Originally posted by Sergey_P
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I am not entirely clear on the concentric coils calculations yet, because I'm aware of the magnetic field peaking near the Tx coil. The most of the induction calculations assume a magnetic dipole and uniform magnetic fields, while in reality field concentrates near wires. Guess the calculations for the balance etc work fine in practice if you keep a bit distance against the Tx coil. I like the proposed concentric coil because it maintains the Tx coil integrity, Q factor, etc.
Reception area is a vector product of Rx and Tx. In case you use differential coils there are zones perfectly illuminated by both Rx and Tx, yet you have a sharp minimum ... due to the vector product.
...
Now about the 4 quadrant...
Let's observe a non-Fe channel of a regular 2 quadrant Rx.
We have a GEB channel that has a zero response for ground vector and a positive response for all metals - considering a positive coil phase.
We have also a Disc channel which has a response ~perpendicular to the GEB. It will have response positive for some metals and negative for others.
They both go to the comparators to be tested against a threshold value, and these form an analog AND function. If both Geb and Disc values are above the threshold, a Geb value is passed through, and detector beeps.
It is important to notice here that it is not necessary that Disc channel passes the threshold to obtain a correct response, but instead it must be positive. In a blurry area between a zero and a threshold value, it is the GEB that must overcome a threshold for a clear response.
To obtain a 4 quadrant Rx we need an analog XOR function, and in practice it is a balanced mixer or a 4 quadrant multiplier. There is a slight problem with both - with Disc and Geb being of the same order of magnitude, the output would be squared, unless... Disc is made binary.
So when Disc is positive the output of such XOR would be equal Geb, and when Disc is negative the output would be -Geb.
Easy way to make this is to supply Disc to a comparator and compare its value with zero. The comparator binary output is supplied to a switch which choses between a Geb and inverted Geb. Such signal is further compared with a threshold value, and passed to the tone circuitry. And we have a solution for discrimination of the opposite quadrants for a non-Fe channel in a 4 quadrant solution. It may even be done as a hardware add-on for the existing rigs.
To get a regular 2 quadrant Rx from this 4 quadrant solution, only the inverted Geb signal is level shifted below zero, and it will work exactly as expected. A Disc polarity switch will operate between a non-inverted Geb value and some negative value. If both Geb and Disc values are positive, a Geb value is passed through when it overcomes a threshold, and detector beeps. Such operation will be needed when using regular coils.
I'll make a model of this soon, but this concept is open for debate.
And yes, I already pimped my IGSL Disc channels gains so that they behave more binary-like, and it results with better sensitivity of borderline materials such as Al foil.
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