The PLL method is a good way of increasing the resolution of low frequency measurements, but the accuracy may be severely affected by the frequency jitter of the PLL output. The circuit described by Kai and Charles, while simple and cheap, will behave quite poorly in this respect. A better loop filter will make a big improvement; see for a good example this Design Note from Linear Technology.

Not too long ago I outlined an even better way of measuring low frequency signals in this post in a thread that was also started by Vivek...


Rob.