Drive Power - Mirage B 1018 G User Manual

At high power levels, three main problems limit linearity and cause splatter. All three problems
are reduced or cured by turning the drive power down.
1.) Output-device current amplification (in this case transistors are used) is a problem.
Transistors, unlike most vacuum tubes, show a gradual decrease in gain as drive level
increases. It is important this amplifier be operated in the area where the gain-slope is linear
or "rounded" in a sine curve to minimize splatter and IMD.
Maximum SSB peak power is approximately 110 watts.
corresponds to an average power of 25 to 50 watts, depending on voice and
compression characteristics.
2.) Output-device voltage saturation caused by incorrect loading. If the amplifier is not properly
matched to the load, the peak collector voltage can get too high. This can not only cause
splatter; it can cause the output transistors to fail.
3.) As base current increases, the base-bias tends to shift negative. Many or most amplifiers with
conventional resistor/ shunt-diode biased amplifiers actually move into class C (a non-linear
mode) as the drive power is increased. This amplifier has active bias, and doe not exhibit this
problem.
In AM operation, carrier output power is normally 25% or more of the peak envelope power as
indicated on a true peak-reading meter. Unmodula ted carrier output power should not exceed 25
watts.

4.2 DRIVE POWER

To preserve signal quality in SSB and AM modes, amplifiers must be operated in the linear
portion of their gain-curves. Since this amplifier has no tuning adjustments, the user can control
only the load impedance (SWR) and drive power.
Many operators think RF power meters should show some fixed-ratio of carrier or "whistle"
power to speech power. This is not correct. The ratio varies greatly with many factors, including
the operator's voice. Pushing up the drive power to "see" a certain output power on a meter that
is NOT a true peak-reading meter is a mistake.
Power should be measured on a true peak-reading meter. Many manufacturers
market meters that are not actual peak-reading meters, even though they call the
meters peak-reading. If you have a meter like that, you will find it impossible to
properly determine drive settings on SSB using the meter.
If you do not have a true peak-reading meter, either an oscilloscope or off-the-air
reports can be used to determine if you are driving the amplifier too hard.
The proper drive power for this amplifier is a drive power allowing output power peaks to reach
approximately 2/3 of the saturated output power, as measured on a true peak-reading meter.
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