> Why not just use the RF PWR adjustment on the front of the transceiver and
> save all that costs and wasted heat dissipated by the attenuator.
It's probably worth distinguishing between the two types of RF power
controls commonly found on moden transceivers. We all know that the RF
Output Contol is used to adjust the nominal power of the transmitter's
output. If it's set for 100W, we expect 100W and no more. Likewise, when
the control is used to reduce power for use with a linear amplifer, the
power should not exceed the desired drive power level.
Some of the better designed transceivers have either a "Drive" control, or
the drive level is set in firmware at the factory (or by the user with a
specific ALC gain adjustment). Examples of the latter include the Elecraft
K3 and Ten Tec Omni VII. With either method, RF drive level is optimized to
prevent excessive stage gain which drives the ALC into gain reduction but
with a relatively slow attack time that leads to power spikes.
A well designed ALC can manage power peaks while not distorting the RF
envelope. In the case of some transceivers like the Ten Tec Omni VI+, power
spikes were managed by super-fast increases in the ALC attack time to the
point where an otherwise well-rounded CW waveform became truncated and
produced on-air clicks. What was really odd is that in later Omni
production units , they removed the CW waveform control all together and
used two fixed resistors. Why have a control, if it does no good? The ALC
was taking complete control of the adjustment. I ended up designing a
two-speed ALC system that managed the peak power of the leading waveform
edge while not distorting the envelope. I believe that mod is still shown
on N1EU's website.
On the Icom 7700 and 7800, one simply dials back on the front panel Drive
control to eliminate any trace of overshoot in CW mode. On SSB, the mic
gain control sets ALC drive level. A seperate drive control was also
designed into the Kenwood TS-2000 and TS-590. By contrast, the Icom 7600
and 756Pro series have no such front panel drive adjustment -- neither does
the Kenwood TS-480.
Paul, W9AC
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