Differential TX driver for TDI?

[eduardo1979]

This schematic shows that a split coil is driven at the same time with the same pulse, and is then combined into a single receive signal. It won't do what you want.

To do what you want requires generating an independent TX pulse for a second driver. That means modifying the PIC code, which means you will need to replicate the existing PIC code. That's not too hard because it mostly just generates timing pulses, and there are a few unused pins available. Then, the second TX coil output will need to drive the (+) side of the preamp (via a clamp) which, fortunately, has a convenient resistor (R13) that can be removed. Finally, you will need to add new timing pulses for the sampling switches to grab the samples for the new negative TX pulse.

None of this is impossible and is quite doable. You will need to make a small daughterboard for the new components and write new firmware. Also fortunately, the coil connector has 5 pins so assigning one to the new TX phase isn't hard, but it might be hard to maintain compatibility with stock coils.

Hi Carl.
You understood perfectly what I wanted.
I thought it would be much easier, but programming isn't something I'm very comfortable with. I considered a solution like using an inverter circuit, AND gates, and a frequency divider circuit to achieve what I wanted. In short, something similar to the AF108 circuit. Thank you very much for your response.

[eduardo1979]

Hi eduardo1979,

are you looking for something like this circuit?

Hi Geomax. After looking more closely at your diagram, it seems to be quite close to what I was looking for. Perhaps Carl can clarify things a bit more.

[pechkata]

Hello Eduardo.
Can you upload the entire schematic Eric?

[eduardo1979]

Hello Eduardo.
Can you upload the entire schematic Eric?

Hello. I don't have the complete diagram. That's all.
https://www.geotech1.com/forums/foru...-small-nuggets
POST #9

[Detectorist#1]

Hi eduardo,
It is not so simple to change of TDI SL in MD with bipolar TX pulses. The changes in the TX part are only half of the problem. This change will needs also changes in RX channels. This change is additionally complexed by your problems with the uP programming.

[eduardo1979]

Hi eduardo,
It is not so simple to change of TDI SL in MD with bipolar TX pulses. The changes in the TX part are only half of the problem. This change will needs also changes in RX channels. This change is additionally complexed by your problems with the uP programming.

Hello Detectorist 1. Thanks for replying. I'm already realizing it's not an easy task, especially the processor programming, which sounds quite complicated to me. I was trying to find a simpler solution, but it's more complicated than it seems. In the AF108 schematic, the preamplifier isn't configured in differential mode, so in my TDI it shouldn't affect the preamplifier input.

[eduardo1979]

Hello Detectorist 1. Thanks for replying. I'm already realizing it's not an easy task, especially the processor programming, which sounds quite complicated to me. I was trying to find a simpler solution, but it's more complicated than it seems. In the AF108 schematic, the preamplifier isn't configured in differential mode, so in my TDI it shouldn't affect the preamplifier input.

Altra sent me this link with a rather interesting method.
https://www.geotech1.com/forums/foru...metal-detector

[algan]

Hi, GeoMax, have you tested this circuit on any pulse detector? If so, what is the difference with this pulse, which one is better in terms of depth or better ground effect with it, what was the benefit?

[GeoMax]

Hi Algan,

no, I have not tested this. I got it from a Russian Detector Forum page. Which one I can not remember. But it was quite a long time ago.

[pechkata]

In the AF108 schematic, the preamplifier isn't configured in differential mode, so in my TDI it shouldn't affect the preamplifier input.

[Carl-NC]

Regardless of what scheme you come up with to run a second TX drive, you still need to modify the firmware to drive the alternate phase of the sampling switches. Since this is required, the overwhelmingly easiest TX solution is to just add a second TX pulse to the micro. Then you will need a small add-on PCB with around 14 components.

A few years ago I started playing with a proportional timing scheme for the TDI-SL, so I wrote all-new firmware for the micro. I started out making it exactly replicate the factory operation, then modified it for my purposes. I think I would be willing to release the factory version which would give you an easy starting point for adding bipolar operation.

[eduardo1979]

Hi Carl. I would be so grateful if you could do me this huge favor. With the schematic for the small additional PCB and the firmware, I'd have everything sorted and my dream would come true. Thanks in advance.

[Carl-NC]

Here are the source code files for what should be a close copy of the TDI. I thought I had also done this for the TDI-SL but cannot find evidence of that. The SL uses a different micro with a different pin-out so there will be some changes need to make it run on an SL -- mostly in pin assignments and maybe register names -- but the core run-code should be the same.

The ISR function has 2 states: the TX-on state and the TX-off state. To make it work with a bipolar TX you simply double this: TXP-on, TXP-off, TXN-on, TXN-off. The TXN-on will turn on the second NMOS via an unused pin, and the TXN-off code will need to have the activated switches changed for the negative polarity. Offhand, I don't think there is much more to it.

If I get some time (unlikely right now) I will make a port to the SL and test it on my SL.

TDI_Std.zip

[eduardo1979]

Hi Carl. Thank you so much for your help.

[pechkata]

Hello Carl
This peak PIC16F684 Is it possible to try it on this board?
With these source code files.