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Re: Fwd: Transponder ideas for future satellites

There are two problems that I see.

1) AGC must be used to prevent over-deviation of the transmitter so weaker
stations will still have a lower SNR.
2) The FM demodulator needs to be very linear to prevent intermodulation of
the 50-150 kHz baseband signals at the ground station.

The only sheme that I have seen that inherently prevents strong stations
from hogging the transponder passband is a time division multiplexed
limiting transponder. However, a station could create problems by
transmitting in all time slots.



----- Original Message -----
From: <rfspace@yahoo.com>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, 26 June 2001 23:42 UTC
Subject: [amsat-bb] Fwd: Transponder ideas for future satellites

> --- In AmsatTech@y..., rfspace@Y... wrote:
> Hi Guys,
> I have been building a transponder for mode L/S and an idea came to
> mind. I want to know if anyone has tried something like this before.
> I wonder how good it would work.
> The current transponders use a linear downconverter to convert a part
> of the band, say 1269, down to 10.7 MHz. The signal at 10.7 MHz is
> then converted to 2401 MHz. There is an AGC that adjusts the gain of
> the whole chain. If one station in the bassband transmitts with
> excessive power then he takes most of the transponder power. Also,
> the efficiency of the TX amplifier is not all that great since it has
> to be linear.
> The idea that I have been considering is slightly different.
> The idea is to convert a section of the band like 1269.1 to 1269.2
> down to 50 to 150 KHz using a single sideband process. This new band
> at 50KHz to 150 KHz is an exact replica of 1269.1 to 1269.2 just
> shifted in frequency. There is an AGC the makes sure that the signals
> at the VLF frequencies remain somewhat constant. There is also an
> impulse noise blanker to get rid of noise.
> The VLF IF is then used to FM modulate a carrier at 2401 MHz. The
> resultant bandwidth is maybe around 400 KHz wide depending on the
> modulation index. This FM carrier is transmitted using a class-C or
> higher amplifier with an extremely good efficiency.
> To receive such a transmission, one would FM demodulate the signal at
> 2401 MHz using a simple FM demodulation chip. The chip could derive
> an AFC voltage to adjust for doppler and LO drift. The output of the
> demodulator would be 50KHz to 150 KHz and this signal would be fed to
> an HF receiver that tunes to the VLF frequencies. The signal at this
> point would be SSB and would be at the exact frequency that it was
> received at the satellite. There would not be any drift associated
> with the downlink.
> It looks like some of the pro's might include:
> 1. The transmitter power is constant and is generated using a high
> efficiency amplifier. The louds stations would not get all the power,
> just most of the FM sideband energy which could result in distortion.
> 2. The receive converter could be relatively simple.
> 3. Downlink doppler would be taken care off by the AFC
> Some of the cons might be:
> 1. Signal is relatively wide and might require more power to get
> decent SINAD.
> 2. The strong stations might distort their audio and still pump the
> receive AGC.
> any other ideas?
> Pieter Ibelings
> N4IP

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