TDI coil- Is it really fast????

[Geo]

Hi.
Today i measured the self resonance of the TDI coils.
Results was not so good... (my opinion).
I used the schematic that bbsailor indicates at his article "
Making a Fast Pulse Induction Mono Coil". I used a probe x10 and a input capacitor 3.3 pF.
First i checked the 7.5" coil. I found a 463 Khz self-resonance.
After it i checked the 12" coil.
I foune a 484 Khz self resonance.
Maybe the large cable have very much capacitance.
After it i measured 2 hand made coils.
One 10" 680uH (i don't remember turns) without shield and 1m coaxial cable. It had a 410Khz self resonance.
The other was a 350uH double coil (as the TDI, i constructed it when i tried the ATiny4).With 1m cable and without shield, it gave a 240Khz self resonance.
So maybe the double coil that White's use at TDI is not so fast as a simple mono coil with teflon wire.
Any comments???

Regards

[Reg]

Hi Geo,

My test is/was simple, I look(ed) at the waveform out of the preamp to see when the signal flatlines sufficiently such that I am not sampling on a rising or falling curve. I used Teflon wire for two different coils I fitted into the same size housing, one coil made using a DF type design and the other a simple mono coil. The DF design clearly flatlined quicker.

I was using the Hay's 8" by 11" elliptical housing for both. Both used the same size and type Teflon wire. When checked the DF allowed for sampling down to about 7.5 usec while the simple mono was hard pressed to operate at 9 usec.

Reg

[KingJL]

DF coils

Reg,
Have you further refined any of the suggested DF coils specs? e.g. relative coil sizes, suggested total inductance. In the instance of 2 separate serially connected coils, does it have any influence on which (large or small) is on the common or driven side?

Regards,
J. L. King

[Reg]

Hi J. L.

Unfortunately, I have not spent any time trying to determine just what might be the best combination or setup. I assumed the outer coil should be much larger in inductance to try to assure maximum depth capabilities. Also, I elected to make the total inductance not that much greater than a typical mono coil.

So, as a short answer, no I have not done that much investigating of what might be best. Since my first attempt exceeded my expectations, I didn't try other combinations.

Reg

[Geo]

Hi Geo,

My test is/was simple, I look(ed) at the waveform out of the preamp to see when the signal flatlines sufficiently such that I am not sampling on a rising or falling curve. I used Teflon wire for two different coils I fitted into the same size housing, one coil made using a DF type design and the other a simple mono coil. The DF design clearly flatlined quicker.

I was using the Hay's 8" by 11" elliptical housing for both. Both used the same size and type Teflon wire. When checked the DF allowed for sampling down to about 7.5 usec while the simple mono was hard pressed to operate at 9 usec.

Reg

Hi Reg.
One problem is that we have opposite results from our tests. But your experience on coils is very very bigger than mine, so i accept what you say.
Because my English don't help me so much, please can you explain me about the "I look(ed) at the waveform out of the preamp to see when the signal flatlines sufficiently such that I am not sampling on a rising or falling curve"??
This month i want to construct a 1x1 m coil for the TDI. Is it possible to make it to work good at 17.5us???

Regards

[Reg]

Hi Geo,

First, there can be reasons we would have different results. One key reason for a possible difference is the damping resistor value. Another could be the wire itself, and finally, the shielding.

There may be other possible reasons, but the ones I have listed are some of the more obvious.

So, even though I may see a faster recovery, it doesn't mean that if I did things differently I wouldn't see a different result.

Now, my DF coil I was referencing to was built with two separate windings and not one idea I posted where I took a larger coil and twisted it into a DF design.

As for what I meant when I referred to flatlining is when the receive signal is observed, one will see the output of the preamp go into saturation when the coil winding is pulsed. Once the pulse is shut off, the output of the preamp will, after a very brief time, will saturate in the opposite direction, and finally will come out of saturation and try to go back to "0" volts.

When the signal is going back to 0V, the signal will take a finite amount of time. If no target is present and the coil is designed well, this rise (or change) to 0V will be very fast. If the coil is stable, then the rise to 0V will be quick and not have any oscillation. Once the signal has risen to 0V and remains there until the next pulse is what I call the flatline.

Signals from targets will cause this rise to take much longer.

Now, the faster coil will change or rise back to 0V (as what happens on the TDI or even the Hammerhead for that matter) sooner than a slower coil. So, the faster coil flatlines sooner than a coil having a slower response.

Let me know if this makes sense.

Reg

[Geo]

Hi Reg.
Sorry me but i just now read your answer.
Yes i understood what you mean with the term "flatline". Thank you

My Regards