the right factor ..?

[Sergey_P]

RX doesn't know anything about TX (IB systems)

[dbanner]

[Carl-NC]

I haven't yet understand why the different approaches to coil TX/RX #turns and offset resonance.

For some background reading study to two common VLF project here in GeoTech. These are the TGSL and the IDX. Lots of threads on building these.

As for why TX/RX off resonance here is a paper that has been posted with math and experiments. Interesting read.[ATTACH]53513[/ATTACH][ATTACH]53512[/ATTACH]

Those papers apply to coupled coils. A metal detector uses uncoupled coils so the impedance reflections discussed in the papers are (ideally) not seen at all.

Some detectors do not resonant-tune the RX coil at all. I think most Tesoros are like this. Those that resonant-tune the RX coil almost always do so a bit off the TX frequency. The reason for doing this is so target phase responses behave well, especially when swapping coils or with temperature and aging. If you resonate the RX coil at the TX frequency then the phase slope is steep and target phases can be squirrely.

[lamoiz]

I did well to ask this question, it seems to me that there are a lot of answers to questions that everyone has. me, I finally have real answers and I realize that sometimes we do without knowing why ..
thank you carl.

[lamoiz]

what type of capacitor is the most temperature stable? teflon, polystyrene ..?

[Carl-NC]

what type of capacitor is the most temperature stable? teflon, polystyrene ..?

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

[waltr]

Those papers apply to coupled coils. A metal detector uses uncoupled coils so the impedance reflections discussed in the papers are (ideally) not seen at all.

Some detectors do not resonant-tune the RX coil at all. I think most Tesoros are like this. Those that resonant-tune the RX coil almost always do so a bit off the TX frequency. The reason for doing this is so target phase responses behave well, especially when swapping coils or with temperature and aging. If you resonate the RX coil at the TX frequency then the phase slope is steep and target phases can be squirrely.

I still haven't got this.
My only experience is with the TGSL which has a resonant TX at 14.5kHz. Then a IB RX coil resonant at 16.2kHz (coil L and Cap).
I thought these are 'coupled".

[Qiaozhi]

I still haven't got this.
My only experience is with the TGSL which has a resonant TX at 14.5kHz. Then a IB RX coil resonant at 16.2kHz (coil L and Cap).
I thought these are 'coupled".

An induction balanced metal detector coil is like a loosely coupled transformer. In a DD coil the two loops are positioned such that pickup in the RX loop is minimized. In a concentric coil there is a nulling loop that cancels the TX signal to minimize pickup by the RX loop.

The RX loop may be tuned to a different frequency than the TX, but it still receives the same [TX] frequency, just at a reduced amplitude.

When a metal target comes close to the coil, the balance between the two loops is upset. The metal target effectively acts like the core of a transformer, and couples some of the TX signal into the RX loop.

[Sergey_P]

When a metal target comes close to the coil, the balance between the two loops is upset.

The geometry of the induction balance is stable. The principle of superposition of fields between the sensor and the target works.

[lamoiz]

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

styroflex..

[WM6]

There is also a question of why rx coil is detuned from tx frequency.

Probably due this:

If both (TX and RX) are tuned at same resonant frequency (both at high Q),
RX frequency tend to lock its oscillation to TX frequency and then small targets
are no more able to decouple such locked coupling and are left out of sensing.

If TX and RX resonances are somewhat apart, coupling between two is loosened
and RX no more tend to lock its oscillation to TX resonance and as a result
small targets are able to affect coil IB (induction balance) and become "visible".

RX resonant frequency should be out of TX Delta F at given Q curve.

[Sergey_P]

and RX no more tend to lock its oscillation to TX resonance

the TX imbalance signal is always present in IB systems

[Qiaozhi]

Probably due this:

If both (TX and RX) are tuned at same resonant frequency (both at high Q),
RX frequency tend to lock its oscillation to TX frequency and then small targets
are no more able to decouple such locked coupling and are left out of sensing.

If TX and RX resonances are somewhat apart, coupling between two is loosened
and RX no more tend to lock its oscillation to TX resonance and as a result
small targets are able to affect coil IB (induction balance) and become "visible".

You're thinking of a BFO system where "locking" can occur. This is different.

As Carl said in post #18:
"The reason for doing this is so target phase responses behave well, especially when swapping coils or with temperature and aging. If you resonate the RX coil at the TX frequency then the phase slope is steep and target phases can be squirrely."

[WM6]

You're thinking of a BFO system where "locking" can occur. This is different.

As Carl said in post #18:
"The reason for doing this is so target phase responses behave well, especially when swapping
coils or with temperature and aging. If you resonate the RX coil at the TX frequency then the
phase slope is steep and target phases can be squirrely."

Thanks.

I accepts this explanation, despite I do not understand quite well
this slANGlish: "slope is steep and target phases can be squirrely."

the TX imbalance signal is always present in IB systems

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

[Qiaozhi]

Thanks.
I accepts this explanation, despite I do not understand quite well
this slANGlish: "slope is steep and target phases can be squirrely."

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.