The osmo-fl2k is an open source project using cheap (<$20) USB 3.0 to VGA adapters as SDR transmitters. The person behind this project was also behind the RTL-SDR, using cheap USB TV tuners as cheap SDR receivers. You can find more information about the project at its website: osmo-fl2k
This page will be documenting my journey in using one of these transmitters.
The transmitter transmits a square wave so there are tons of harmonics. To safely and legally use it, the signal either needs to be filtered or directly connect the transmitter to the receiver with the proper attenuation to protect the receiver. This second option is the one I took. Using designs from Dan Watson (Blog, Twitter) I ordered a VGA to SMA breakout board and Pi Attenuator boards from OSH Park.
Using an online pi attenuator calculator, I calculated resistor values to make a 30dB attenuator with input and output impedance of 50ohms. Below is the schematic with the resistor values I used as well as photos of the completed boards.