New ground exclude?

[FrancoItaly]

Hi All,


I do not know if this topic has already been covered, but I follow much more the LongRangeLocatorsForum, I have some experience in VLF and PI bulding and the biggest challenge for me is to make the search head especially the null point. So I thought to limit the null point for example to 1V and to add to the following amplifier stages a signal phase-shifted by 180 degrees. The same system can realize the ground exclude. We can add a signal phase shifted by 180 degrees with respect to the signal caused by the ground and with amplitude proportional to the variation of the output signal, thus if the output signal increases due only to ground this increase is neutralized by the phase-shifted signal. Still need to understand the characteristics of the signal in the presence of metal.


Best Regards

[Davor]

... and real soil. It is seldom 180° but easy soils are close to it.

[FrancoItaly]

The normal ground exclude works on the basis that it has a precise phase shift with respect to the transmitted field, it seems to me correct then cancel it with a signal of equal intensity but out of phase by 180 degrees. Obviously it is possible to vary the phase of the signal cancellation and this will be the command of ground exclude.

[Davor]

Err... no. What you may exclude this way is a so called "air signal" which is cyclostationary against the Tx (it has a fixed amplitude and phase relationship). By means of varying distance to the ground you lose the cyclostationary relationship. Because of it you need a separate mechanism that will not depend on the proximity to ground.
What you propose is a mechanism by which we achieve induction balance, not ground balance. Mind you, there is nothing wrong with induction balance.

[sled]

Such compensation to reach a "null" in the induction balance can be found on the Quasar and Kpot Mole metal detector:

http://fandy.ucoz.org/QuasarARM/Quasar_ARM_SCH.pdf (the SIN_C signal, grid square 5D)
http://radiodom.ucoz.ru/_si/0/82158727.jpg (the DAC1 signal, on the lower left where the pre-amp is)

[FrancoItaly]

Hi sled,
Very interesting, but what you think about the system of ground exclude?
Greetings from Ticino

[FrancoItaly]

Hi Davor,
Maybe I simplify it too much, but the principle of the ground exclude is that the signal caused by the ground has a fixed phase shift with respect to the TX signal, it changes only the amplitude moving the head toward the ground or moving it away. I can use the rectified signal of the output for determining the amount of signal phase-shifted by 180 degrees to be applied to the RX amplifier.

[asolone]

This concept is already applied on the search coils for nulling the tension on the c. scope coil

[Qiaozhi]

Hi Davor,
Maybe I simplify it too much, but the principle of the ground exclude is that the signal caused by the ground has a fixed phase shift with respect to the TX signal, it changes only the amplitude moving the head toward the ground or moving it away. I can use the rectified signal of the output for determining the amount of signal phase-shifted by 180 degrees to be applied to the RX amplifier.

The RX signal has a fixed phase-shift relative to the TX due to the way the coil is nulled. When the coil is placed above "neutral" ground, some of the TX signal is absorbed, and therefore only a decrease in amplitude is seen at the RX. The phase relationship remains fixed. This part of your theory is correct, but is only true in an air test. In the real world, even on very lightly mineralized ground, there will be a small phase-shift.

Re-reading your posts, I think what you're trying to describe is a method of automatic ground balance, whereby you're attempting to measure the phase-shift between the zero-crossing of the RX signal, and real ground. Then create a signal in anti-phase to cancel out the ground signal. Is this correct?

[FrancoItaly]

No, is similar to the manual Ground Exclude, here it acts on the phasing of the signal of cancellation so that it is exactly out of phase by 180 degrees with respect to the ground, while the amplitude depends upon the of the amplitudethe output signal. I have in mind a VCA circuit, for example the glorious CA3080. In this way any variation in amplitude of the signal due only to the effect of the soil is automatically canceled. Continuing in reasoning you could add a signal out of phase by 180 degrees with respect to the iron to greatly reduce the influence of ferrous objects, always using the output amplitude. The difference compared to normal VLF where a synchronous demodulator extracts the desired signal is that here you inject a signal opposite to cancel unwanted signals.

[Qiaozhi]

No, is similar to the manual Ground Exclude, here it acts on the phasing of the signal of cancellation so that it is exactly out of phase by 180 degrees with respect to the ground, while the amplitude depends upon the of the amplitudethe output signal.

That's not how ground exclusion works on a standard VLF (unless you're not explaining it clearly enough). The sync demod samples the RX signal at (or near) one of the zero-crossings, so that the amplitude-only variations are ignored, and only amplitude changes due to phase-shifts caused by the target are allowed through. The quadrature (90 degree) sample determines whether the amplitude is increasing or decreasing, this providing a means of discrimination between ferrous and non-ferrous targets.

[WM6]

Probably we need to rethink here something in LRL manner - as (remote) air balancing of material world?

[FrancoItaly]

I admit my ignorance, but do you think my idea can work?

[Qiaozhi]

I admit my ignorance, but do you think my idea can work?

The difficulty will be separating the amplitude changes that are due to ground response, from changes due a metal target. If I understand your theory correctly, then it seems unlikely it will work as intended. However, there's always one way to find out the answer ... do the experiment. Even if the result is not as you expect, you'll be higher up the learning curve.

[WM6]

I admit my ignorance, but do you think my idea can work?

Not sure, but it is pleasing that you are trying different new approaches to solve problem.
Worth to check, if it suit well to your expectation and probably special condition too
(this under presumption, that this is linked to your experimental work on remote forum?).