Dr. John Schwacke RF 12-bit A/D board update

Terry Fox tfox at knology.net
Sat May 16 01:02:43 CDT 2009

John and Mel came over to the house last Tuesday, and John supervised the
construction of one of his 12-bit A/D boards that hooks to a Digilent Nexys2
FPGA board.  After some construction issues, the board was tested, and
success was declared.  Another board will be built shortly.

This board has a 27MHz LP filter, variable-gain amplifier (VGA), and an
AFEDRI8201 12-bit 80msps A/D converter, along with a couple RF switches.  It
plugs into two of the PMOD jacks of a Digilent Nexys2 FPGA development card.

It can cover HF, especiall if you put a bandpass filter in front of it.
using the "bypass" mode, it can actually tune in 2-meter repeaters.  While
building the new board, John had the old board listening to local repeaters
here in Charleston.

I am working through getting the proper FPGA code right now with John, and
hope to bring the board for a show & tell at Tacos on June 6, along with
maybe a quick demo at the AMRAD board meeting.

John was originally using GNU radio under Linux for the user interface, but
also has some Windows code using the FLTK GUI toolkit.

John believes that the board can be reproduced for under $80.  While it's
not a Mercury or QS-1R (16-bit A/Ds), it costs much less, and allows
experimentation of direct-sampling A/D systems with both FPGA and host code
at a reasonable cost.

Incidentally, the board was constructed using a plastic stencil to spread
solder paste with a small spatula.  The surface-mount parts are then
carefully placed where they belong.  The board is then placed on a regular
cooking skillet, which heats up the paste.  Oops, we forgot a part, then I
dropped the board as I removed the board from the skillet.  Another ten
minutes or so to reposition the parts that had jumped around thanks to
gravity.  Then, the skillet was turned back on, and viola!! a nice clean
board.  One part tombstoned, and there was one solder bridge on a chip.  Two
minutes with a small-tipped iron, and they were taken care of.  After some
voltage checks, the board was plugged into the FPGA card, and took off.  I'm
sold on the solder paste and skillet method, if you have a stencil.

More later.

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