On Fri, 2006-12-29 at 08:52 -0500, K4IA@aol.com wrote:
> Interesting that you leave the transmit power at 100w and control the output
> by reducing the audio drive. Wouldn't that underdrive the output and
> introduce non-linearities? I have always reduced the Orion power output to
> 25w and
> then adjusted the audio drive to just below the ALC lighting up. I don't
> remember having to change settings from band to band.
>
> What say list? Which is the better way to control the PSK output?
>
> k4ia
> Craig "Buck"
> Fredericksburg, Virginia USA
>
Reducing the output in a linear transmitter such as a SSB transmitter
has the same effect at the PA whether reduced by audio gain, audio
input, or reducing a mid stage gain by ALC. And often any reduced output
has better linearity that full tilt operation. In most real world
amplifier systems the power output limit is defined by "acceptable"
distortion products, not transistor dissipation. There are some that do
create more distortion products at low level than at high level, but
they hardly meet the usual definitions of a "linear amplifier." Most are
significantly cleaner at low power.
I believe the Orion uses an ALC threshold to set output power. ALC
detects output power (and in some radios, reflected power) to create a
voltage that reduces the gain of a stage or two that operate at low
level. I never changes the supply voltage or bias of the PA.
Since every stage may have a different gain for each band, using ALC to
set power is handier than just reducing the drive manually. ALC is using
feedback to control, manually reducing the drive with the Orion power
set to full power is open loop output control. Generally the ALC
detector has minimal variation with frequency.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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