Saturday, October 31, 2009

Homebrew 5 W linear amplifier for the Softrock SDR

Enough of this QRPp!  Now that we know the radio works for sure,  lets add some power for those QSB moments.
The 5W amp is in progress and being tested on 40/20m. First tests into dummy load show about 3- 5W out with 1W of drive. Using NB6M's "miniboots" design, here goes,  and "yes I know this really needs to go on copper" with proper shielding.

Notice the LPF is already inline, and output reads close to 5W / CW (with 1W drive from the Softrock SDR)
Parts for this 5W amplifier will run you about $8-$10 depending on where you source your components.
Adding several LPF's for each band combination will require mode capacitors and toroid inductors. 
The most expensive part (in my case) was the relay (DPDT tx relay switch) at about $2.50. The amplifier design is based on the very popular and very affordable IFR510 MOSFET.  Add your choice of LPF for the output stage and make sure to mearure for harmonic suppression.

Be sure to measure for proper harmonic suppression with the LPF inline.  A good IMD test can be performed using Rocky's built-in 2-tone generator.

Miniboots reference:
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Softrock 6.3 nearing completion.

Oh - the joy of SMT.  With the Softrock 6.3 kit, theres a few more interesting hours of soldering ahead.  The double sided board, has al lthe smt on the bottom.  Here the board is taking shape, wiht the PSU and some initial smt components, including the VXLO in place.

Here the board is nearing completion with the LPF inplace and all the components in place.  The bottom has a few more smt's added to the mix.

A few headers make the external connections a breeze.  This way you can work on the boad, make corrections without worrying about damaging the external wiring already soldered in place.

And so the first day on-air arrives.   With things wrapped up and headers all wired, the radio can get on the air. This time with a 20m quaterwave vertical (wire) antenna no higher than 20ft. Sitting on the antenna tuner, one can easily watch for smoke, while ensuring there's a nice match.
Given the space limitations at my QTH, I choose to make sure things are matches by adding the antenna tuner.  The Softrock earning it's place in the "shack with a view". 
Here goes with a few CQ's on PSK31 (20m)

What do you know -the signal makes it over to the east-coast.  Software:  Rocky, Antenna = wire vertical at sea level.

And after a few hours of calling CQ - the results look like this.

Softrock txrx 6.3 (1W) nearly completed

The Softrock 6.3 txrx kit will test a few more of your smt skills, but with a little persistence and after a few hours of building, the board is coming together nicely. The main board acts as motherboard for the rx BPF's as well as multiple Power Amps. The PA modules can be purchased separate for each band-combination you require (eg. 40/80, 30 - 17, 15 - 10) The smt components on this design is all mounted on the reverse side with through hole components on the top. Tony's design is very good and so is the quality of the manufacturing of the boards. As always with kits and projects, once you start, the thing has to get done, and you can't wait to get it on-air. No different here.
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