Over on Hackaday.io Ted Yapo has written about his frequency marker generator which uses Osmo-FL2K. Osmo-FL2K is a recently released hack that allows cheap $5 - $15 USB 3.0 to VGA adapters to be used as an SDR transmitter. We have several posts on it available here. The FL2K-SDR frequency marker generator tool simply generates an AM voice that reads out the current frequency on all of the 118 US AM channels. He writes:
A frequency marker generator is a very useful tool for calibrating antique or experimental (even crystal!) medium wave (aka AM broadcast band) radios. While an RF marker can be as simple as a fixed frequency oscillator, that's not the most convenient way to calibrate multiple frequencies. This project provides a software-defined-radio solution for a deluxe marker generator using a cheap USB-to-VGA dongle. The output signal consists of 118 individual AM "stations" in the MW band - one for each assigned channel in the US - each announcing their own frequency in a synthesized voice.
In the future Ted hopes to be able to generate a full band of old radio shows that could be sent to an antique radio, which is sure to help in improving that authentic antique feeling.
If you're interested in Osmo-FL2K his hackaday.io page also has information on throughputs that he got from different USB interfaces on his PC, as well as a PCB breakout board to convert the FL2K-SDR VGA port into SMA outputs.