Making Phase Lock Loop lock onto low input frequencies is easy. Most of the references frequency counters come from PLL that are locked onto a time source from the time observatories. There have been many excellent articles in QST magazines and many excellent books written about the subject.

The best accuracy you would expect any other method is about 2 percent, but with the PLL method you can get 0.1 percent accuracy.

Regards,

Charles Bannister