This project is based on prior auto tuner work by Ralph, W5GJV. However, instead of a Foster-Seely type phase discriminator like Ralph used, I chose to use a double balanced mixer (DBM) phase detector with the voltage sample shifted approximately 90 degrees. Doing so shifts the DC output (IF port) indicating the phase error so that there are not two zero crossing points within the useful phase error range, but rather one zero crossing within a +/- 90 deg range, with 0VDC corresponding to a 0 deg phase error.
The phase detector is based on one used by M0BMU in his tuning indicator circuit. The control circuitry is similar to W5GJV’s, but with some changes as needed to deal with the single ended phase detector output, required DC amplification, etc.
I created a short (16 min) video demonstrating the automatic tuner on 630 meters: