signal strength

[mikebg]

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.

That means you use a primitive block diagram because:

1. Your preamp operates without AGC to maintain gain near to saturation. Then you can't use maximal possible gain and maximal sensitivity to small and deep targets,

2. Your TX has no P-I control to suppress amplitude modulation when TX coil moves. Then your RX receives a modulated by ground AIR signal, that is awful, and

3. There is no automatic balance in input of your RX. Then it cant operate at maximal possible sensitivity.
IMNHO, if this is true, you can't see noise and even interference in preamp output.

I posted in the forum images of preamp noise at wideband and narrow band preamps when AGC maintains maximal sensitivity.

http://www.geotech1.com/forums/showt...898#post165898

http://www.geotech1.com/forums/showt...545#post167545

You really achieved such output?

[Qiaozhi]

That means you use a primitive block diagram because:

1. Your preamp operates without AGC to maintain gain near to saturation. Then you can't use maximal possible gain and maximal sensitivity to small and deep targets,

2. Your TX has no P-I control to suppress amplitude modulation when TX coil moves. Then your RX receives a modulated by ground AIR signal, that is awful, and

3. There is no automatic balance in input of your RX. Then it cant operate at maximal possible sensitivity.
IMNHO, if this is true, you can't see noise and even interference in preamp output.

I posted in the forum images of preamp noise at wideband and narrow band preamps when AGC maintains maximal sensitivity.

http://www.geotech1.com/forums/showt...898#post165898

http://www.geotech1.com/forums/showt...545#post167545

You really achieved such output?

Please post the full schematic of a PI detector that you have personally designed using these theoretical techniques, and that has been successfully tested in the field.
We are waiting .....

[Davor]

Mike, you can't push only a single side of a story just because it works somewhere else. Metal detectors operate over somewhat larger dynamic span than short wave radio does, so perhaps some broadening of perspective might be in order.

You often propose AGC as a solution to all problems, yet, what kind of AGC you think about is not entirely clear. Most of the AGCs as seen in Ham solutions choke on such strong signal you encounter in metal detectors. There are a few possible options that could cope but one more exotic than the other one, using vactrols, digital potentiometers and what not. And to what end?

When using a metal detector, most of the time there is no target to observe, so the system should graze right on the noise floor anyway. Once the target comes, and boy it comes sharply in, your system must be ready for the full span, only to recover ASAP to the noise grazing state. No AGC could ever cope like that. There are attack and release times attached to the AGC concept that would render your AGC-equipped rig useless. 200ms release time? You miss two targets with that.

Shortening attack and release times to zero you obtain a simple clipper. Well hello to the metal detecting technology!

We are dealing with signals that come with such steep power law transfer characteristic you simply don't want to convert them into sound in their raw form. You need to compress them, one way or another. Binary sound is perhaps the worst form of it. Proportional sound is a somewhat compressed form of the target response turned into sound and it sounds immensely better. VCO also, within reason.

There is one technology you probably missed in Ham radio world that comes to mind, a SSB voice compressor. You have a SSB modulator producing SSB at some low-ish frequency, say 455kHz, followed by a clipper (!), followed by a simple filter and a demodulator. Albeit crude compression is involved in such compression, intelligibility of such voice is good, spectrally all the spikes are there as in the input voice signal, but oddly equalised in amplitude - the voice signal is severely compressed, and surprise! there is no AGC to be seen.

Whole point is that if you render an AGC to a clipper with no attack and release times - you get what we already have. Even the SSB IF in form of a zero IF of the metal detectors.

[Carl-NC]

Speaking from a VLF point-of-view, AGC has very limited use. We're dealing with a small target signal on top of a potentially large ground signal, so there is limited headroom to AGC the gain up higher. And, as Davor points out, you would need a fast-attack AGC to avoid instant saturation, and even then you are likely to get AM-to-PM modulation that screws up the TID. If the AGC has discrete gain steps, you will also get signal discontinuities that screw things up. This is one of those Good Ideas that doesn't quite work out when you actually build it.

What you really want is a way to null the ground signal. Literally remove it at the coil, so that the preamp never sees it. This is also fraught with pitfalls and is (so far) a Good Idea that doesn't quite work out when you actually build it.

[Mark-VA]

Are you bring the new unit to the ******* this week?

Depends on the design. My current design has a 2-stage preamp, each 20dB. Any more than that screws things up.

[Carl-NC]

That was the plan, but today I completely cooked my receiver board. O.E. Now in the process of tediously converting another board.

[6666]

My current design has a 2-stage preamp, each 20dB.

Is that the magic number Carl ?

[mikebg]

Please post the full schematic of a PI detector that you have personally designed using these theoretical techniques, and that has been successfully tested in the field.
We are waiting .....

George, it is time to use term WIDEBAND or BROADBAND instead PI detector. Even MINELAB makes this.
I answered to the above your question two years ago:
http://www.geotech1.com/forums/showt...428#post116428

That means CHINELAB Unlimited had enough time to test this:
http://www.geotech1.com/forums/showt...783#post101783
http://www.geotech1.com/forums/showt...445#post106445

Moreover, the CHINELAB Unlimited knows the reinvented block diagram of the most sensitive metal detector:
http://www.geotech1.com/forums/showt...264#post160264

If you send me via email this PDF file, which costs only $ 2, I'll send you more details:
http://www.geotech1.com/forums/showt...343#post167343

[Qiaozhi]

George, it is time to use term WIDEBAND or BROADBAND instead PI detector. Even MINELAB makes this.
I answered to the above your question two years ago:
http://www.geotech1.com/forums/showt...428#post116428

And on the 30th August 2010 I replied:

As many enthusiastic designers have discovered, with metal detectors there can be a large gap between the initial theory and what happens when you try to put that theory into practice. Your learning experience will be improved if you try building something.
http://www.geotech1.com/forums/showt...442#post116442

Obviously, nothing has changed.

[deemon]

And on the 30th August 2010 I replied:

As many enthusiastic designers have discovered, with metal detectors there can be a large gap between the initial theory and what happens when you try to put that theory into practice. Your learning experience will be improved if you try building something.
http://www.geotech1.com/forums/showt...442#post116442

Obviously, nothing has changed.

But on the other hand , one good scientist said - "Nothing is more practical than a good theory"

[Gwil]

If I may contribute another quotation:

"There is no sadder sight in the world, than to see a beautiful theory killed by brutal fact."

-- Thomas Huxley, biologist (1825-1895)

[bklein]

Are you bring the new unit to the ******* this week?

And what is ******?

[green]

Prospector Al made a comment in a thread I was reading. If the detector doesn't detect a nickel at 15 or 16 inches with a 11 inch coil put it back on the shelf or sell it on ebay. Might have remembered the diameter wrong. When building or buying a metal detector what are some non field tests a person should perform. How much would the distance change for different size and type coils for the nickel test.