the right factor ..?

[Sergey_P]

Then it should be named (II) Induction Imbalanced and not (IB) Induction Balanced system?

There is no perfect system. In addition to IB tuning errors, there is also TX generator noise.
It is better to use coils without resonance with equal phase characteristics of ground impedances ( Qcoil=tg () ground )

[WM6]

If you look at a Bode phase plot of the frequency response
you will see that the slope of the curve is at its steepest at resonance.
Further away from resonance the phase curve becomes less steep.
By "squirrely", Carl means that the response can be unstable as you get
closer to the resonance point.

Only Nexus tune their coils close to resonance to achieve a stronger signal response.
The downside is that the coils are complex, time-consuming to manufacture, and expensive.
All other metal detector manufacturers tune their coils off-resonance to make them easier
to manufacture, to maintain consistency and stability, and reduce cost.
Resonance tuning is not conducive to volume production.

Thanks for your additional efforts with clarification.

[dbanner]

Let us be very clear on this issue. Squirrelly! No hell, no.
What I've figured out, or more aptly put, is that there is certain range of acceptable response, without which no profit can be had.

[dbanner]

The steep slope is a result of the temperature response of the coil Q.

[lamoiz]

Probably ceramic NP0/C0G. Then PPS and polypropylene. Does anyone still make polystyrene?

if you have to choose between 2 types of capacitor in a coil. between ceramic or mica, which one to take for good thermal stability / held in farad / freq

[lamoiz]

what is the impact on a circuit, between a resonance lc series or lc parallel ..?
Carl ?

[Carl-NC]

what is the impact on a circuit, between a resonance lc series or lc parallel ..?
Carl ?

There is a noise impact from the opamp input noise current though the RX coil. A parallel-resonant coil has a higher noise impedance at resonance whereas a series-resonant coil has a lower noise impedance at resonance so theoretically the series cap should be lower noise. However, the series mode had a much lower Q and I suspect the overall noise impact is minimal at best. Also, using a series cap makes the coil parametrics more critical, which means if you want to make a bunch of different sizes & styles you'll need to pay close attention to matching the parametrics.

[lamoiz]

There is a noise impact from the opamp input noise current though the RX coil. A parallel-resonant coil has a higher noise impedance at resonance whereas a series-resonant coil has a lower noise impedance at resonance so theoretically the series cap should be lower noise. However, the series mode had a much lower Q and I suspect the overall noise impact is minimal at best. Also, using a series cap makes the coil parametrics more critical, which means is you want to make a bunch of different sizes & styles you'll need to pay close attention to matching the parametrics.

I ask this question, because yesterday I tested an Idx coil with parallel capacitor, on a quasar. On this unit, we can see the consumption of the tx winding, then that it fixes the frequency at the maximum resonance, on the peak of consumption. But with parallel LC it does not work. Then, I did the same test with another winding and the capa in series .. and it works perfectly. Thanks Carl, I learn from your valuable advice everyday.