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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
--- End forwarded message ---

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