signal strength

[Tinkerer]

(What would the voltage be if the silver dollar was at the diameter of the coil 260mm?) Wasn't very clear. I meant if the coin was at a distance equal to the diameter of the coil. Would the signal amplitude from the silver dollar in the center of the above coil be similar to a worse case ground signal, lot less, or a lot more?

This goes into the realm of the Ground Balance.

If the ground is magnetically permeable, the coil field will be distorted by the permeability and the target will see lesser field strength, therefore the return signal will be less.

[green]

(Bu, since e have your attention, prof., ould you be so kind and look at my calculations and give your opinion?)
The math is over my head but I'll study it. I have looked at Rx return signals but not with that much Tx power. When I get a distance near the diameter of the coil the signal gets noisy. That is a larger signal than I would expect. I think I could see 1.7mv I'm trying to learn.
Thanks green

[green]

I guess the main thing I would like to know is how to simulate ground on the bench. What to use and the signal amplitude with coil 1 inch above ground compared to a known target in the center of the coil. Some way to test if my detector would work at the beach or the gold fields before I got there.

[Tinkerer]

I should mention that this is a high power TX and a TINKERERS TX and RX board, not a traditional PI.

I use a pile of red fired clay bricks to simulate "hot" ground in the lab, but only the final field test with actual field conditions will be the final proof.

[mickstv]

I should mention that this is a high power TX and a TINKERERS TX and RX board, not a traditional PI.

I use a pile of red fired clay bricks to simulate "hot" ground in the lab, but only the final field test with actual field conditions will be the final proof.

Hi Tinkerer, are you able to post a couple of CRO shots with and without the clay bricks. So we can all see what sort of response they produce.

[moodz]

Maybe this helps:

I was just balancing a coil and have the signal available at the output of the preamp.

With a gain of about 10, the signal of a silver dollar at the center of the coil is about 1.2V.

This means the actual signal is about 120mV

The coil is about 260mm outer diameter.

I add the pictures below:

First picture: output of the preamp, no target.

Second picture: output of the preamp, with a silver dollar at the center of the coil.

Look at the changes produced in the signal shape, by the target.

Enjoy

Tinkerer

This is a classic experimental error sometimes referred to as "a priori" .... ie you "know" which waveform is the dollar and which is not when you test the system. If the target signal is very weak and buried in the ground AND you do not know it is there ... then present your results based on DIG or DONT DIG for classifying the waveform. Another test is to show your two waveforms to a complete stranger off the street and ask them only which one is detecting a target. They wont know better than 50%.

[mikebg]

Not B, even not dB/dt, but d2B/dt2

mikebg, ah, the intrinsiquies of translation. Green meant the silver dollar at a distance of one coil diameter, from the coil.

But, since we have your attention, prof., would you be so kind and look at my calculations and give your opinion?

All the best. Tinkerer

You do a numerical analysis for TX coil, whose diameter can be modified by designer. The analysis shows that a larger diameter of TX coil will produce more strong field in depth.

But you need an analysis of eddy current in the coin, which has a much smaller diameter and you can not increase it (only Mr Green can this :-). How many times weaker magnetic field of the coin we have at distance 10 coin diameters?
You can do an experiment making a little flat spiral coil as the diameter of a coin, for example 20mm, and cinnect it to the TX or a function generator. Measure how amended RX signal altering the distance from zero to 200mm.

In general, the numerical calculation that you do, is not convenient for analysis. The analysis is more informative when done with formulas rather than with practical numbers. But even if the analysis is made with formulas, it is not as understandable as an analysis in Frequency domain. The EMV that causes eddy current in target is proportional to the rate at which magnetic flux is changing, so instead of care from the magnetic induction B (calculated in your analysis), we should see the derivative dB/dt. Derivative in time domain means multiplication with frequency in FD.

