satellite linear transponder how is it implemented?

Hello, Some satellites have linear transponders onboard. That is, they can repeat simultaneously more than one "channels" as long as they are not on the same frequency.
I am mostly talking about analogue voice channels, not digital technology.
How is this technology implemented? What RF components compose this?

Voice channels are normally narrow, so the satellite has a bandpass filter that picks up a bandwidth of frequencies that are your voice channels, then down converts them to a 70 MHZ IF with a 5 MHZ bandwidth. They then will be down converted again, filtered out into individual channnels where AGC will be used to equalise their levels. Then added, upconverted to 70 MHZ then upconverted again to the output channel and amplified to the output power by a TWT, which is meant to be linear.
Frank

Can't this scheme be more simple, instead of downconvert twice, to downconvert directly to vhf from uhf? I believe this would require high dynamic range converters though to cope with the absense of signals equalization.

Yes and yes. This is how ham radio satellites work. No individual ALC per "channel", so fair operation regarding power level is required.

Thanks!
However most amsats I know operate like simple repeaters, they do not have linear transponders onboard.
Anyway, I got the point.
Thanks a lot!

I think just up-mixing or down-mixing followed by suitable amplifiers and filters is used. I don't think wide 'base-band' is used as an intermediate stage, at least in linear transponders. The 'linear' refers to frequency domain rather than the more commonly used amplitude domain. Given the distances involved, all signals are likely to likely to be relatively weak so although not impossible, overload is not likely to occur.

Brian.

There is at least one guy that has done it the complex way http://www.leijenaarelectronics.nl/l...ronics_009.htm
It is mainly intended to plug into cubesats, but not restricted only to this.
He uses downconversion and then upconversion again. See the datasheed.

I think they did it that way to allow crystal filters to be used so it had a sharp cut-off out of band. It only has a 30KHz bandwidth, probably because it uses such low input and output frequencies anyway. It doesn't provide baseband signal conversion, that requires the signal to be recovered and remodulated. I'm pretty sure the link I'm using right now uses a linear frequency shift, 27GHz uplink and 30Ghz downlink.

Brian.

This is what AMSAT says:" Most amateur voice satellites use linear transponders (there is only one known orbiting FM repeater accessible in VK at the time of writing)."

Interesting! Thanks for the info

The word 'voice' is important, some satellites use "store and forward" techniques but that only works in digital modes and requires the data to be decoded, saved and re-modulated on the down link.

Brian.

It would be interesting to know what short of characteristics a linear transmonder must have, in it's simplest form (direct mixing from band to band, no baseband or IF down/upconversion). Obviously this has to have a high dynamic range, which means that mixers and preamplifiers / power amplifiers must have high dynamic ranges.
As far as I see it:
For mixers, high level (+17dbm LO) mini circuits can be used.
For filters, passive LC could be the choise for high dynamic range.
For Preamplifiers, something like the PGA-103+ could be proven usevul as a very low noise high dynaic range preamp.
For power amplifiers, some kind of linear amplifiers have to be used but I have no idea what.