Is this brilliant or bogus?

If it indeed works, nonlinearity is present in any oscillator without exactly unity gain. It has to saturate at some point, and mixing occurs at that point.

Interesting configuration; it looks like C is just a detector for the oscillations going through coupled oscillators A & B.

A and B appear to be working push-pull on the 180 deg. phased inductor windings.

I suspect the designer was fooling around around with various circuits, because it looks like some kind of hybrid, between an unbalanced inductor locater, and a heterodyne frequency-pulling locater.

Scott

Looks bogus to me. The TL074 is a relatively high Zin OpAmp, so it doesn't look like it should oscillate on its own.

However, it could be a receiver for a separate oscillator system, looking for path differences.

...Jim Thompson

It does turn out that the JFET opamps are low gain, around 200.

But I suspect this is some sort of a blocking oscillator type of thingy. Still do not understand where the second frequency and an audio difference frequency would come from.

If it is because of saturation, surely other considerations would dominate over tiny inductance changes.

Also suspect the lack of any supply bypassing may be a factor.

I carefully read the article. Only problem is that I either do not believe it or else do not understand it.

Cannot believe this would be a very sensitive way to measure delta-L.

BTW, Analog Devices has a new $5 chip that does the job properly. Links at

My guess is that there is capacitive coupling between opamps (Faraday shield might do it) that causes circuit to oscillate as "sort of" a flip flop, free running. Thus no need for nonlinearity as no mixing takes place. A change of tone would indicate proximity to metal as the coils "see" the metal and some inductance change.

George

Have you shelled out the $6 or whatever it is for the magazine article to read the description of operation? If so, what does it say?

The author (Rev. Thomas Scarborough) seems to know what he's doing-- the coupling between the coils obviously plays a part.

Best regards, Spehro Pefhany

The really strange thing about this circuit is that there is no obvious way to get much current through the coils- you have a few puffs of input capacitance and maybe some hidden diodes to work with.

I take it the schematic matches the pictorial?

I presume VC1 is supposed to trim the difference? Strange.

Fresh AA cells are pretty low-Z. It would be interesting to see some waveforms.

Best regards, Spehro Pefhany

The small metal detectors you find in the hardware store consist basically of a single oscillator (using a transistor and a multi-tapped-coil on a ferrite rod). As the coil gets near metal (usually only 2 to 3 inches range), the inductance of the coil changes and you get a frequency and amplitude change in the oscillator.

For your case, there is coupling of the coils and perhaps some capacitive coupling with the Faraday shield. Since I see no obvious mixing nonlinearity or obvious way to have two oscillators, I had to assume it was a single oscillator. Granted, it would not be very sensitive for L change caused by metal proximity. My guess would be 3 to

George

Considering that the 2 coils are apparently driving into a fet input ! I can't imagine that it was *designed*.

Maybe it was something that was perceived to 'kinda work' in which case it appears to be more by luck than judgement.

Graham

It could be a kind of phase-shift oscillator. Jfet amplifiers have high input impedance, but also high gain.

I've experimented with cmos ring phase shift gain-tuned vco's, and found parallel capacitance (low-pass) makes a more stable oscillator than series coupling phase shifters. A series inductance is a low-pass configuration. Changing either gain or inductance changes frequency, and a low-pass configuration is more likely to be soft-limiting than a high-pass design.

But not a good, stable design, dependent on the input capacitance and gain of the amplifier.

Scott