But it turns out that this is not enough because the RX signal depends on derivative of eddy current, that means second derivative d2B/dt2 of TX current. The TGT signal from conductivity forms in FD as multiplication with frequency squared. This is shown visually in another thread:

http://www.geotech1.com/forums/showt...399#post167399

It is seen from the illustration, that the linear change of excitating magnetic induction B (ramp function) does not create an RX signal because the second derivative d2B/dt2 is zero.

Tinkerer, let we think when d2B/dt2 is small!
When eddy currents subside more slowly. Slow decay happens when the metal has less resistance, ie it has large conductance: silver, copper, aluminum, gold.
A large time constant means weaker signal in the high frequency region. So when Mr Green seek a coin with diameter as his TX loop, it is better he to build an ELF metal detector. This is because of S/N ratio.

The GND signal at conductive soil is relatively strong in the high frequency part of the spectrum because the soil has low conductivity (high resistance). In this case, eddy currents in earth deay more quickly, which means that the d2B/dt2 is high. Therefore, when we reduce the TX frequency (or increase sample delay at PI), the S/N ratio increases because the GND signal decreases more quickly.

If this thread where started by a radio amateur, the title will be "S/N RATIO" instead "SIGNAL STRENGTH".

[Tinkerer]

Hi Tinkerer, are you able to post a couple of CRO shots with and without the clay bricks. So we can all see what sort of response they produce.

Hi mickstv,

Good question.
Let's look at the realm of GROUND BALANCE or GB.
The GB is such an important matter that I will start a new thread for it. There I will post the screen shots.

Tinkerer

[green]

What I know about metal detectors is what I've searched on the internet and a little experimenting. What I say is what I think not what I know. Detecting targets in the ground is more important than in the air. The ground has a big effect, how much I don't know. The signal strength at the diameter was target at a distance of 1 diameter was looking for a ratio. I have a hard time measuring it and was wondering what it should be. I think I need a ratio of signal to ground to determine if the detector will work. Example
dime at one inch/ground at one inch. Maybe Carl limiting preamp gain to 100 gives an answer. Bricks are mentioned as a target. I think I would still need to know brick signal amplitude/ground signal amplitude. I would guess the ground varies a lot but would hope there might be a bad but not worse number for different ground types. Maybe 1/2 preamp output with gain = 100. Just some thoughts, would appreciate correcting my thinking.
green

[Overtheedge]

If this thread where started by a radio amateur, the title will be "S/N RATIO" instead "SIGNAL STRENGTH".

Which then makes me ask the question that comes out of a weak-signal VHF ham mind.
In order to improve the S/N ratio, we often tighten up the bandwidth. Many of the digital modes use a computer sound card to extract intelligence from way below the audible S/N ratio. In VHF and up, we provide maybe 10dB pre-amp, then filter tight before further signal processing.

Weak signal techniques might need to be applied at all frequencies. Use just enough pre-amplification to pull the signal out of the mud and tighten the bandwidth early in the signal processing flow.

Or maybe I don't grasp something.
eric

[mikebg]

Eric, you don't grasp that in practice there is no noise. The sensitivity of a conventional metal detector is not limited by noise, but by a large AIR&GND signal which makes TGT signal as modulation with very small index. We can't increase enough the gain of preamp because the AIR&GND signal will saturate it. To increase the gain, we should suppress the AIR&GND signal in input.

[Overtheedge]

That was what I kinda thought, but I read the word S/N (signal to noise) ratio and I think noise. 40+ years in RF communications (job and/or hobby) and I tend to get stuck thinking in those terms. So it is more like a DX ham operator trying to pull another DX op out of a dogpile.

It would seem the spherics would be easy to minimize. Emphasis on "seem". There is basically no difference in signal strength or phase between 5cm above the ground and 1m above for spherics.

But it appears I have a lot more to learn so I need to read more and comment less.
eric

[Carl-NC]

Eric, you don't grasp that in practice there is no noise. The sensitivity of a conventional metal detector is not limited by noise, but by a large AIR&GND signal which makes TGT signal as modulation with very small index. We can't increase enough the gain of preamp because the AIR&GND signal will saturate it. To increase the gain, we should suppress the AIR&GND signal in input.

