TX Cascade for Multi-Frequency

[JoyJo]

Greetings to all!
In the Stalker IB project, we have already achieved full-fledged operation at one frequency using a fully resonant system: the TX cascade is a series resonance, the RX cascade is a parallel resonance.
Now we want to master the multichassis from which we want to get the following benefits:
1. Improved ground control. Determination of the frequency dependence of the soil conductivity and the frequency dependence of the magnetic susceptibility of the soil.
Knowing the frequency dependence of these parameters, it is possible to predict the behavior and response of the soil, and accordingly compensate for its influence.
2. Get a more informative indicator of the number of VDIs

To implement this task, we plan to use a TX bridge stage and a non-resonant RX coil.
Here is a preliminary diagram.
This scheme is not new. I met her in one of the patents.
Ideally, get work done on 3-4 frequencies at the same time.

What can you say about such a circuit implementation?

[Carl-NC]

The H-bridge as you show it will not work. Besides the fact that Q3,Q4,Q6 are upside-down, it requires a more complex design which was not revealed in the patent (I know because I wrote the patent). If this is your first attempt at MF then I suggest a more basic approach to start with: a single-supply H-bridge and only 2 frequencies, either sequential or simultaneous. Personally, I think sequential is a little easier.

[JoyJo]

I am very glad that I had the honor to communicate with you! You are a legend!
Regarding circuitry, yes, this is the first attempt.
I have so far used the H-bridge to work at one frequency. I used the DRV8833 chip. The results were quite predictable.
As a control of the power of the TX field, I chose a way to adjust the voltage. The TPS63070RNMT chip is selected as the voltage source for the TX. With the help of the MC, the resistance of the resistors in the TPS binding is regulated and it is possible to obtain a voltage from 3.4 V to 9V. The entire metal detector is powered by two 18650 lithium batteries. At a low frequency, the voltage will decrease, and at a high frequency, it will increase. So far, this works in a single-frequency version.
And why does it make sense to stop at only two frequencies?

[Carl-NC]

Yes, I have also built a variable frequency TX circuit, using the DRV8872 with a boost voltage from 7.5V to 30V. I did the same thing you did, scaled the voltage with frequency.
I have built a 3-frequency version of the circuit in the patent that ran at 4kHz, 16kHz, and 64kHz. I consider 3 frequencies to be a practical maximum, I have seen no real benefit of using more than that.

[Daves]

Hi guys,
can you write the patent number?

[Carl-NC]

US11022712

[JoyJo]

Wow! Wow that's cool))) We think the same way))) And the real benefit was obtained from the underlying scheme and the element base? For the TX coil, I used 16 coils of wire with a diameter of 0.75 mm in the experiment. I think you consistently radiated TX frequencies, but how was the processing done? I understand that this is your secret, but nevertheless, it is very interesting. And what is the main benefit of multi-frequency? fighting the influence of the soil or the exact vdi? At the moment, my familiar owners of vanquishes and equinoxes claim that the detection depth is lower at the multi-frequency.

[JoyJo]

I implemented a similar scheme for the TX cascade. But, I also provided for the possibility of turning on the DRV to work at the same frequency in the sequential resonance mode. The choice is made by sealing the jumper.

[JoyJo]

But I want to use a lower voltage of 3.5-9V. To be able to be flattered by two 18650 batteries.

[JoyJo]

And what is the reason for the choice of such frequencies 4, 16, 64? This is 2 to the power of n, where n is 2, 4, 6.

[Carl-NC]

No, arbitrary choices. But they are evenly spaced at 1:4:16.

[JoyJo]

And how can a multi-frequency signal be processed? And according to the vdi, it is not clear: each frequency will have its own vdi, but there is no point in displaying three vdi values. And when averaging, the vdi will not be accurate.

[Carl-NC]

The easiest way to process is to treat each frequency like a separate metal detector. Separately ground balance each one, then scale their VDIs to the same scale. For example, the White's V3 scales each frequency (2.5k-7.5k-22.5k) to 6.592kHz, which they had been using for many years.

[JoyJo]

Do you need a multi-channel ADC for this? We now have an input stage on one operational amplifier, then the signal goes to the ADC, and then to the microcontroller.

[Carl-NC]

Are you wanting to use analog demodulators or direct sampling?