators (IC1a, pin 4 and
IC1d, pin 11) are available via RV16 and RV8 at the D-range connector for metering pur-
poses.
Summary of Contents for T830 Series
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