Transmit amplification mod

[golfnut]

simple IB geb CH sampling can only average out a single phase. As soil has a range of phases, So the background can only be reduced by a large percentage, but there is always some there. So user cant drop the sens level Really low - otherwise it would beep at ground.

With dsp techniques it should be possible to look at the range of gnd signal phases and to average them all down. Real target threshold could then be correspondingly lowered.


Or another approach may be to allow the GND signal to come thru on 2 channels - and fed into a diff amp. So the gnd signal ( with a range of phases ) would dissappear.

To get a real amplitude delta the Rx CH's could be on different frequencies, then almost all targets would give a different amplitude resonse and the diff amp would catch it and beep.

[Davor]

Most of the falses are simple 1/f noise (drift), and with proper proportional tone generation the ground that creeps in a bit is not a problem. In PI you have "EF" channel that greatly reduces 1/f noise. Most people believe it only removes earth field, while in fact it acts as auto-zero chopper amp that reduces 1/f drift as well.

@Sean_Goddard, my case against use of DSP-s is their immense power thirst. Also their vanishing nature - as soon as you get used to a nice micro - it disappears.
BTW, Aziz is banned from this forum, which is a great loss.

[Funfinder]

Of course powerful field strenghts are important to penetrete the soil - there is absolutly no doubt -
with too weak x-rays machines also no doctor could work - this is the same structure with tighter material.

Even clouds already start to weaken EM-waves - water-dust.

Creating strong signals is not the problem, also not because of the additional weight for batteries -
the point is how to receive attenuated on a very stable basis those fields and analyze them
on a very highly powerful contrast basis.

Seen from the signal strenght its like listening to a radio program directly near the
transmition tower and detect it if the signal has 0,001% changed because a bird
was flying inbetween tower and receiver.

And if detection depths of 5 meters with a 30cm DD coil should be reachable
it will need such high powerful signals incl. ultra-sensitive analyze-circuits.

And it doesn't matter if the soil gets "saturated" by the EM-field - it must be strong
enough so it goes through it like through butter until a certain distance.

Of course the more mineralization, the higher the "field-pressure" will be
and if the find is so small or rusty so it melts with its surrounding soil it
needs either special (small) coils or even the highest stable detection circuit
won't see enough contrast within the embedded EM-field-structure-situation.

The main reason so far for amplification would be if RX and TX coils are apart (20 meters etc. distance).
Otherwise the detector would receive no longer any signal at all if the TX signal is "normal" weak.
With such a configuration of course a car or tank or large metal-construction will be detectable
10meters or further away if it gets inbetween RX and TX. But only if there is enough TX power.

For usual DD coils the situation is different and very strong EM-fields also must not overload
the RX-stage or other parts of the detector.


btw. wirelessguy simple inserting some stand alone amplification stage before the coil
like for amateur-radio is no good idea - it will completly overload the receiver-and analyze circuits
with a high change to destroy it. You need some attenuators and even then its not shure if
it works because the amp may heavily distorts the original coil EM-waves.
You can experiment with some very weak amps (5-50 mW) but there is no guarantee
if it improves depth or if it destroys anything.

[golfnut]

You can have additional amp in front end - you just need a real time AGC circuit.

As you sweep a plough blade the gain is slashed, this way nothing down wind saturates.


The benefit is you keep the target phase dexterity
The recovery speed is maintained

[Funfinder]

Gain is a good point, as long as it matches with the complete contrast-spectrum.

I found out that it doesn't need a large coil to reach extreme depth as long as all the
components (analyze circuit, cable, frequency-transformer!, coil, coil-housing) are really ultra stable.

[Sean_Goddard]

I've seen military radio which can resolve signals down to -127dBm that's effectively at Earth Noise levels, and sonar in submarines which can ear and identify the type of ship leaving harbour in New york whilst the sub was off the Irish coast!! So when simple target characterisation is all you require, 20th order filters implemented in a few lines of code.

I love the Blackfin DSP's and they are around 10 years old now. Low power, multi core, graphical programming available etc.

Why was Aziz banned, anyone?

[Qiaozhi]

Why was Aziz banned, anyone?

http://www.geotech1.com/forums/showt...ziz#post193990

[kt315]

What am I missing?

a copy INSIDE THE METAL DETECTOR. http://www.geotech1.com/forums/forum...September-2012

[Funfinder]

I've seen military radio which can resolve signals down to -127dBm that's effectively at Earth Noise levels, and sonar in submarines which can ear and identify the type of ship leaving harbour in New york whilst the sub was off the Irish coast!! So when simple target characterisation is all you require, 20th order filters implemented in a few lines of code.

