FPGA QAM Generation

I recently drew up a board to connect a fast DAC to FPGA dev boards for the purpose of RF generation. I chose the DAC900E from TI as it's fast (165MS/s), pretty accurate (10-bit), and extremely easy to interface (latches values in on clock edges, doesn't require any setup or external stuff).



Normally I'd cut a simple one-layer board like this on my CNC mill, but I wanted to try a low cost hobbyist PCB fab I read about via Sparkfun. The did take a very long while, but came out beautifully. Also, when the boards finally arrived, I was pleased I had picked a DAC with so few external requirements to solder. The resistors are there to prevent an output buffer short in case I soldered badly, by the way - I don't own a hot air rework setup.



Right off the bat, I was pleased to see that my sine-generation test code worked great at various frequencies and bit depths (aliasing visible in the 2nd pic, its harmonics in the 3rd):



With a bit more hacking, I got the FT245R interface to work such that I send I and Q values via USB, the output value is calculated from an LUT (I originally was doing the math out, but hardware multiply is very slow), and sent to the DAC. In these pics, you can see that I was generating a 500-hz tone over an AM carrier at 9.375 MHz (the 3rd image is the AM-demodulated signal shown on the spectrum analyzer).



My code is on Github here. It's by no means robust or feature-rich, but it does work well in my opinion. When I have time, I'd like to see how far I can take this design. Assuming I don't find any unforseen roadblocks, the next step will be to make a board with two DAC900Es for I and Q generation that feed into a GHz-range QAM modulator that takes its carrier from a PLL on the FPGA, which will theoretically give me full TX control over a wide range of freqs.