Is this brilliant or bogus?

[Don Lancaster]

http://us1.webpublications.com.au/static/images/articles/i1013/101343_5mg.jpg

Why would there be two frequencies?
Where is the nonlinearity for mixing to take place?

What, exactly, does each opamp do?

What prevents the two oscillators from pulling or locking?

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Scott Stephens]

http://us1.webpublications.com.au/static/images/articles/i1013/101343_5mg.jpg


Why would there be two frequencies?
Where is the nonlinearity for mixing to take place?

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.

What, exactly, does each opamp do?

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

What prevents the two oscillators from pulling or locking?

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

--
**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!
http://home.comcast.net/~scottxs/

POLITICS, n.
A strife of interests masquerading as a contest of principles.
The conduct of public affairs for private advantage. - Ambrose Bierce

Don't ever give up what you want in life. The struggle is worth it - Ayn
Rand

**********************************

 

[Jim Thompson]

http://us1.webpublications.com.au/static/images/articles/i1013/101343_5mg.jpg

Why would there be two frequencies?
Where is the nonlinearity for mixing to take place?

What, exactly, does each opamp do?

What prevents the two oscillators from pulling or locking?

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

 

[George]

http://us1.webpublications.com.au/static/images/articles/i1013/101343_5mg.jpg

Why would there be two frequencies?
Where is the nonlinearity for mixing to take place?

What, exactly, does each opamp do?

What prevents the two oscillators from pulling or locking?

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

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

 

[Spehro Pefhany]

http://us1.webpublications.com.au/static/images/articles/i1013/101343_5mg.jpg

Why would there be two frequencies?
Where is the nonlinearity for mixing to take place?

What, exactly, does each opamp do?

What prevents the two oscillators from pulling or locking?

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

 

[Don Lancaster]

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

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.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

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

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

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

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

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 http://www.tinaja.com/whtnu05.asp



--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Spehro Pefhany]

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

200 V/mV typical, according to the datasheet.

But I suspect this is some sort of a blocking oscillator type of thingy.

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?

Still do not understand where the second frequency and an audio
difference frequency would come from.

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

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.

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


Best regards,
Spehro Pefhany

 

[George]

George wrote:

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 http://www.tinaja.com/whtnu05.asp

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

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
6 inches for that size coil. Did they give a range in article?

George

 

[Pooh Bear]

http://us1.webpublications.com.au/static/images/articles/i1013/101343_5mg.jpg

Why would there be two frequencies?
Where is the nonlinearity for mixing to take place?

What, exactly, does each opamp do?

What prevents the two oscillators from pulling or locking?

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

 

[Scott Stephens]

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.

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

--
**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!
http://home.comcast.net/~scottxs/

POLITICS, n.
A strife of interests masquerading as a contest of principles.
The conduct of public affairs for private advantage. - Ambrose Bierce

Don't ever give up what you want in life. The struggle is worth it - Ayn
Rand

**********************************