GROUND BALANCE ( without the ground :-) )

[JoyJo]

VLF detectors have quadrature demods, so they effectively take the sine and cosine of the incoming signal to create the resistive (R) and reactive (X) signals. for GB we want the R signal to be zero in the presence of ground, even if the strength of the ground varies. So we adjust the sine() term to be zero.

Maybe I'm wrong, but the rotation of the vector is carried out according to such formulas. I'm not saying, I want to understand))
x1=x*cos(angle)+y*sin(angle);
y1=y*cos(angle)-x*sin(angle);

[JoyJo]

Is it possible to consider the following method: to pre-normalize the signal amplitude in front of the synchrodetector. That is, there will be no increase or decrease in the amplitude of the signal when the sensor approaches the ground or target.
Will this approach weaken the impact of the soil?

[Carl-NC]

Maybe I'm wrong, but the rotation of the vector is carried out according to such formulas. I'm not saying, I want to understand))
x1=x*cos(angle)+y*sin(angle);
y1=y*cos(angle)-x*sin(angle);

There are different ways to implement GB. The most straightforward way is to simply rotate the phase of the demods until the R channel is exactly 90° to the ground phase at which point the ground R signal is zero. That looks like this:

R_GB = mag(GR)*sin(ph(GR) + ph)

and you adjust ph until the sin() term is zero.

Another way is combine a little bit of the ground X signal with the R signal until the result is zero. That looks like this:

R_GB = mag(GR)*sin(ph(GR)) + k*mag(GR)*cos(ph(GR))

and you adjust k until the result is zero.

I'm not sure what you mean about pre-normalizing the signal amplitude. Adjust the preamp gain so the demods always see a constant signal level?

[JoyJo]

Maybe it will be clearer this way?

[Carl-NC]

I think it will get really noisy especially at low signal levels. Digital radio systems sometimes use a similar approach whereby small signals are gained up and large signals attenuated, and an "RSSI" signal is included with the main signal so you can recover the real signal strength later. But these radio systems use phase-modulated encoding where changes in amplitude don't really matter. With a metal detector, the amplitude is used for the audio response so amplitude steps can cause a lot of noise.

[pito]

[moodz]

Thats basically how a VLF detector is done ? So zeroing Uy ground balances the detector?
?

[JoyJo]

It's a classic. It looks like a Fortune metal detector, only it is built on a PIС.

[pito]

So zeroing Uy ground balances the detector?
?

yes, rotator it is a phase shifter

[JoyJo]

I think it will get really noisy especially at low signal levels. Digital radio systems sometimes use a similar approach whereby small signals are gained up and large signals attenuated, and an "RSSI" signal is included with the main signal so you can recover the real signal strength later. But these radio systems use phase-modulated encoding where changes in amplitude don't really matter. With a metal detector, the amplitude is used for the audio response so amplitude steps can cause a lot of noise.

Yes, that's right, it has a similarity with the FM receiver: when there is no transmitting station, that is, noise in the dynamics. But, there will always be a signal in the metal detector due to the imbalance in the DD coil.

[JoyJo]

yes, rotator it is a phase shifter

Fortune has enough problems associated with such a scheme.

[pito]

this is not a Fortune, it is my own design and it is working ok, rotator/phase shifting is done by stm32f103

[JoyJo]

What kind of detector do you have? There are not so many of them on stm32f103.

[moodz]

Here is a video showing operation of the "ground balance without the ground" .

The demo is a VLF IB and have not had time to verify but it should/may work on PI detectors also.

It uses an algorithmic method that I have discovered. ( ie not look up tables or fairy dust )

Enjoy.

https://youtu.be/idVP0gRar54?si=lGSRHIzjKnjHTJ8g

[moodz]

Here is a video showing operation of the "ground balance without the ground" .

The demo is a VLF IB and have not had time to verify but it should/may work on PI detectors also.

It uses an algorithmic method that I have discovered. ( ie not look up tables or fairy dust )

Enjoy.

https://youtu.be/idVP0gRar54?si=lGSRHIzjKnjHTJ8g

In the video ... after the auto ground balance is done there is still some residual response to the large ferrite.
This is now fixed with the addition of a missing step in the code.

So I used to use a rotary encoder to adjust the GB manually ... not needed now.