Modular Metal Detector

[Carl-NC]

Here is a simplified diagram of the MMD system:



It is simplified in that it does not include the "ground" signals or the module control/clock inputs. There is a transmit module for driving a single TX coil. There can be (2) RX coils and (2) preamp modules; either preamp can be connected to either RX coil, so these are possible:

• RXA drives both PreampA and PreampB
• RXB drives both PreampA and PreampB
• RXA drives PreampA, RXB drives PreampB
• RXB drives PreampA, RXA drives PreampB

There are (4) complete X/R channels, each channel has a demod module and a filter/gain module. Each channel can be selected to read from either PreampA or PreampB. The X&R outputs of the filter modules are routed to a header to feed an off-board ADC.

The purpose of the MMD design is to compare different preamp, demod, and filter topologies. For example, you could use RXA and PreampA to drive 4 different demod designs and look at all 4 at the same time, to compare performances.Or, you could run it as a multifrequency design with up to 4 frequencies.

[Carl-NC]

Here is a schematic of the motherboard:



You can see why I led with a block diagram, as the schematic is just a bunch of connectors with mostly label-based routing. Hard to read.
And here is the layout:



It's very simplistic, only 2 layers and most of the routing is for power. The selection options are intended to be 0.1" male pins with suitcase jumpers. Clock and control pins are simply routed to 0.1" headers next to the modules, with the intent that jumper wires are used to connect them to a micro eval board like an STM Nucleo.

Below are a schematic PDF and Gerber zip file for the layout.

MainBoard.pdf
MainBoard.zip

[Tinkerer]

Is this schematic for only PI RX preamp

This preamp schematic is specifically for the AMX PI.
The AMX PI is using a bi-polar square wave current TX. There are several versions of bipolar square current wave TX circuits on this forum. Like Moodz, King, Tinkerer. There might be several more I don't know of. The Minelab 7000 also uses a bi-polar square current wave TX.

[Carl-NC]

Here is the schematic for the power module:


It is a simple linear regulator (there are 2 chip options) followed by a rail splitter. There is also a reverse polarity protection NMOS transistor.
The layout is this:


Overall size is 25 x 37.5 mm.

Here is an alternative power module that uses a true ground and an inverting regulator:


Layout fits on the same size module:



PowerModule.pdf
PowerModule.zip
Power2Module.pdf
Power2Module.zip

[Daves]

Carl-NC can you share your Transmit, Demodulator and Filter designs?

[Carl-NC]

Here is the VLF transmit module:



This combines a sinusoidal oscillator (Q1, Q2, etc) with an H-bridge square wave driver (IC1). You would populate one or the other but not both. They are powered by a boost converter which can provide up to 30V.

A caveat with this design is that I recently found a flaw in the DRV8872, in that it has a 200ns jitter in the output transitions. This creates a large transition noise in the RX signal. Supposedly there are other H-bridge chips that have a different internal design that avoids this problem but I have not tried them yet. Meanwhile, I am designing a discrete H-bridge driver.

A TX module can also be designed for PI (either monopolar or bipolar) but I have not done that yet.

TX_VLF.pdf
TX_VLF.zip

[Carl-NC]

Here is the VLF Preamp module:




There are various options, like making the first stage inverting, non-inverting, or differential; parallel or series resonance; inverting or non-inverting second stage; or flipping the polarity of the output signals. Many of the components can either be SMT or thru-hole so that changing values is easy, especially if you use sockets. Note that none of the component values are real, they are all TBD.

PreampVLF.pdf
PreampVLF.zip

[Carl-NC]

Here is the PI Preamp module:




Same comments as with the VLF Preamp module. The PI includes flyback protection diodes and some switches for experimenting with switched isolation.

[ATTACH]n419761[/ATTACH]
[ATTACH]n419762[/ATTACH]

[Carl-NC]

Here is Demod module:



It can be configured as a SHA demod with 1, 2, or 3 pole filter; a sampling integrator type, including zero-droop feedback; and two types of switched-cap demods. Again, most values are TBD.

[ATTACH]n419770[/ATTACH]
[ATTACH]n419771[/ATTACH]

[Carl-NC]

Here is the Filter module:




This is the one module I have not put much thought into. It is intended to be a final gain stage with low-pass filtering before feeding the signals to the ADC, but on thing missing is a way to offset signals if needed. If you want to maintain analog processing then this module could be changed to do the first and second derivatives.

[ATTACH]n419775[/ATTACH]
[ATTACH]n419776[/ATTACH]

[Daves]

Carl-NC thank you, I will review it and return back with my questions.