DEEPER PI DETECTION DEPTH

[Midas]

A PI transmit pulse is a wide band pulse, which has different content of energy in each frequency spectrum.

That seems to be a pretty key point worth discussing further. Any idea how to work out the amount of energy allocated to each frequency range? Presumably its strongly related to the dI/dt of the pulse.

Thanks for the info on ML FBS. Only 3 real frequencies, pretty cheeky marketing.

[WM6]

Does such a toroid really couple well to the loop?

-SB

If you do not like toroid, you can use ferrite pot too.

[simonbaker]

Look at this old (from BFO times) Russian construction:

http://babelfish.yahoo.com/translate_url?doit=done&tt=url&intl=1&fr=bf-home&trurl=http%3A%2F%2Flib.qrz.ru%2Fnode%2F6561&lp=ru_en&btnTrUrl=Translate

If you use more than one signal source (as in your drawing), there is only matter of synchronisation.

Does such a toroid really couple well to the loop? I'm wondering about the orientation of the flux lines.

-SB

[Davor]

Yep, that's about that.

[WM6]

Look at this old (from BFO times) Russian construction:

http://babelfish.yahoo.com/translate...rUrl=Translate

and for some further ideas at this patent too:

http://www.freepatentsonline.com/5055793.html

If you use more than one signal source (as in your drawing), there is only matter of synchronisation.

[Davor]

Good to know. Do you know of any schematic of this implementation circulating around? I have great expectations with this principle, and why learning from scratch?

[WM6]

That would depend on pulse duration only. You get a true impulse response with step voltage source. E.g. like with switching power supplies.

System response would still be highly dependent upon coils loading, and I see even this going into favor of step voltage supply: cold driving transistors, flat frequency response, preserved phase response.

OT:
I worked as an RF engineer at a transmitters factory, doing mostly MW/SW designs, and the most ingenious thing at the time was a Harris transmitter which had a copper rod as a summing device for a multitude of driving elements, and operating as multiple transformers in series. (see picture) In case any of the drivers broke, it was SHORTED automatically to avoid damage to the device, and maintain continuous operation of the whole transmitter. So the broken driver of a coil shorted it in order not to hamper current flow through the rod. Just brilliant. Mind you, it was a well above 80% efficient - even with broken devices.
If I stretch the principle just a tiny bit, when I want to detect a tiny response from something deep in the ground, the last thing I wish to do is couple any kind of resistance to it.

Nothing new, Davor, this basic idea of your considerations was patented and implemented in Gardiner detectors decades ago.

[Aziz]

So what your sim needs now Aziz is a realistic non-constant and unpredictable ground effect component, then you can play around trying to remove it. From what I gather Minelab are doing this already by using some clever maths to combine the low frequency response with the high frequency response. They are actually use 28 frequencies which might be more to increase the wank factor than because its necessary or who knows perhaps it really does help.

Here's Minelabs consumer level explanation of their technology:

Find Every Target Type & Size with Every Sweep Generally, high transmit frequencies are more sensitive to small targets and low transmit frequencies give more depth on large deep targets. FBS simultaneously transmits and analyses a full band of multiple frequencies from 1.5 kHz to 100 kHz and is therefore sensitive to both very small and large deep targets at the same time. This means you only need to cover the ground once and can be confident you’re not leaving ANY valuable treasure behind.

If that's to be taken literally and isn't just sales patter then once your model is complete you should see some improved sensitivity to long TC targets by using a lower frequency that isn't captured in your current model. Perhaps as a result of the increased attenuation effect of the ground at high frequencies.

Hi Midas,

I have found interesting discussions going on on this topic:

http://www.dankowskidetectors.com/discussions/read.php?2,23116

and

http://www.findmall.com/read.php?66,1615365,1615365#msg-1615365

Could interest you and the other members.

Cheers,
Aziz

[Aziz]

I have tried to put the main capacitor on the coil and only a very small fine adjustment capacitor on the board. However, I got HF oscillation noise due to the cable between.

Could this be caused by a bad choice of cable?

How can this problem be avoided or fixed?

Tinkerer

Hi Tinkerer,

remember, the coil lead is a small inductor. You have a small adjustment capacitor in the control box, the coil lead inductor, the coil inductor and the bigger tank capacitor.
If you put all these parts into the spice model, you will see the inevitable HF oscillation.

Just make a simulation model and try different things and you will find the solution.
Aziz

[Tinkerer]

Well, the real reason, why the standard VLF coils have a high inductivity should be due to to get the resonant capacitor small enough, which is placed in the coil of course. And they don't require a high current flow through the TX coil so a thin wire can be used for it without becoming the search head to much heavy.

Even a 100µH coil could be used as a TX coil. That's convenient, if the upper frequency of a dual-frequency VLF is high enough.

The efficiency is very high, if the most of the TX energy is held locally in the coil's resonant tank.

Aziz

I have tried to put the main capacitor on the coil and only a very small fine adjustment capacitor on the board. However, I got HF oscillation noise due to the cable between.

