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Re: AO-40 Satellite RF Architecture Question

Hi Joe,

Joe Leikhim wrote:
> 1. Why is AO-40's transponder described as having receivers and
> transmitters when nothing is apparently demodulated? There is an IF to
> which I would assume downconvertors and upconvertors are interfaced like
> any other bent pipe transponder..

What we usually call a receiver is not a demodulator and a transmitter
is not a modulator. However, we convert down to baseband, i.e. 10.7 MHz IF
with some sort of bandwidth limitation.

> 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?

Probably because this is an old technology, unfortunately not widely known.

HELAPS stands for High Efficiency Linear Amplification by Parametric Synthesis
and this concept was the subject of a doctoral dissertation by Dr Karl Meinzer, 
DJ4ZC, past president of AMSAT-DL and P3-D Project Leader. Karl's concept has 
been proven a while ago on the Mode B and J transponders used for OSCARs 7, 8, 
10 and 13 and on AO-40 (Mode-B and Mode-S). This technique results in a very 
power  efficient, and therefore cool-running wideband amplifier.

> 3. Did HELAPS achieve the efficiency and linearity required for the
> mission?

Yes, with the exception that the FSK signal from RUDAK which is feed into
the IF suffers a little bit when there is other traffic. PSK should be fine.

One year ago Tom Clark discussed HELAPS very detailed, better than I 
could do it here, so here is Tom's email from last year..

73s Peter, DB2OS


Subject: Re: [amsat-bb] Using Class C Amplifiers for SSB
Date: Tue, 02 Jan 2001 22:23:56 -0500
From: "Tom Clark (W3IWI)" <tac@clark.net>

Jon Ogden wrote:

> Well, the only way I've seen it done (with a Class C amp) is in a Dougherty
> Amplifier configuration.  This type of amplifier uses a Class C amplifier as
> a peaking amp to amplify the signals at their peaks.  During other parts of
> the modulation cycle a linear amplifier is used.  The two are phased in such
> a way as to make the whole thing work.  It's a pretty fascinating way to
> develop a more efficient linear amplifier.  I've got a paper on it somewhere
> that I should scan in someday.  The biggest problem is that due to the
> matching/combining structure, it is inherently relatively narrow band
> (narrow band meaning less than 10% bandwidth).

FYI -- the linear amplifier design by DJ4ZC which has been used in most of our
satellites uses a scheme invented/patented by Karl similar to the Doherty
amplifiers. Karl calls his scheme HELAPS, standing for High Efficiency Linear
Amplification by Parametric Synthesis.

In essence, consider the signal you want to amplify as consisting of a phase
component and an amplitude component; assume that there is a 3-port black box
does the vector analysis on the input signal and with two output ports --
A(mplitude) and P(hase).

The P component (as with any FM/PM signal) can be amplified in a non-linear,
limiting (i.e. Class C) amplifier to achieve high efficiency..

Now let's feed the A signal into a good old AM Plate Modulator (some of us
remember that the plate is the "collector" in a thermionic glow-FET) to vary
supply voltage feeding the hard-limiting P amplifier, and hence re-modulating
P amplifier.

Voila -- we have a high efficiency amplifier nearly linear where the RF power
developed by the hard limiting P amplifier and the signal amplitude information
comes from the AM modulator.

In Karl's designs, the "plate modulator" is actually a Class-D switching power
supply. Because the transponder bandwidths are several hundred kHz, the
power supply operates at frequencies up to a couple of MHz (i.e. several times
bandwidth). Transponder bandwidths have been dictated in part by the speed of
Schottky switching diodes and power MOSFETs  and the loses in the associated
"swinging" inductor in the Class-D modulator.

Because the P amplifier's "plate" voltage is changing, the amplifier does not
a constant load impedance (a flaw in ordinary AM transmitters which caused the
Doherty design to happen) and power is lost. In Karl's design, the "Parametric
Synthesis" magic black box not only separates the A & P components, but it
intentionally pre-distorts the A & P signals so as to cancel the distortion
introduced by the amplifier. With this scheme, Karl's design ends up with
distortion products that are ~30 dB down.

You certainly wouldn't want to operate next to a station with -30 dB IMD in the
CQWW contest. But if there are many nearly equal signals in a satellite
transponder passband, the level of IMD simply raises the noise floor a little
for everyone. The LEILA active notch filter in the AO-40 IF chain was designed
try to kill off alligators that hog too much of the transponder's power and
bad IMD for all other users.

73 de Tom, W3IWI

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