DEEPER PI DETECTION DEPTH

[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]

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

[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/di...ad.php?2,23116
and
http://www.findmall.com/read.php?66,...65#msg-1615365

Could interest you and the other members.

Cheers,
Aziz

[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.

[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]

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]

Yep, that's about that.

[simonbaker]

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

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

[WM6]

Does such a toroid really couple well to the loop?

-SB

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

[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.

[Aziz]

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.

That's a very trivial task.

Make a simple PI switcher spice simulation. Sense (view) the TX coil current and do a FFT for the pulse period time span only (that's important, otherwise, the whole frequency response will be distributed around the harmonics of the pulse frequency). LTspice offers this feature.

To make things easy, use a 1 kHz pulse frequency and make the FFT for let's say 10 ms .. 11 ms (after the circuit stabilizes late time of course).

You can see the frequency spectrum distribution of the TX current pulse. Notice, it contains the pulse-on response too (more low frequency contents). The flyback period current, when decay quickly to zero is a high frequency stimulation.

Aziz

[Tinkerer]

Attached is the FFT of the switch OFF transient. This is the target stimulation, sampled after switch off.

What does it tell us?

Tinkerer

[Tinkerer]

Below is the FFT of the full cycle TX

Tinkerer

[Tinkerer]

And here is the FFT of the TX ON time.
All these are simulations. The real TX pulses have quite a bit of Mosfet switching noise, specially when high Amps are switched.

Tinkerer

[Tinkerer]

And here is the PI TRADITIONAL Full TX cycle FFT