BFO With Wobbulator

Talk about a wobble causing a flutter. After referring to a wobbulator in one of my posts, I received a number of requests to explain it. Surprisingly, I don't see much about wobbulators on the Internet -- so, I'll offer a circuit which employs a wobbulator -- a BFO metal detector that I designed (pictured). In a comparison with six commercial designs of the 60's and 70's (before BFO went out of fashion), it out-performed four. But first, the wobbulator. A BFO metal detector uses two high frequencies running side by side (oscillators IC1.A and IC1.E in this case, which both run at about 250kHz). One of these frequencies (IC1.E) shifts when metal is picked up, while the other (IC1.A) remains the same, and so a difference frequency is created. This, one hears in a pair of headphones. That's the theory, anyway. In practice, however, there's a thing called frequency lock, where those two frequencies "lock on" to one another -- they get stuck to each other like magnets. The difference frequency thus fails to materialise until the difference is comparatively large. It's a nuisance, because it may severely affect sensitivity, and is not easy to overcome. However, a wobbulator, since it introduces a wobble into one of the frequencies, unsettles the frequency lock, and may do a lot to "bust out of it". OBSERVATION: A wobbulator might benefit almost any kind of detection circuit -- including the Matchless Metal Locator recently described on this blog. IC1, in this circuit, should ideally be a CD40106BCN or a HEF40106BP to obtain optimum performance -- otherwise R3 may need some alteration. The potentiometers' metal cans would ideally be wired to 0V. Click on the image to enlarge. The details of the coil are at Tri-Coil. . NOTE: You may re-publish this design, on condition that you acknowledge the designer, Thomas O. Scarborough.

POSTSCRIPT: See Silicon Chip magazine March 2023 for a six-component BFO design in which I use regeneration to double the typical detecting range of BFO.