Saturday, August 22, 2009

1296 2W Amp Update

I got the 2 watt 1296 amplifier working Friday evening. I have had some problems with it when I was working on it. Let me give you a history before I go on.

The active device is a Motorola SML7008, which is known to work on 1296, but with slightly reduced RF output. This was aquired on eBay as a kit.

A couple weeks ago, I installed the circuit on an aluminum pallet ready to install in the enclosure that I planned on using. After it was assembled, I installed a couple of SMA connectors, connected it to a 20V Dell laptop power supply, terminated the input and put my power sensor on the output.

When I powered it up, the unit oscillated. I tried all of the usual fixes - conductive foam, etc... to no avail. The PC board that came with the kit used single sided material, with 2 mounting holes for the board and the active chip is flange mounted with 2 holes.

The fix was...I installed some flashing on the top side of the board, wrapped it around to the bottom, and screwed the board down against the flashing to the aluminum pallet. This is what eventually fixed the amp.

Now, when I was testing the amplifier earlier, I discovered the transverter power output was a tad bit low. This amplifier likes 10 milliwatts on the input for full output. I'm getting about 2 mW. The output spectrum isnt as clean as I'd like either, so I'm planning on building a 3 resonater filter and will place that on the output of the transverter, and a MMIC amplifier following that, to drive the 2 watt amplifier. I'm guessing the filter will have about 3 dB or so of loss, so the MMIC should work out just fine. I will probably be using a MSA-1105 for the active device. I still need to look up the device specs, gain, etc., but I believe it should work out. Once I get this done, I'll be packaging the thing up and will try some line of sight contacts with it using one of my 10 element yagi's. Of course I'll keep you posted as to how it comes along.

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