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Auteur Topic: Using surplus VSAT power amplifier modules on 10 GHz  (gelezen 10046 keer)
Eene de Weerd PA3CEG
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« Gepost op: Mei 21, 2011, 11:06:02 »

Hallo,
Ik kwam een artikel tegen op microwave valinet, wat gaat over her gebruik van Modules op 10 GHz uit de satelliet opstraal units.

Working in the satellite industry, I decided to take a closer look at the inside of one of the units in a slowly-growing pile of "dead" ODU (Out-Door Units) that we have laying around that we occasionally scavenge for bits and pieces of hardware when fixing other units.  Most likely, these units have been junked due to moisture or mechanical damage.  Electrical problems seem to be (mostly) due to damage on the power input circuitry and the RF output section, being "deeper" and farther behind more regulators, seem to survive.  Since a "new" unit isn't all that much more expensive than repairing an already-old unit, those that don't have "simple-to-fix" problems are thrown into the boneyard.

These are 1-watt units designed to operate in the 14-14.5 GHz Ku uplink band and they contain a 7 GHz VCXO with a prescaler and PLL that locks to a reference operating at 1/128th of the 14 gig transmit frequency (or 1/64th the VCO frequency) provided by the indoor unit.  The output of this (OQPSK-moduled) VCXO is then doubled, filtered and amplified with a few MMICs and then sent to a Teledyne TBQ-3018 GaAsFET power MMIC which, at 14 GHz, has a saturated output power in the 2-ish watt range.

In staring at the data sheet, I noticed that one of the graphs showed that it was capable of about +27.5dBm at 11 GHz so I decided it was worth investigating their usability at 10 GHz.  Wielding a hacksaw, I removed the portion of the circuit board with the power module (and a MMIC driver amp) - plus a section of the cast-aluminum shield - and put it in a die-cast box with some SMA connectors for testing.

A preliminary page showing the gory details can be found here:

http://www.ka7oei.com/10gig_1watt_vsat_PA.html

* * *

In testing at 10.368 GHz, the upshot is this:

- At a drain current of 600 milliamps at 8 volts, the 1dB compression point is a bit over 27dBm with around -5dBm drive at the input connector.

- At a drain current of 1000 milliamps at 8 volts, the unit will saturate at about +32.5dBm with a drive level of around +2dBm.  This level was maintained for only about 10 minutes as the diecast box and the 20dB power attenuator were getting a bit warm.

- Setting the drain current at 800 milliamps at 8 volts, the unit was run for 18 hours at +30dBm - this, with just the small diecast box for heat-sinking and being partially covered to allow it to get even hotter to simulate an outdoor "summer" environment.  The unit wandered within a few 10th of a dB of +30dBm (probably due to the less-than-perfect input cable from the signal generator) with no degradation noted during the testing period.  The drive level at the unit was -2dBm.

- It cannot be operated without the cast-aluminum shield - unless you want it to be a free-running oscillator!  With the shield and its RF absorbing material in place the unit appears to be unconditionally stable with a very clean output spectrum.

The "final" configuration consisted of the TBQ-3018 being driven by one of the original on-board MMICs, so I'm not quite sure how far below the nominal 30dB gain the '3018 is at 10 gig, but the combination of this MMIC and the '3018 provides around 30dB of power gain.  No attempt was made to "tweak" anything down to the 10 gig band - but then again, there was little to "tweak"!

The next step is to build a bias supply and drain regulator to keep the
'3018 alive and happy:  Since it's earmarked for possible beacon use, a bit more heat-sinking will be attached to it as well.

We have a smaller number of dead 2-watt units and similar devices of different design, but I've not yet looked at them to see how suitable they might be for use at 10gig.  Occasionally, these units appear on EvilBay and from different manufacturers:  All I can speak of is the model noted on the above web page.

If nothing else, it's fun, anyway...

73,

Clint
KA7OEI

 
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