Could this be caused by a bad choice of cable?

How can this problem be avoided or fixed?

Tinkerer

[Aziz]

Hi all,

what's the reason for the increased interest in this topic thread?
The number of viewers went up.

I rather would like to see more contributions. This ain't a team work otherwise.
And:
Patent Trolls! Keep Out!

Aziz

[Davor]

It just hit me

Using a thick metal ring, copper or aluminum, and a ferrite-current pliers-like-transformer, just like in the above example, you could forget about the capacity problem in PI coils forever. Such contraption would act as a coil of the ring diameter, but with number of turns equivalent to the number of turns on a ferrite thingy. In case of aluminum it could be self supported as well, and as a bonus you'll be able to forget about shielding alltogether.

Did I earn a lollipop or what?

A step forward... I could have a pair of separated search rings (with no IB overlapping) and supply them with in phase for Tx and counter phase for Rx balance. Using aluminum rings it could make a very lightweight construction, with 100% metal used for both Tx and Rx, and in perfect balance as well.

As Aziz said before, patent hogs beware! This idea is NOT for commercial use.

[Davor]

But the high frequency region is more power efficient (saturation of the TX coil current does not happen in the high frequency region).

That would depend on pulse duration only. You get a true impulse response with step voltage source. E.g. like with switching power supplies.

System response would still be highly dependent upon coils loading, and I see even this going into favor of step voltage supply: cold driving transistors, flat frequency response, preserved phase response.

OT:
I worked as an RF engineer at a transmitters factory, doing mostly MW/SW designs, and the most ingenious thing at the time was a Harris transmitter which had a copper rod as a summing device for a multitude of driving elements, and operating as multiple transformers in series. (see picture) In case any of the drivers broke, it was SHORTED automatically to avoid damage to the device, and maintain continuous operation of the whole transmitter. So the broken driver of a coil shorted it in order not to hamper current flow through the rod. Just brilliant. Mind you, it was a well above 80% efficient - even with broken devices.
If I stretch the principle just a tiny bit, when I want to detect a tiny response from something deep in the ground, the last thing I wish to do is couple any kind of resistance to it.

[Aziz]

So what your sim needs now Aziz is a realistic non-constant and unpredictable ground effect component, then you can play around trying to remove it. From what I gather Minelab are doing this already by using some clever maths to combine the low frequency response with the high frequency response. They are actually use 28 frequencies which might be more to increase the wank factor than because its necessary or who knows perhaps it really does help.

Here's Minelabs consumer level explanation of their technology:

Find Every Target Type & Size with Every Sweep Generally, high transmit frequencies are more sensitive to small targets and low transmit frequencies give more depth on large deep targets. FBS simultaneously transmits and analyses a full band of multiple frequencies from 1.5 kHz to 100 kHz and is therefore sensitive to both very small and large deep targets at the same time. This means you only need to cover the ground once and can be confident you’re not leaving ANY valuable treasure behind.

If that's to be taken literally and isn't just sales patter then once your model is complete you should see some improved sensitivity to long TC targets by using a lower frequency that isn't captured in your current model. Perhaps as a result of the increased attenuation effect of the ground at high frequencies.

Ground effect circuit simulation is a non-trivial task. It is even not understood well enough in the MD business.

It's way easier, to solve the ground effect problem in the firmware (software solution).

Regarding the ML's 28 FBS:
The clever math is called FFT (Fast-Fourier-Transform). If they really use 28 frequencies, they must use a 64-point FFT, which delivers 32 frequency demodulation components. The 0 - operating frequency range can't be used and very likely some of the upper frequency ranges as well. So 28 out of the 32 frequency components were used at the end.

No problem, you could have more frequencies if you have enough processing power. But you could do it with less frequency components too.

The Minelab's consumer level of explation is correct. To see the proof, just have a look at the bode plot I'm talking about for a long time now.

Well, we didn't talk about the frequency response of the transmitter. Every transmitter type has it's own frequency response. A single frequency VLF uses a single frequency and the total TX energy is focused at this frequency.
A PI transmit pulse is a wide band pulse, which has different content of energy in each frequency spectrum.
If I compare VLF with PI, the PI type isn't efficient as most of the emitted TX energy isn't processed at the end.

Some food so far to think about.

Cheers,
Aziz

[Aziz]

But if a target pops out of the background, maybe the info is readable?

Good observation. Would be hard to have optimal efficiency at two frequencies, unless double resonant coils (messy).

Interesting...

Of course if we made the TX circuit resonant at some frequency(s), that relationship could be changed.

-SB

-SB

Well, the real reason, why the standard VLF coils have a high inductivity should be due to to get the resonant capacitor small enough, which is placed in the coil of course. And they don't require a high current flow through the TX coil so a thin wire can be used for it without becoming the search head to much heavy.

Even a 100µH coil could be used as a TX coil. That's convenient, if the upper frequency of a dual-frequency VLF is high enough.

The efficiency is very high, if the most of the TX energy is held locally in the coil's resonant tank.

Aziz