Pulse induction metal detectors.Is more than 1 pulse necessary?

The signal due to the target object will be very very much smaller than the signal due to the transmit pulse. I think a typical oscilloscope might not be able to handle the overload of the transmit pulse without messing up its behaviour for a long time so that it could never see the tiny target signal. I think you will need a good electronic (MOSFET?) switch to disconnect the coil from the receiver during the transmit pulse, and then the receiver will need very high gain, quite likely more than an oscilloscope would normally provide, so you may need a pre-amplifier.

There may be a benefit in averaging the echoes from multiple transmit pulses, as a way to reduce the effect of interference and noise, especially if you test it near or in a building containing electrical wiring and appliances. Really you probably ought to test it outdoors, far from any building or cable.

There is a wealth of fascinating information on metal detectors on youtube. For example, search using the query

1. You will find two main types, Concentric Coil vs Double D: Metal Detecting 101 - DD vs Concentric Coils
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2. You can make your own detector. Here is a Double D version: DIY DD metal detector 1 - first success
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3. This compares the performance of a Concentric Coil and Double D:

1. Some detectors can discriminate between different metals: Metal Detector Discrimination Explained (with a Garret AT Pro)
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2. The coil frequency affects the results: Metal detecting frequency tests. Do they make a difference? Some interesting results
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I did some research and found the technique is called "Pulse Induction", or PI. It is a very complex tecnique and relies on the detection of microvolt level signals immediately after a strong transmit pulse. As you mentioned, it is very sensitive to external low frequency noise, such as atmospheric noise, 60 Hz radiation from power lines, radiated noise from flourescent tube lighting circuits, etc.

There is a wealth of information available. Here are some selected articles:

1. KBA_METAL_DETECTOR_BASICS_&_THEORY.pdf
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2. Building the Geotech Baracuda (Rev-A).pdf
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3. TA5.pdf
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4. Pulse Induction Metal Detector Circuit | element14 | Sensors
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5. TreasureSweeper – For the Metal Detecting Enthusiast
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6. Pulse induction or VLF detectors
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7. DIY Mono Coil for Pulse Induction Metal Detector
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8. Field Coil DIY project for PI Detector
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9. Mirage Pulse Induction Metal Detector DIY steps 1, 2 and 3
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In data luglio 2021 alle ore 08:15:57, Steve Wilson snipped-for-privacy@not.com ha scritto:

Thanks for your hints,links and for having to do with my poor english ,

going on with patience I just found that effect of eddy currents(or something after current pulses) are visible on oscilloscope(2 mV/div)! That was possible only arranging the 2 circular coils in a double D fashion,say as olympic rings. Before reading anything about the double D arrangement, it's advantages became evident: if the two coils centers are exactly aligned the effect of current pulse appears huge on 2nd coil,say it has positive sign,anything right after this huge pulse is hidden by ringing; if the coils are side by side the sign of the pulse on 2nd coil changes,there is still big ringing ; moving again the centers toward alignment,the negative pulse decreases,and when the entering field equals the outgoing field, the pulse disappears,so does the ringing,and effect of eddy currents appears from noise. In 2021 i just noticed what engineers probably got clear in the 50's, on tube oscilloscopes

You may have found a new method of PI. None of the references I found mention double D coils. Please continue updating us on your progress.

In data luglio 2021 alle ore 07:18:49, Steve Wilson snipped-for-privacy@not.com ha scritto:

It sound very strange to me,when i started gathering informations I met soon lot of references to double D coils.Anyway,I'm not shure that the only reason is suppression of the exciting magnetic field.