Practical or differential integrator?

[eduardo1979]

Hello everyone.
I know this topic has been discussed several times before, but after reading the posts, I've noticed that not everyone agrees, and that's causing me some doubts. I would like to ask this question again to get the opinions of the experts and finally have my doubts resolved. I also read about this topic in the ITMD3 book, but my doubts still remain. I am developing a pulse induction (PI) detector for finding very small gold nuggets, and for this, I need a very fast detector with effective earth field elimination (EFE). My question is whether it's better to use a simple integrator or a differential integrator? I know that a simple integrator requires an inverting amplifier after the preamplifier, just before the demodulators. I also know that the capacitors and resistors in the integrator must be matched in pairs with a tolerance of 1%. Again, the detector is for finding very small gold nuggets.
The cost of the operational amplifiers doesn't matter. I have no problem investing in high-quality electronic components. I just want to build a good metal detector, because that's something I'm very passionate about.
Thank you in advance.

[Carl-NC]

The difference between them is probably not measurable. I prefer the simple integrator because fewer matched caps makes it more manufacturable.

[Detectorist#1]

Hi eduardo1979,
See my posts in this topic "https://www.geotech1.com/forums/forum/technology/pi-technology/419502-one-more-variant-to-create-pi-md-with-reliable-fe-reject"
This is my first attempt to make PI MD very sensitive to small targets and with good EFE and GB. The two RX coils are connected as "gradiometer" and in "Fe" mode the detector reacts ONLY on Fe targets. In "Fe" mode, the detector works un passive mode - no TX pulses. Have in mind that the sensitivity for Fe targets in this mode is reduced in comparison with "ALL" mode. I hope, some ideas from this project will be useful for you.

[Detectorist#1]

Hi eduardo1979,
It will be interesting to use "Super DD" coil as for GPZ-7000 detector, but with gradiometric (non in-phase) connection of the two RX coils. This will allow further reduction of the "Minimal delay" and to increase sensitivity for small targets!

[eduardo1979]

Hello detectorist1.
Thank you very much for your suggestions. Your project seems quite interesting and very complex. I have never built a metal detector based on a magnetometer-gradiometer, so I have no experience with those types of detectors. My question was only about the type of integrator I should use in my project. I was wondering whether to use a simple integrator or a differential integrator. I am working on a pretty interesting PI project, and I had this question, which Carl answered for me, and I would like to thank him for that. My PI has ground balance, two audio tones, and conductivity discrimination (HI-LOW); nothing too sophisticated, for now. All of this was made possible thanks to the great help from the ITMD3 book and the forum members.

[Detectorist#1]

Hi eduardo1979,
It is possible to assembly more simple version of this MD without PCB inside the search coils and with "super DD" balanced coil. If you find ready case for "super DD" coil, the task will be more more simpler. Soon, I will post the scheme for this more simple solution. Concerning integrators, I agree with Carl - no practical difference in results between singe and differential integrator if time-constants are similar and suitable for the size of the coils and the speed of moving of the coils.

[eduardo1979]

Thank you for your valuable contribution to the metal detector community. I'll be looking forward to the publication of your simplified schematic so I can analyze the different components in detail and understand how it works. For now, I'll continue working on the PCB design for my own detector, since I started it last week and I'd like to finish it and test it as soon as possible.

[Detectorist#1]

Hi eduardo1979,
I wish you success with your current design pointed to find small targets. As you know, this task is not easy - when the solution is sensitive for small targets, the PI MD is very sensitive to ground features. Also, PI detectors without complex software treatment haves very long recovery time after strong target's signal. This eliminates recovery of the small targets in trash areas. It will be good if you post the scheme of your PI MD that you build now. We will have possibility to help to you with some comments.

[Detectorist#1]

Hi eduardo1979,
When you design PI MD for very small targets, have in mind that the most sensitive sensors in electronics uses differential input connection with two RX convertors from an input physical quantity to an electrical signal. This again points that using of "Super DD" configuration of the coils have to be used when your target is to design very sensitive PI MD for very small targets while also having good sensitivity to medium-sized targets at a reasonable distance of one to two feet. Some pin-pointers haves good sensitivity for one grain targets at distance of one inch, but they aren't capable to discover medium-sized targets at distance of one or two feet.

[eduardo1979]

Hello Detectorist1.
I'm using a 4-layer PCB and not a differential input. I'm using a single-ended preamplifier and a mono coil from the GPX series. Right now, I'm comparing the results of my self-built detector with the Whites TDI SL. The results are very similar; the only thing I've noticed is that the MOSFET in my detector gets hotter than the one in the TDI, and I don't know why. I'm using an ESP32 powered at 3.3V, and the TDI uses a PIC microcontroller powered at 5V. The gate driver for my MOSFET is the same as the one in the TDI SL. I'm still investigating this issue.

[Altra]

HI Eduardo are you using a 2 ohm resistor like the TDI in series with the Tx coil?

[eduardo1979]

Hi Altra.
I also tried using the original Dual Field coil from the TDI. The problem remains the same. The MOSFET still gets slightly less hot than with the GPX coil, but it still heats up.

[Altra]

TDI's used a series resistor to limit the the Tx current to coil. Like this

[eduardo1979]

Hi Altra.
I'm using the same configuration as the TDI SL. I'm not using a resistor in series with the coil, as in the original TDI.
The truth is, I don't understand why the MOSFET in my detector gets hotter than the MOSFET in the TDI SL, even though I'm using the same gate driver, the same frequency (2.8kHz +/-), and the same pulse width (100µs). Perhaps Carl could help me.

[Carl-NC]

Does your MOSFET have a heat sink?