Tuned coils

[Mark-VA]

Wow, learned something new again.

[Dave J.]

Is this (and the previous nexus article) to suggest that a particularly wound Coil can only support 1 (and only 1) Best central resonant Frequency ?
- no matter the Voltage supplied or Parallel Capacitor or series Resistor ?

... How about a Coil with multiple Winding-# Connection Leads - for Multiple Frequencies ?

1. Nope. There are well known examples to illustrate this. The FTP-Fisher "Fratbros" DD searchcoils are used on mainstream products running at 7.7 kHz and also 19 kHz; and in the lab for experimental projects we sometimes run them down to about 3 kHz. There are limitations of course, you'd never use those searchcoils in a 1 kHz or a 100 kHz product.

2. In an induction balance, that'd be a nightmare to actually do and make work. Things like that are commonplace in RF work where induction balance is not part of the picture.

[jlsilicon]

Is this (and the previous nexus article) to suggest that a particularly wound Coil can only support 1 (and only 1) Best central resonant Frequency ?
- no matter the Voltage supplied or Parallel Capacitor or series Resistor ?

... How about a Coil with multiple Winding-# Connection Leads - for Multiple Frequencies ?

[golfnut]

If you have Tx and Rx coils with same amount of turns and Cap values, this may be best case.


I think this because phase pulling from ground proximity should be the same on both - so may be ok.

may be reasonably easy to try with donor HW like .

IGSL

[Sun_rise_pt]

The resonance that I refered do not have to do with the sender block, but only with the receiver.
In fact the noise increases when you exit the resonance point, ie: Noise is a random disturbance. But if the circuit is tuned to receive certain frequency it is specially prepared to receive it and reject any others. This will undoubtedly be the best S / N.
But ... (there's always a but ...) Unfortunately ... It is necessary to discriminate responses of metals with a synchronous detector that tells us the phase shift...
Then there comes a commitment to consider the sensitivity of the circuit when it is resonant and the reading of the phase that requires to leave a little of this ideal situation ...

This is just my opinion translated by some years of practical (hands-on) experience.

[Dave J.]

Sunrise, I don't dispute that the Nexus product "works", it's one way to build a metal detector. However when it comes to the technical essay on the website, a lot of it is erroneous and should be ignored. I don't have the time to go through it and discuss it point by point, but when I say a lot of it is erroneous, I do know what I'm talking about and I'm doing a public service by encouraging experimenters not to rely on it.

The whole discussion in this thread favoring resonance smacks of vacuum tube era thinking. It ignores related design issues like noise figure, system bandwidth in synchronously demodulated architectures, phase stability, manufacturing reproducibility, and how modern discriminators actually work.

In the metal detector industry, Fisher abandoned resonant receiver coil circuits in most of its products during the mid-1980's, and some other manufacturers began doing the same at about the same time. Resonant receivers have been obsolete for 20 years now. Some major manufacturers began moving away from resonant transmitter coil circuits in the early to mid 1990's, but resonant and nonresonant transmitter circuit topologies are going to coexist for a long time before the resonant topologies finally disappear.

--Dave J.

[Sun_rise_pt]

I have found experimentally that, in terms of a slight mismatch at the optimal frequency of the Rx tank circuit (that receive same TX freq.) have a better discrimination with some disadjust. Obviously, this causes a phase shift, and you can easy discriminate the angular deviations caused by the different responses of the metallic targets constituent.
However if we fail to consider the part of the discrimination, and that the target response has exactly the same characteristics in the signal returned (Rx) (in frequency compared to Tx signal ) the best "antenna" that will be that is perfectly tuned and with a better Q factor. Even for a beginner this is a base rule, that makes sense, coming from far away, since the time of Heinrich Rudolph Hertz, Oliver Lodge, James Clerck Maxwell, many others... ....and could not be denied.

I believe in the Nexus... Is it reasonable for anybody to understand that a manufacturer has available a good research lab that develop your product to compete with those of other manufacturers, which are already numerous. For this, we assume that for a manufacturer seriously not "shoot" to the discredit for making public wrong statements...


-----------
Use a filter!

"Amateurism" doubtful & light of knowledge often proliferates wildly. Opinions will always experienced people are the best help when you put the hands on a new project.

