AMX TX

[KingJL]

The superD coil assembly sounds very interesting. What kind of RX frontend did you use?

Tony, Currently experimenting with direct sampling using low noise differential input buffer with 26db gain and Audio DAC... a work in progress. First issue I had was a suitable coil cable... now that is solved

Did you use any ground compensation?

Only ground compensation is in cpu code... call it DSP if you want, but I call it preprocessing before DSP.

Is there any way to simulate the SuperD coil?

Probably, but that is beyond my capability... I find it difficult to simulate any IB coil setup with the simulator(s) that I have available.

Adding to the AFE mentioned above: I am following Carl's approach to the AFE and subsequent processing closely. I like the "current mode" configuration and I have always been a proponent of the AD797 (A few years ago, I actually modified my HH2 implementation to use a two stage preamp with a AD797 @ 12x as the first stage... I piggy backed 2 boards to implement the 2 stage preamp and supply the extra supply current that AD797 needed).

[Tinkerer]

@moodz,
I have actually built your circuit (using different components of course). I have been experimenting using a SupedD coil with 300uH @0.6 ohm TX (Rx are 375uH each). Current is maintained at 500 mA; TX voltage at 1.31V (higher than your 1.23, but probably due to slightly higher RDSon for the supply FETs that I used). Have tested with TX pulse trains of 20 KHz 10 KHz and 5 KHz. I have used no dead time in the TX signal train (which CAN in itself lead to "tilt"). I have experienced NO shoot-through issues and no tilt issues. What I love about the alt2hbridge circuit is the elegant simplicity and versatility of the circuit that does not require extraordinary efforts to overcome shortcomings present in other h-bridge circuits. If you stay with the relatively low current implementation (0.5 mA), issues like shoot through do not seem to appear (higher current devices also have higher gate capacitance, which affects switch on/off timings). There is more "bang for the buck" with a higher frequency TX train than higher TX current! And to minimize any effects of a "tilt" keep the low side resistance to an absolute minimum... that is where the losses occur... if you need resistance for current sense, put in the high side where it does not involve the coil discharge current! I am an advocate of improvement where there is actual realized "improvement"... I am also an advocate of "if it ain't broke... don't fix it"!

The superD coil assembly sounds very interesting. What kind of RX frontend did you use? Did you use any ground compensation?
Is there any way to simulate the SuperD coil?

[KingJL]

... so I checked the "original" proof of concept circuit from the museum of projects layin around and it works as advertised.

1.3 volts supply / 0.5 amps peak ( ie 1 amp swing ) / 520 volts peak / 1 microsecond flyback time.

The coil is a minelab 0.5ohm 300uH mono.

There is no current drawn with the coil disconnected so I am going to say that there is no shoot thru in the supply fets.

The mosfets are nothing special IRF840 for the coil fets and IRL1404 for the supply switches.


The CPU outputs a single TX pulse train at 15 Khz to a TC4428 mosfet driver chip that produces the inverting gate drive .. shown in the pic below.

Measuring the tilt ... couldn't get a reading of any tilt ... so I am going to say it dont matter.

PS .. if anyone can point out why tilt does matter AND point to a detector design ( not theories ) that uses tilt control I might reconsider that opinion ...

moodz

@moodz,
I have actually built your circuit (using different components of course). I have been experimenting using a SupedD coil with 300uH @0.6 ohm TX (Rx are 375uH each). Current is maintained at 500 mA; TX voltage at 1.31V (higher than your 1.23, but probably due to slightly higher RDSon for the supply FETs that I used). Have tested with TX pulse trains of 20 KHz 10 KHz and 5 KHz. I have used no dead time in the TX signal train (which CAN in itself lead to "tilt"). I have experienced NO shoot-through issues and no tilt issues. What I love about the alt2hbridge circuit is the elegant simplicity and versatility of the circuit that does not require extraordinary efforts to overcome shortcomings present in other h-bridge circuits. If you stay with the relatively low current implementation (0.5 mA), issues like shoot through do not seem to appear (higher current devices also have higher gate capacitance, which affects switch on/off timings). There is more "bang for the buck" with a higher frequency TX train than higher TX current! And to minimize any effects of a "tilt" keep the low side resistance to an absolute minimum... that is where the losses occur... if you need resistance for current sense, put in the high side where it does not involve the coil discharge current! I am an advocate of improvement where there is actual realized "improvement"... I am also an advocate of "if it ain't broke... don't fix it"!

