[amsat-bb] Re: Question about vacuum and power module for lineartransponder

Michael Fletcher oh2aue at kolumbus.fi
Tue Dec 29 03:27:47 PST 2009



>>> Hi AMSATs,
>>>
>>> After many work on my transponder design I decided to put some work in 
>>> extending my lab.
>>> One of the things I liked to have is a vacuum test chamber, to test electronics 
>>> under vacuum conditions.
>>> 
>>> I have looked around and I see all kind of classes of vacuum pumps, and their 
>>> equal increasing price.
>>> My question is how deep the vacuum has to be to be able to do a valide test on 
>>> (space) electronics working under space vacuum conditions ?
>>>
>>> I also would like to know if anyone has experience in SSB mode with the new 
>>> power modules types from Mitsubishi. Like the RA60H1317M, which can give 60W 
>>> power on 2m with only 50mW input power. 
>>> I am looking for some PA behind my LE005-R2 transponder design, to get a small 
>>> sized high-power linear transponder. The transponder gives 200mW PEP, so the 
>>> 50mW is no problem at all.I just wonder how linear these Mitsubishi modules 
>>> are, when using in linear mode.
>>>
>>> In the datasheet it says that they 'may älso be used for linear mouldation", 
>>> which doesn't give many guarenty. Any experience with these modules in linear 
>>> (SSB) mode are welcome.
>>>
>>> 73 de PE1RAH, William Leijenaar
>>> For my small mode-uv transponder design, please check: 
>>> www.leijenaarelectronics.nl 
>>> ---

>> Hi William.
>>
>> A better solution would be to build a class AB linear from 2 suitable
>> FETs. An even better solution but one requiring a lot of development
>> would be to separate the modulation into phase and amplitude and then
>> modulate the gate with  phase and drain with amplitude. Efficiency >80%
>>
>>
>> David

> Hi David, G0MRF
>
> What you suggest is the HELAPS principle of Envelope Elimination and
> Restoration that was used in all linear transponders from OSCAR-6 ,OSCAR-7
> OSCAR-10 and OSCAR-13 with an efficiency of about 85 %
>
> http://www.amsat-dl.org/dl/HELAPS_en.pdf
>
> The above is a complete description of the HELAPS principle written by
> Dr. Karl Meinzer DJ4ZC with all functional schematic diagrams of the
> linear transponder Mode-B on board of OSCAR-7
> 
> I hope it helps William PE1RAH
>
> 73" de
>
> i8CVS Domenico

William,

regarding vacuum, you need to be able to simulate the space environment
as accurately as possible. This means you need to be able to create a
high vacuum of the order of 1E-7 Torr or better. Going all the way to 1E-9
Torr and beyond would be great and the Big Guys even use ion getters to
grab the last floating molecules captured to make the chamber resemble
real space...

AMSAT has experience e.g. with failure of a particular type of RF capacitor
that fails _only_ in high vacuum, so the only way to segregate the good
ones from the bad ones is (thermal) high vacuum. And when finding such a
failure and having corrected it (you think), you _must_ take the time, 
patience and money to _redo_ the test to ensure your results. At the same
time you need to slowly cycle through your planned temperature cycle, e.g.
-40 ... +60 degrees C or whatever.

Regarding HELAPS (or whatever other type of efficiency and linearity
improving technology you want to use), this is the only way to go with
a) limited power available, b) maximum RF output power needed and c)
limited thermal dissipation possible in space.

The old AO-07 is a favourite of mine. Here is the block diagram of the
Mode B transponder (in Finnish) showing the components used for each
stage:

http://www.kolumbus.fi/michael.fletcher/ao_07b.gif

For those interested, here is breadboard of an early 10 GHz HELAPS
experiment:

http://www.kolumbus.fi/michael.fletcher/X_band_HELAPS_2.jpg

Back then, AMSAT-OH was not feeling confident in building a flightworthy
10 MHz SMPS HELAPS modulator, but since then I have been looking at a
12-polyphase modulator concept. The reliability issue here is a statistical
one with the component count rising pretty rapidly. I just have personal
dinosaur thing about wanting to experiment with this stuff in the analogue way ;-)

73, Michael, OH2AUE



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