Beat Frequency

Viewing various videos of my metal detector designs and concepts on YouTube last night, constructors tended to make one or two mistakes. Here's one. If you have a beat frequency which shifts in the presence of metal, keep it low. The difference between 1000Hz and 1005Hz will be undetectable, while the difference between 0Hz and 5Hz will be obvious. OBSERVATION: However, the lower the beat frequency, the bigger the problem of frequency lock, and the bigger the problem of frequency drift. If you run a metal detector that is locked onto 0Hz, you could lose, say, 20Hz before you break out of lock. Drift is a practical impediment, if the beat frequency drifts, say, 20Hz over a minute. Also, one metal detector may have more than one beat frequency, and one beat frequency may not be as good as the next. One of my "covers" is shown on the right.