Here on the net, as in life, I think it is advisable for each person always use a good filter. There are a lot of honest information here... But there is also the opposite.

[golfnut]

My interpretation of what is said..


They speak of the target phase pulling the Tx tank off and then the Rx tank off. Rather than listening for eddy currents in the target.


If you have a tx tuned cct with a wide bandwidth - resistive coil, dielectric loss, poor quailiy cap etc a small pull on the tank would not be seen.

If the tank is q'ey and narrow band (low loss coil and cap pls low resistance coil etc) the same target pull would be easier to see - greater shift.

I work in an antenna design company and we daily adjust antenna BW for reasons discussed..
s

[Dave J.]

Pretty sure that's what he means, Jerry.

However I recommend that you simply ignore the Nexus essay, rather than attempt to understand it (since so much of it is nonsense).

--Dave J.

[Jerry]

I agree with corretamante tuned LC circuits, thinking that the S / N ratio will come out much stronger.
The noise exists in the entire frequency spectrum whatever the tuned LC the set of Rx "antenna".
It seems natural that this circuit is prepared as much as possible to be aware of all the disturbance amplitude and / or phase detection ie: (in essence) The unbalance that occurs in exactly the same frequency of TX. A circuit perfectly tuned to "hear" specially this frequency (from TX) and not any other, will not make sense to be the ideal?

A very well written article by a manufacturer of MD's with great reputation, is accessible to all hobbyists are on the page below:

I leave the suggestion of a special reading of the end of the article referring the matter of resonance ...

Then please read:

http://www.nexusdetectors.com/scienc...detectors.html

Well I thought everything was making good sense until I read to the end of that article.
I have worked with resonant circuits for many years but do not have a clue as to what
total electro-magnetic resonance means.

"The best metal detectors regarding
depth penetration and discrimination
accuracy are the IB detectors tuned in
total or as close as possible to total
electro-magnetic resonance."

Is he talking about tuning the RX as close as possible to the TX? What is meant by Total Resonance?

I am confused on this one.

Jerry

[Dave J.]

The Nexus "science" essay is full of errors and misunderstandings.

--Dave J.

[satdaveuk]

Talking as a personal point of view, have been building tuned coils for quite sometime now, far as performance on the field goes theres no noticeable difference between either tuned or non tuned coils.
The advantage of the tuned coil is you can make multiple coils for the same project, where as if you use untuned then your knackered because the chances of getting the same specification every time with diy manufacturing is almost zero.

Will add though can only speak on projects ive built todate, spent alot of time on this very subject with experiments based the TGSL and that was my conclusion.

I do wonder when these manufactures make tuned search coils use the same value caps on each search head or do they vary depending what the final inductance was.
With the pro tools they have proberly the latter but does anyone no for sure?

Regards

[Sun_rise_pt]

I agree with corretamante tuned LC circuits, thinking that the S / N ratio will come out much stronger.
The noise exists in the entire frequency spectrum whatever the tuned LC the set of Rx "antenna".
It seems natural that this circuit is prepared as much as possible to be aware of all the disturbance amplitude and / or phase detection ie: (in essence) The unbalance that occurs in exactly the same frequency of TX. A circuit perfectly tuned to "hear" specially this frequency (from TX) and not any other, will not make sense to be the ideal?

A very well written article by a manufacturer of MD's with great reputation, is accessible to all hobbyists are on the page below:

I leave the suggestion of a special reading of the end of the article referring the matter of resonance ...

Then please read:

http://www.nexusdetectors.com/sciencemetaldetectors.html

Re: Tuned coils

Just off means that a detector transmitting at 6.5KHz will have its receive coil resonated to about 4KHz. This is quite a long way from resonating at the transmit frequency. Being tuned does however mean that a lot of noise is rejected.

Re: Tuned coils

"The coils are commonly tuned lower in frequency than the transmitters frequency although they could also be tuned higher."

This just jogged my memory... when I visited a manufacturer a few years ago, I specifically asked if the RX coils were tuned to the TX frequency. Their response was that they are tuned just off of the TX frequency... I don't recall their explanation, other than I didn't quite understand it. Now I understand. Thanks!

- Carl