Re: Question about vacuum and power module for lineartransponder
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
My question is how deep the vacuum has to be to be able to do a valid test on (space) electronics working under space vacuum conditions ?
It depends. We finally got a space vacuum chamber that can pump down to .00001 Torr. Beginning at atmospheric 760 Torr. So it can pump to nearly one-hundred-millionth of atmospheric pressure.
The roughing pump (typical lab pump) can probably get it down to .01 which I always thought would be plenty good for space testing.... A high-cost turbine pump runnning at 27,000 RPM takes it the rest of the way down. I had always figured that .01 Torr (one one-hundred thousandth of atmospheric woiuld surely be good enough for most putposes.. Until this year, when we did a thermal lab and forgot to turn on the turbine pump.
We were demonstrating the properties of radiative heat transfer (the only way to get rid of heat from components under a vacuum after you remove all the convection cooling by air. Turns out, not until we got that final factor of 1000 by turning on the turbo pump, did we actually get the resluts we expected.
Remember, I posted that here back in late November or so. That is, the inability of aluminum to radiate away any heat (it is only 3% as effective as Black). At that time, the point of my posting, was that of-course, the aluminum has to be CLEAN! Just the surface oil from handling completely masked the effect. But it was during this test that we also found that convection cooling still existed even at .01 Torr. Though we did not document it... We simply turned on the turbinge pump.
Notice, we were looking for a 3% value compared to a 90% value of heat transfer. When even that 90% is completely masked by convection cooling which is orders of magnitude still higher. So that is why the closer to pure vaccum the better.
But almost all ectronics that are going to get hot are going to be colored BLACK or other high emissive coating, and so the radiative cooling will probably be effective at higher pressures.
So. The answer is, what are you testing for? Any component that has entraped air or volatiles inside shouldn't be flown anyway (electrolytic caps). Anything else that might have entrapped air would change its characteristics under a vacuum... But with most modern electronics, I woiuld guess that it is the cooling effects you are most interested in... But then anything that should get hot, you are going to use a conductive heat sink on it, and so again, vacuum wont make much difference.. Etc..
I don't know the right answer, but those are our experiences...
Good luck. Bob, Wb4aPR
participants (2)
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Michael Fletcher -
Robert Bruninga