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Re: AO-40 Satellite RF Architecture Question



>2. Why do I see little discussion of the HELAPS (sp?) power amplifiers?
>What are these? Are they an Envelope Elimination and Restoration
>technique? How does HELAPS work?

There are several ways to make linear RF power amplifiers more
efficient.  Most have their origins in the 1930s through the 1950s.

The Doherty technique has two semi-linear RF amplifiers in parallel
with different bias points. For small signals, one amplifier is linear
while the other is biased into cutoff. Larger signals that saturate
the first amplfier are sufficient to push the second amplifier into
its linear region.

The Kahn, or Envelope Elimination and Restoration (EER) technique uses
a Class C power amplifier that handles a hard-limited (phase only) RF
signal.  The RF envelope is stripped off and amplified at baseband
using a high efficiency switching techniques (similar to those in
switching power supplies). The envelope is placed back on the signal
by collector (or drain) modulating the class C RF power amplifier.

The Chireix or Outphasing technique uses two class C RF power
amplifiers in parallel, fed by a phasing network that uses the
envelope of the drive signal to control the relative phase of the RF
signal fed to each amplifier. At low signal amplitudes, the power
amplifiers are fed limited versions of the input signal that are
nearly 180 deg out of phase, so they nearly cancel at the amplifier
outputs. At high signal amplitudes, the amplifiers are fed nearly in
phase.

Many combinations of these techniques have been used over the years,
first in AM broadcast transmitters and later in many AMSAT satellite
transponders. They have not been used much in commercial satellite
systems, especially since the switch to digital. These techniques are
inherently complex and require great attention to detail. Even so,
it's difficult to achieve wide bandwidths and/or very low
intermodulation ratios.

The trend in digital modulation and coding for satellites has been to
spend bandwidth to save power, so those bandwidth restrictions are a
problem.  And over the past 25 years or so, much work has gone into
digital modulation and multiple access schemes that avoid the need for
linear satellite transponders; offset-keyed QPSK and TDMA is one
popular combination. Even multiple CDMA signals, which being
noise-like do benefit from linear amplification, can tolerate quite a
bit of intermodulation distortion because of their inherent ability to
tolerate interference.

Phil
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