I'm thinking of building an HF transmitter for data (AX.25 or PSK31). Rather than using an IF stage, I'm thinking of generating the RF frequencies (or phases) directly with a Direct Digital Synthesis chip.
The chip I'm thinking of is the Analog Devices AD9834. It can output a 0 - 37.5MHz sine wave (that covers most of the HF bands). It picks one of two programmed frequencies based on a digital input (high--one frequency, low--the other frequency). Thus, I can get 2FSK out of it. I can do the same thing with another digital input and two programmed phases for PSK. The output power is 20 mW, so I'll need to hook it up to a power amplifier.
I've never heard of anyone building a digital transmitter this way. Usually an intermediate stage is involved. Is there something I'm missing?