I love the Blackfin DSP's and they are around 10 years old now. Low power, multi core, graphical programming available etc.

Why was Aziz banned, anyone?

Good point to think deeper about it.
So far I'm not shure if we can compare those two things.

btw. I meant small but still very high energy radiating and receiving coils.
Compare it with a good enough X-ray camera that also needs a good amount of power.

Receivers on "earth noise level" are like some of those passive LRLs - extremly unreliable.


What is important:
The electro-magnetic "beam" has to be concentrated like a small stripe for scanning the ground ultra-deep.
Thats why a row with 4 or 6 small but still very stable and powerful coils would be better than
some huge 1m coil.

And some computer-software would be needed to analyze the different and all the time changing energy-levels
for creating a good readable output.

[Gwil]

Davor and Qiaozhi have said why Tx noise considerations mean that simply increasing Tx output more and more doesn't help. On the other hand, the less Rx gain you need the better. That leaves the question of how much output can be used in different kinds of ground, that is, how much is too much?
I tried to estimate the magnetomotive force produced by various detectors (I know mmf isn't the whole story, but it's a start). For example, the TGSL with a 6mH coil seems to give about 1.3 amp turns (peak). Some results are much higher - in this article

http://jb-ms.com/Baron/gb.htm

the legendary George Payne describes coils that would produce around 6.4 - 7.0 amp turns peak with typical drive voltages.
It seems that in some types of ground the Tx drive does need to be limited. Nautilus detectors had variable output power, and there was a Treasure Baron "Black Sand Module" that when installed reduced the coil current, although I don't know by how much. There is some information on this subject here:

http://compass-metal-detector-forum.548136.n2.nabble.com/Coil-field-strength-by-George-Payne-td6185238.html

Is it only black sand that causes a problem? In my part of the world (SE England) black sand, and the associated gold, is rare to non-existent as far as I know, so in principle can I increase Tx drive as much as I like?

[Qiaozhi]

Let's consider an analogy ... If you are driving in the fog at night (with dipped headlights) and it's difficult to see very far ahead; putting your headlights on full beam doesn't help one bit. All that happens is that you get dazzled by the light reflected from the fog. However, if you switch back to dipped lights, and turn on your fog lights, then things become clearer. Much the same thing happens with a metal detector in trashy or highly mineralized ground. Increasing the TX power doesn't help you see the good targets any better, because the signal from unwanted targets makes it difficult to detect anything amongst the clutter. You need to find a "fog light" solution.

There are times when a higher TX power can be useful - and that's when the ground has very low (or no) mineralization, and minimal trash. In that case, the extra TX power can help you to go deeper. One solution is to provide a Boost switch, which can only be used in certain special cases. A similar thing happens when users turn the Sensitivity control on full, and then complain about excess chatter from the trash. Simply turn it down. Yes, there will be a loss of depth, but the ground conditions may not be suitable. There have been many "iron see-through" claims made by some manufactures, but it's only smoke and mirrors. You need the right settings to suit the conditions.

[sinclairuser]

Davor and Qiaozhi have said why Tx noise considerations mean that simply increasing Tx output more and more doesn't help. On the other hand, the less Rx gain you need the better. That leaves the question of how much output can be used in different kinds of ground, that is, how much is too much?
I tried to estimate the magnetomotive force produced by various detectors (I know mmf isn't the whole story, but it's a start). For example, the TGSL with a 6mH coil seems to give about 1.3 amp turns (peak). Some results are much higher - in this article

http://jb-ms.com/Baron/gb.htm

the legendary George Payne describes coils that would produce around 6.4 - 7.0 amp turns peak with typical drive voltages.
It seems that in some types of ground the Tx drive does need to be limited. Nautilus detectors had variable output power, and there was a Treasure Baron "Black Sand Module" that when installed reduced the coil current, although I don't know by how much. There is some information on this subject here:

http://compass-metal-detector-forum.548136.n2.nabble.com/Coil-field-strength-by-George-Payne-td6185238.html

Is it only black sand that causes a problem? In my part of the world (SE England) black sand, and the associated gold, is rare to non-existent as far as I know, so in principle can I increase Tx drive as much as I like?

hi gwill, dont believe the hype, some cornish beaches and river silts have plenty of black sand, and some gold particles but mostly black sand.

[Gwil]

Thanks for that, but unless there has been a major tectonic event that I didn't notice, Cornwall is not actually in the south-east. Interesting though.