In most of the designs I'm involved with, AIR+GND isn't the problem; the preamp(s) is not close to saturation, even with a fairly strong ground. We could increase the preamp gain, but typically the chatter gets so bad it is unusable. The same is true in trying to apply more post-demod gain, too much chatter. So, in my experience, it is mostly a noise problem.

[golfnut]

I read the word S/N (signal to noise) ratio and I think noise. 40+ years in RF communications (job and/or hobby) and I tend to get stuck thinking in those terms.

Eric,
These are generally fairly simple zero IF receivers. The IF filters are often bandpass, centered at about 10Hz, their BW is limited in attempt to reduce noise as you say.
The signals come though, post mix, at low Hz due to the sweep speed over the ground, fast sweep long shaft - 12Hz? - No motion - mixed signal comes through at 0Hz with the image at DC.
Medium sweep maybe 5 - 8Hz.


The main problem is that the ground signal comes though in the IF filter passband and appears as a co-channel type of signal.



Most IB VLFs have 2 Rx channels,

GEB - CH is phase correlated with the gnd signal - with 180deg offset in mixer LO signal so that 1/2 of detected geb signal is above 0v and 1/2 below 0 axis in order to average the gnd to zero
( this CH detects target alloys too, but to lesser extent)

Disc CH is set to max the sens on the expected target alloy - by opening up the mixer at the time - wanted alloys should be there
( it will detect gnd signal too, but to a lesser extent, as not phase correlated with gnd signals)



Both Rx CH outputs are amplitude compared to a common reference. If and only if both CH outputs are above the common ref you get a tone.



The weakness is the GND CH, it needs to have almost nothing coming out of it while there is no target -Perfect GND null, in order that a small target will give a small output above the threshold. The Disc CH will give a bigger output and will easily trip the common threshold.


Perfect GND balance is possible with a manual GND tune but only for The ground where the balance was done - as you walk away the GND signal will grow (as different ground) the level out will grow and trip the threshold and false. The user is left with having to either raise the trip threshold or retune GND bal.


With a quality auto gnd system on a mid or top machine the GND balance is done for you maybe hundreds of times per sweep. So this GND co channel signal is very low almost all of the time - this leaves the user to set a very low alloy trip threshold. So now, tiny real alloy targets are above the very low level GND residue - and allow a real alloy to trip at these low levels - more sens and depth for the same Tx power..


In the summer I fell foul of the need to regularly tune out the GND signal as best you can.

I was on a field in UK - naturally iron rich (ironstone/red soil/furnace slag) - I did not reset my GEB properly one time.

After not getting much, I dug in a regular brass Yale door key- I didnt get it at 3 1/2" ! ! !

After resetting the GEB best I could, I buried the key - flat - at 9 1/2" and I could get it on a regular sweep. In this condition - noise and preamp noise etc then becomes significant - albiet to a much lesser extent. With a normal radio these things are top drawer.

S

[Overtheedge]

Think I've got it now. Not really noise but rather co-channel interference from the ground signal the MD induced to re-radiate.

An analogy might be standing and shouting on a canyon rim and hoping to use the echo to determine the distance to a spire midway across the canyon. Too many extra echoes masking the desired echo.

By zero IF receiver, I'm assuming you mean Direct Conversion (DC) receiver centered on the oscillator frequency. The frequency conversion stage output being bandwidth limited to reduce noise prior to further signal processing. And then we are faced with filter Q realities. Wow.

And ground is not homogenous so a lot of different decay rates, many clustered but with some outlayers and still find the needle in the haystack just because it is more conductive. Wow. The ground return is like the spray paint hiding what is underneath. The heavier the coat, the more hidden it is until it can't be found except by excavation.

This MD theory and practice is a whole different animal completely. Between ITMD and this forum, I should get it figured out.
eric