I built a Q meter based on the ring down method that used an atmel processor. It was originally in the Silicon Chip magazine but you need a subscription to see the back issues. It was republished in EPE magazine in October and November 2007 and the PCB could be gotten from them back then. I etched my own. It works pretty well but is designed to read no higher than a Q of 120 mostly due to processor tech available. I suspect it could be expanded to higher values now that faster processors are available.? One nice thing is that the source code is available.
The EPE articles are on archive.org so you can see how the author designed it.
