User's Manual

M889-00

Circuit Operation 2.5

2.3.1 Power Control

The DC voltages from the directional coupler representing forward and reflected power

are buffered by the two voltage followers, IC3 pins 1, 2 & 3 and pins 5, 6 & 7. Their out-

puts are summed at an integrator (IC1 pins 8, 9 & 10), which drives the series pass con-

trol element (Q16).

Forward and reflected power are summed so that, under high output VSWR, the power

control turns the PA down. This is because the control loop adjusts for the same DC

voltage from the directional coupler that would have been present if there were no

reflected power.

2.3.2 Driver Power Level

The maximum output power of the T889 can be limited by placing a ceiling on the

driver output power level using RV69 (accessible through the side cover). For example,

if RV69 is set for a maximum output power of 50W, the range of adjustment using RV63

(front panel power adjust) will be 20 to 50W.

2.3.3 Thermal Protection

At excessively high temperatures, the output power will automatically reduce to a pre-

set level (set by RV74), thus preventing the PA from overheating.

A thermistor controlled voltage divider (R68, R72) applies a voltage to a comparator

with hysteresis (IC1 pins 12, 13 & 14).

The output current from the comparator is summed into the power control network via

RV74 so that the power level to which the PA must turn down may be set.

2.3.4 Forward And Reverse Power Alarms

If forward power drops below, or reverse power rises above, presettable limits, alarms

may be triggered.

The alarm outputs are open drain configuration and are low under normal conditions

(i.e. forward and reverse power levels are normal).

IC1 pins 1, 2 & 3 and pins 5, 6 & 7 form comparators with thresholds adjusted via RV48

and RV52 respectively. The inputs are from the forward and reverse power signals from

the directional coupler, buffered by IC3 pins 1, 2 & 3 and pins 5, 6 & 7. Thus, the power

levels at which the forward and reverse power alarms are triggered are defined by RV48

and RV52 respectively.