Oscillator power!!!!

[Tinkerer]

OK, we are back to TX power.
I have been experimenting with increasing TX power and reducing RX amplification because I have observed that very high amplification adds problems like the earth's magnetic field response etc.
From TX power and RX amplification, we get a certain amount of dynamic range.

To define the dynamic range I use a 1 Euro coin, getting the following approximate air test response:

1 Euro at 7cm from coil, the signal amplitude is about 5V
1 Euro at 15.5cm from coil, the signal amplitude is about 1.25V
1 Euro at 50cm from coil, the signal amplitude is about 5mV

Driving the opamp with 5V supply, this is about the maximum dynamic range I can use. More amplification will mean the amp will saturate with a 1 Euro coin further from the coil.

As it is, this input will make a decent detecting range while using a 10bit ADC.

Comments and feedback are welcome.

Tinkerer

[pebe]

As I read disputes and argues over in this thread, I feel that participants are irascible retired elders who do not understand that the subject of the dispute is minor and not important.
Instead of identifying a serious problem in which we all to think, experiment and design collective, you are wasting time and energy for some old radio terminology.
As the title of this thread is "Oscillator power", let's dispute and consider what should be optimum TX power for a metal detector. The problem is important because the correct answer will show how should operate the ultimate metal detector.

Yes, of course let’s get back to the original thread, but I cannot let your comments go unanswered.

I don’t know if they were aimed at anyone in particular so I can only speak for myself, but ‘irascible’…? No. ‘Retired elder’…? Yes. Is the discussion important…? Yes.

‘Old radio terminology’…? Perhaps when you have grown to become a ‘retired elder’ you will have learned the importance of correct terminology. Because it is essential when dealing with a subject such as electronics that people should understand what is being said by others, without ambiguity. That can only be if we all sing from the same hymn sheet – ie. use standard terminology.

I would like to remind you that you were the first one to go off topic in your posting #26 about antennas. I did not enter the discussion until post #43 after WM6 had submitted a post about BFOs. I attempted to point out to him where he was in error. There was no lost temper on my part – just a straightforward exchange of ideas.

If you think that is being ‘irascible’ then you have a pretty poor command of the English language!

[Geo]

Hi.
From time to time i hear that if i will increase the power of the oscillator i will make the detector to goes deeper!!!!.
Today i supplied the output transistors of my Sovereign with more voltage so the wave output to be about 50% more. I did not see any difference to the depth of the detector (air test). I want to learn if this method help to increase the depth inside the earth.
Any info?????

Regards

Hi.
I have read 76 answers!!!
Finally, who is the conclusion???.
As i read it is difficult to take good increase at the depth of the detector by increase the Tx power. OK!!!, but who is the minimum Tx power???

Regards

[Qiaozhi]

Instead of identifying a serious problem in which we all to think, experiment and design collective, you are wasting time and energy for some old radio terminology.

Crikey! Talk about the pot calling the kettle black!

‘Old radio terminology’…? Perhaps when you have grown to become a ‘retired elder’ you will have learned the importance of correct terminology. Because it is essential when dealing with a subject such as electronics that people should understand what is being said by others, without ambiguity. That can only be if we all sing from the same hymn sheet – ie. use standard terminology.

Does everyone remember this discussion? ->
http://www.geotech1.com/forums/showp...07&postcount=6

[Tinkerer]

Hi.
I have read 76 answers!!!
Finally, who is the conclusion???.
As i read it is difficult to take good increase at the depth of the detector by increase the Tx power. OK!!!, but who is the minimum Tx power???

Regards

Ok, so lets look at the problem.
Lets consider we want to double the depth of the detector of post 76.
These results were obtained with a coil of 50cm diameter.

We know that the signal response diminishes dramatically beyond a distance of 2 radius from the coil. This is the target distance of 50cm for the 50 cm diameter coil above.

So, to double the detecting depth, we double the coil diameter to 100cm.
what happens now, is that the magnetic field density is reduced by a factor of 4. (roughly 2000cm square x 8000cm square)

Therefore we need to increase the field density by a factor of 4.

Lets say we used 250mA coil current, so now we increase to 1A.
But, with the larger coil diameter, we need to reduce the amount of turns to keep the same inductance.
For example, the 50cm diameter coil has 16 turns for 300uH, the 100cm coil will have 8 turns for the same inductance. Ampere x turns / area = field density.

So we would have to increase the amps 8 times.
We could also increase the amplification, but usually the detectors are designed with the
signal amplification near the saturation limit.

In other words, to double the detecting distance, we need to redesign the whole TX and RX circuit.

There are many other factors to be considered in this redesign to make it work. If you really want to get that much involved, I would consider this to be an interesting academic
challenge. I always have fun solving problems.

Is anybody else up to the challenge?

Tinkerer

[mikebg]

[Quote = pebe; 118030]
If you think that is being 'irascible' then you have a pretty poor command of the English language! [/ Quote]

Pebe, you are correct and I agree with your opinion. However I'm irascible at birth. I always seek flaws in new postings and in circuit diagrams.
Remember your reaction when I used a strange term "dismodulation of TX" (not used in standard terminology). However it describes correctly the action of a PID controller.

[Quote = Qiaozhi; 118036]
Crikey! Talk about the pot calling the kettle black! [/ Quote]

Qiaozhi, the kettle is black because is heated by smoky flame. Instead old kettle technology, now we use a glass pot heated with microwaves and can see what happens inside.
In our case we see an invisible magnetic field relatively far from current loop.
(Oho, I rediscovered the correct term for posting # 26 "Relatively far region" ie
z> D / 2

[Quote = Qiaozhi; 117854]
Antennas have dimensions comparable to the wavelength of the signal of interest. [/ Quote]

This is not correct for all cases. A ferrite antenna always has dimensions not comparable to the wavelength. Despite this we call it "antenna".
See below the symbols used in radio world for receiving ferrite antenna, TX coil, RX coil and monocoil (operating as transmit and receive coil). Is it correct to use such arrows in MD circuits to describe TX coil, RX coil and monocoil?
The old radio term TX (transmitter) at metal detecting should be "driver", or "exciter". The correct term for TX COIL is "Excitating winding" and for RX COIL is "measuring winding". Such terms are used in NDT with eddy currents.

