Modular Metal Detector

[Altra]

A better close up

[Altra]

All the micros I use now have motor control HSPWM modules. These work very well for vco audio. I'll try an h-bridge.

Found this file on my hard drive. Don't remember the source. It has a nice h-bridge for cw. These mosfet driver IC's (IR2104) will not work on PI because of the isolation diode going to coil on the highside n-fet. The bootstap circuit in the driver needs to be connected to the source and load. If anyone knows a way to make it work ?

[Carl-NC]

Shoot through on the transitions can be a problem. Ideally use a micro with motor drive timers that have adjustable dead time, then you can tweak it.

[Altra]

In a discrete single voltage design what's the most important factor? Cross conduction? I been using the ML Sovereign PN h-bridge for a while. It works but generates a lot of ringing and noise on the transitions. Too bad these motor drivers seem to have issues.

[Carl-NC]

So far, a discrete H-bridge.

[Altra]

Here is the VLF transmit module:

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.

Hi Carl,
Did you ever find a solution for the jitter?
Thanks

[Daves]

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

[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]

[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 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 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 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

[Daves]

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

[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

[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.