Learning about how the DD coil works (to eventually build my own detector driver)

[nickw]

I am going to build a custom multi-frequency VLF metal detector, with a driver and DSP chip and all that, for scuba diving. I'd like to use an off-the-shelf coil that has good accuracy and discrimination, so I got a Garret AT Pro DD 8x11. Please respond with appropriate scorn if the Garret was a bad choice. I chose a commercial coil so that I didn't need to worry about the black art of making a good one.

First step is to characterize this coil so I can select the right amplifiers and filters and things. Specifically I need to know its frequency response and how much gain I need to put on the RX amplifiers. The Garret AT Pro 8x11 coil is about 35 ohms and 2.4mH on the Tx coil, and about 4 ohms and 1.6mH on the Rx coil.

How I planned to characterize the coil: put a 3-40khz, 10V sine into the TX coil with a 50 Ohm function generator, and use a good oscilloscope with 1x leads to measure the signal out of the Rx coil in the presence of metal.

How I thought it would work: I would see a millivolt level signal from the Rx coil that changed amplitude and phase when some metal was a few feet to a few inches away.

What I saw: a millivolt level (like 10mVpp) signal with magnitude and phase that changes like I thought it would. It does detect, but only when the metal is very close. The metal needs to be right up next to the detector or it doesn't affect the output of the Rx signal.

According to reviews, the Garret AT with the 8x11 coil can detect small coins 30+ inches away.

• I may have done my measurements wrong or perhaps Garret uses the coil in a different way that I thought.
• I may have Tx and Rx backwards and need a more powerful source to drive the Tx coil.
• The coil may need a resonant circuit on the Rx loop tuned to the specific frequency to create magnetic coupling between the loops

Does anyone have experience with the signals that the Garret detectors produce or how their coils are supposed to work? I'd appreciate any wisdom since I am starting at zero.

 

[Rudy]

I think the Tx & RX coils are backward?

 

[TorqueMaster]

Good luck with your experiments. All I can say is -- our modern detectors with that size coil have a usable range closer to 8-10 inches on coins, not 30. Maybe it was 30 cm?

There is/was a website/forum dedicated to tinkering at the hardware level as you are, but I can't find the bookmark I made a long time ago. Hopefully someone else knows what I'm talking about, and can post it.

 

[Cherry Picker]

Sounds good.

Speaking of coils. I've had a concept coil in my thoughts for some time. I call it the Pulse Coil. It runs in all-metal mode, you are able to choose from a DD or concentric detecting pattern. The pulse comes from the idea that you don't swing the coil but it sends out pulses, which you control. A slow pulse would go deeper and a faster pulse would give more accurate target ID'ing. Frequency would also be adjustable. Not just a few to choose from, but adjustable.

Since it would be a non-motion detector it would pulse scan a target 100s of times a second, and through AI, be able to give a far more accurate ID. The detection pattern could be adjusted from the full size of the coil down to perhaps a few inches for best separation in the trash.