[Skippy]

Geo. As you have seen, gaining extra depth requires significant effort and a large gain in transmitter power and/or receiver gain. Many other problems may also arise, such as the need to improve the balancing of the detectors' coil assembly. The 50% increase in TX power you mention would only give approx 8% increase in depth (air test, it may be less in real ground),which you may not notice. A better solution would be to fit a larger size search coil to your detector, if available. Be aware that his option may have negative consequences, such as reduced sensitivity to smaller targets, more target masking etc.Hope this helps.

[Geo]

Geo. As you have seen, gaining extra depth requires significant effort and a large gain in transmitter power and/or receiver gain. Many other problems may also arise, such as the need to improve the balancing of the detectors' coil assembly. The 50% increase in TX power you mention would only give approx 8% increase in depth (air test, it may be less in real ground),which you may not notice. A better solution would be to fit a larger size search coil to your detector, if available. Be aware that his option may have negative consequences, such as reduced sensitivity to smaller targets, more target masking etc.Hope this helps.

Hi.
I made the experiments and i closed the box of the detector.
My Tx stage has not the ability to give more than 50% increase at the output signal.
So, my detector wanted a new design if i would like to double the depth. I have big coils, square coils ... etc. I opened this thread only as academic discussion

Regards

[Geo]

Ok, so lets look at the problem.
Lets consider we want to double the depth of the detector of post 76.
These results were obtained with a coil of 50cm diameter.

We know that the signal response diminishes dramatically beyond a distance of 2 radius from the coil. This is the target distance of 50cm for the 50 cm diameter coil above.

So, to double the detecting depth, we double the coil diameter to 100cm.
what happens now, is that the magnetic field density is reduced by a factor of 4. (roughly 2000cm square x 8000cm square)

Therefore we need to increase the field density by a factor of 4.

Lets say we used 250mA coil current, so now we increase to 1A.
But, with the larger coil diameter, we need to reduce the amount of turns to keep the same inductance.
For example, the 50cm diameter coil has 16 turns for 300uH, the 100cm coil will have 8 turns for the same inductance. Ampere x turns / area = field density.

So we would have to increase the amps 8 times.
We could also increase the amplification, but usually the detectors are designed with the
signal amplification near the saturation limit.

In other words, to double the detecting distance, we need to redesign the whole TX and RX circuit.

There are many other factors to be considered in this redesign to make it work. If you really want to get that much involved, I would consider this to be an interesting academic
challenge. I always have fun solving problems.

Is anybody else up to the challenge?

Tinkerer

Hi Tinkener.
The larger coil at many cases is a very good solution. But what is happening if you can't use bigger coil??? Rx stage at all detectors, working near to saturation point. I believe that Tx stages are low because the factories wants to keep the radiation of the coil at low point. This was the reason that i lookd to experiment with the increase of the Tx signal.

Regards

[Qiaozhi]

I believe that Tx stages are low because the factories wants to keep the radiation of the coil at low point.

Most commercial designs are a compromise. In this instance the TX power is low because of the law of diminishing returns.

Yes, you could increase the power ... but:
1. This might saturate the ground matrix, causing too many signals from small surface trash.
2. The RX pre-amp will most likely be overloaded by the larger amplitude signals.
3. Balancing the coils becomes more difficult.
4. Increased cost of manufacture.
5. Reduced battery life.
6. Potentially less reliability due to higher stresses on the components.
7. Only minimal increase in depth.
8. More erratic in operation and frustrating to use in the field.
9. Increased retail price of the product.
10. Dissatisfied customers.
11. Reduced market share.

Most so-called high-power detectors will perform well in an air test, but when used in the field it's a different story. At least it can help to produce a good youtube video.

[WM6]

Most so-called high-power detectors will perform well in an air test, but when used in the field it's a different story.

.

Yes, very different. And don't forget that device must comply with emission regulations.

[Qiaozhi]

Yes, very different. And don't forget that device must comply with emission regulations.

Do you have a link to the emission regulations regarding metal detectors?

[WM6]

Do you have a link to the emission regulations regarding metal detectors?

Maybe is good start here (then EU Directives and then country regulations):

http://www.mod.uk/NR/rdonlyres/BB5A9...tionPolicy.pdf

Regulations are done almost always in general not for specific device, because there are spectrum of frequencies which is protected at first line not so much ecology (end user).

[Qiaozhi]

Maybe is good start here (then EU Directives and then country regulations):

http://www.mod.uk/NR/rdonlyres/BB5A9...tionPolicy.pdf

Regulations are done almost always in general not for specific device, because there are spectrum of frequencies which is protected at first line not so much ecology (end user).

This appears to be a document regarding health and safety issues while exposed to non-ionising radiation. It does not specify any limits or restrictions regarding electromagnetic emissions.

Do you know of any document that specifically states any restrictions?

[WM6]

This appears to be a document regarding health and safety issues while exposed to non-ionising radiation. It does not specify any limits or restrictions regarding electromagnetic emissions.

Do you know of any document that specifically states any restrictions?

As far I know national authorities (national EMF control agencies) act (adopt EMF regulations) according recommendations of ICNIRP . There are many documents / recommendations which are than adopted in national regulations ( I am sure that in UK too):

http://www.icnirp.de/