GROUND BALANCE ( without the ground :-) )

[pito]

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

[JoyJo]

yes, rotator it is a phase shifter

Fortune has enough problems associated with such a scheme.

[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.

[pito]

So zeroing Uy ground balances the detector?
?

yes, rotator it is a phase shifter

[JoyJo]

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

[moodz]

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

[pito]

[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.

[JoyJo]

Maybe it will be clearer this way?

[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]

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?

[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);

[Carl-NC]

Can multi-frequency, simultaneous operation on 3 or 5 frequencies contribute to alleviating this problem? It seems to me that multi-frequency will not solve the soil issue completely.

In some cases, yes. Non-viscous ground (ferrite-like) has a loss angle and magnitude that is largely independent of frequency. So, assuming there is no null vector component, you can subtract two frequencies and cancel ground without knowing its loss angle or strength. Viscous ground, though, has a loss angle that slightly increases with frequency so that won't work.

[JoyJo]

VDIs are mostly taken from the 2nd derivative which will have removed all of E2 and most of the static effect of E1. You'd think that leaves just the target vector, except that mineralization can also rotate the target vector in a dynamic way. I don't know if any detectors try to correct for this, I expect that MF models attempt to do so.

Can multi-frequency, simultaneous operation on 3 or 5 frequencies contribute to alleviating this problem? It seems to me that multi-frequency will not solve the soil issue completely.

[boilcoil]

This is too complicated for me, but I think that in order to fix the system correctly, you need at least 2 points - air and ground. Without any of these points, the system is undefined.