This repository contains some of my experiments on FL2000-based USB3-to-VGA adapters, using the osmo-fl2k library  to use one as a general purpose digital-to-analog converter.
To install the software, first follow instructions in osmocom site  to
install the library and then run
make in this directory. The software works
on Ubuntu Linux and probably other distributions as well, but other operating
systems haven't been tested yet.
Currently, there is only a simple WSPR  transmitter which was originally written in a small software radio hackathon as an exercise on using the library. A command-line interface for configuration and some additional features were added later.
For example, to transmit on 160m, 80m and 40m, use a command like this:
./fl-wspr f 1.8381e6 f 3.5701e6 f 7.0401e6 ppm 140 s $(python3 wspr_encode.py CALL KP20 3)
Replace CALL with your callsign and KP20 with your Maidenhead locator. Replace 3 with your transmit power in dBm (if using an amplifier). Change the ppm value to correct for frequency error of your adapter.
To increase transmit power, the R, G and B outputs can be all connected in parallel. This should provide about 2.5 mW (0.7 Vpp, about 3 dBm) into a 25-ohm load, decreasing on higher frequencies. An LC low-pass circuit can do both anti-alias filtering and impedance matching, so that the output can be fed into an antenna.
Phase shift between different outputs is also supported, so the program can directly generate a balanced, I/Q or a three-phase signal. For example, to generate 3-phase RF to feed a "tripole" antenna  for circular polarization on 80m and swap handedness on each transmission, do:
./fl-wspr f 3.570123e6 p1 120 p2 240 ps 1 s $(python3 wspr_encode.py CALL KP20 3)
Code here is licensed under GPL, since it depends on the GPL-licensed osmo-fl2k library and SM0YSR's wspr-tools . See COPYING.