[Willy Bayot]

Measuring the tilt ... couldn't get a reading of any tilt ... so I am going to say it dont matter.

PS .. if anyone can point out why tilt does matter AND point to a detector design ( not theories ) that uses tilt control I might reconsider that opinion ...

moodz

For more explanations, See post of Tony here:
https://www.geotech1.com/forums/foru...352#post411352

[green]

I added a coupling command, Tx to Rx with my spice circuit. Big change, tilt doesn't work. Removed Rx damping resistor, tilt worked but oscillates. Wanted to learn something, see I have alot to learn.

[Willy Bayot]

PS .. if anyone can point out why tilt does matter AND point to a detector design ( not theories ) that uses tilt control I might reconsider that opinion ...
r
Did you try to change the energy absorption of the coil by passing a large target over it?
This is not a theory,.
A tiny variation of tilt generated by this type of change generates in turn a large signal offset change after all the receiver stages.
The variations of XMIT current tilt are not really disturbing but the large variations of signal offset at the receiver end are really disturbing the DSP.
Everything was OK while testing on the bench.
It is a surprise we got a few months ago on our real square wave project as soon as we went over a real ground.
It is not even when there is a large target going under the coil, it is also happening while the coil is swinging with its slight variations of height over the ground.
This behaviour is specific to the square wave system and does not appear on traditional triangular wave systems.

Now, if you still want to ignore this fact, go ahead, you will have the same surprise as us!!!

[ivconic]

I'm curious about Carl's comment on that...
...
"...TC4428 mosfet driver chip that produces the inverting gate drive..."
Is it relevant at all? Inverting or not?
I just checked with the local suppliers, the IR2103 is currently the only driver that can be found, albeit at a ridiculous price.
It has high and low sides. In this case, the low side would be used.
But I wonder; why use of driver?
Can't logic level fet be used without driver?
IRLZ44NN comes to mind, I have those...
I worked with them and made constant current regulators and powered them with a signal directly from the Atmega...

[moodz]

... so I checked the "original" proof of concept circuit from the museum of projects layin around and it works as advertised.

1.3 volts supply / 0.5 amps peak ( ie 1 amp swing ) / 520 volts peak / 1 microsecond flyback time.

The coil is a minelab 0.5ohm 300uH mono.

There is no current drawn with the coil disconnected so I am going to say that there is no shoot thru in the supply fets.

The mosfets are nothing special IRF840 for the coil fets and IRL1404 for the supply switches.


The CPU outputs a single TX pulse train at 15 Khz to a TC4428 mosfet driver chip that produces the inverting gate drive .. shown in the pic below.

Measuring the tilt ... couldn't get a reading of any tilt ... so I am going to say it dont matter.

PS .. if anyone can point out why tilt does matter AND point to a detector design ( not theories ) that uses tilt control I might reconsider that opinion ...

moodz

[ivconic]

At least 3 samples per response cycle, in order to look at curvature. Way back, I proposed a 2-frequency system of 5kHz and 25kHz, and sampling every 10us. That gives 2 samples per short pulse and 10 samples per long pulse. The long pulse is used for ground & iron ID and large gold, the short pulse for small gold.

Yes, you can do it with analog demods. The above scenario requires 12 demods but you could cut back to even 6 (2+4).

The motor controllers I've seen are basic H-bridge with no means for flyback preservation, so they shunt the flyback through the body diodes back to the power supply. Therefore you can never get a high slew rate. Might work for VLF, in fact I've use the DRV8872 H-bridge in a VLF design.

Damn! I am out of luck today! All the misses!
But you must admit that it looks so tempty!

[Carl-NC]

At least 3 samples per response cycle, in order to look at curvature. Way back, I proposed a 2-frequency system of 5kHz and 25kHz, and sampling every 10us. That gives 2 samples per short pulse and 10 samples per long pulse. The long pulse is used for ground & iron ID and large gold, the short pulse for small gold.

Yes, you can do it with analog demods. The above scenario requires 12 demods but you could cut back to even 6 (2+4).

The motor controllers I've seen are basic H-bridge with no means for flyback preservation, so they shunt the flyback through the body diodes back to the power supply. Therefore you can never get a high slew rate. Might work for VLF, in fact I've use the DRV8872 H-bridge in a VLF design.