Let's made a PC-base metal detector with usb interface !!!

[Aziz]

Hi Aziz

what is basic circuit idea behind this? I am not sure if I am understad functionality of this frequency response diagram mapped to real world of circuit.

Hi WM6,

the circuit is a simple filter with two resonant frequencies (current will be passed, voltage will be amplified). The search coil (TX coil) is part of the filter network.

It's a very simple implementation. See below the circuit principle. The capacitor Cs and Inductor LTX is the series resonant tank (for the higher frequency f2). The capacitor Cck and choke Lck is the parallel resonant tank (dominating the frequency f1). The LRX is the receive coil as an induction balanced configuration. I have put both stereo output signals from the sound card to lower the signal source impedance (from 30 Ohm -> approx. 15 Ohm). The Resistors Rs (R1 & R2) are typically very low (1 Ohm). But a single voltage source is also possible.

Cheers,
Aziz

[WM6]

Thanks Aziz, it is more understandable what do you have in development now.

Do you assume that orientation and distance between Tx and Rx coil can be critical?

[Aziz]

Do you assume that orientation and distance between Tx and Rx coil can be critical?

Hi WM6,

this must be investigated further. Induction balanced coils are always critical, if you want to process the phase information (or synchronously demodulating). This is also very critical to ground balancing/mineralization and discrimination.

But I am not interested in the phase relationship anymore to make things more easy and robust. This is the reason for the dual frequency VLF detector effort.

****************
* It seems, that the big effort was really worth. *
****************


Aziz

[Aziz]

Hi all,

I have to make a small correction regarding the latest posts:
The term choke is only valid, if there is flowing a DC current through the coil. But we haven't a DC current in the coil. So it's becoming only a coil (with or without magnetic core).


Ok, at the end, the good solution becomes only a band-pass filter with overshooting pass characteristics at the corner frequencies f1 and f2. To achieve the overshoot characteristics, we need high-Q factor parts (see http://en.wikipedia.org/wiki/Q_factor)

We could also implement the same filter in the receive part. This would be very nice to suppress a lot of EMI noise. Band-pass filter is more convenient to implement a multi-frequency analysis, which could be done very easy (via chirp wave form).

I have a new implementation of the filter, which should be much better than the previous one (see below).

Cheers,
Aziz

[WM6]

I have a new implementation of the filter, which should be much better than the previous one (see below).

Cheers,
Aziz

Hi Aziz.

Any progress?

I saw somewhere on one of forum that you are thinking about PLL solution (like NE565) couple years ago. Is PLL frequency metering no more actual?

[Aziz]

Hi Aziz.

Any progress?

I saw somewhere on one of forum that you are thinking about PLL solution (like NE565) couple years ago. Is PLL frequency metering no more actual?

Well,

I am total bankrupt, yet again. It could take some time to make some progress as I need some more parts. I have spent my budget for a new and big hard disk (that was overdue). Oh damn, January is also critical as I get always more annual bills to pay.

PLL:
I don't need a PLL. The sound cards I am using are capable of to synthesize frequencies up to 45 kHz. This is fully in the range of VLF or PI pulse clock frequency.
The sound output line is fully coherent with the sound input line. This means, they internally share the same crystal clock source. Cheap and bad sound cards may have different and not cohorent clock sources (I have one, which isn't appropriate for this application).

Dual frequency VLF:
I have wound some ferrite ring core coils to try the new design. I also found my amplifier module now. So I can try the new design in the coming days.

The latest news prevent me doing this earlier. So I was quite busy in acting in the underground community.

Aziz

[WM6]

The latest news prevent me doing this earlier. So I was quite busy in acting in the underground community.

Aziz

Very well, we do need good multifrequency detector to leak some more wikidoc.

[Aziz]

Very well, we do need good multifrequency detector to leak some more wikidoc.

CLASSIFIED due to reason 1.4 (b) and (d).


Aziz

[WM6]

Hi Aziz,

interesting sound card:

Do you think that it can be software controlled?


http://www.dealextreme.com/details.dx/sku.22475

[Aziz]

Hi Aziz,

interesting sound card:

Do you think that it can be software controlled?


http://www.dealextreme.com/details.dx/sku.22475

Forget this sh1t.
The ones, that should work well begins from 80 EUR.

We will see new 192 kHz sound cards soon. Let's give the industry the time for making it damn cheap.
Aziz

[Aziz]

192 kHz Sound Card

Hi all,

look at this (see specifications):
http://www.emu.com/products/product....6&nav=features

Looks quite interesting. I think, other competitors should also release some more sound cards in the coming future. The above one is quite big at the moment. But it's USB bus powered (no external power supply required).

It then get's quite interesting using it for high bandwidth PI detector (direct sampling).
I'll keep an eye on the market.

Aziz

[Aziz]

Another Dual Frequency Transmitter Proposal

Hi all,

look at this dual frequency transmitter example. It is optimized for two frequencies only and achieves better efficiency. It can produce really high voltages (>100 V), if your voltage source can deliver enough power (small power amp, low source impedance).

The two frequencies and their spectrum power content can be dimensioned by the four parts: L1, Cp, Cs and LTX (search coil). Note: L1 is a center-tapped coil wound on a ferrite ring core. All filter parts must be high-Q parts (low losses).

This type of transmitter is optimally suited for a DSP starter kit with built-in codec. You need a power amp, a receiver front-end, this filter and a sophisticated DSP software.

You can also drive it with digital clock feed (containing the two fundamental frequencies) instead of sine waves. Should give more power to the coil.

Cheers,
Aziz

[WM6]

Hi all,

look at this dual frequency transmitter example. It is optimized for two frequencies only and achieves better efficiency. It can produce really high voltages (>100 V), if your voltage source can deliver enough power (small power amp, low source impedance).

The two frequencies and their spectrum power content can be dimensioned by the four parts: L1, Cp, Cs and LTX (search coil). Note: L1 is a center-tapped coil wound on a ferrite ring core. All filter parts must be high-Q parts (low losses).

This type of transmitter is optimally suited for a DSP starter kit with built-in codec. You need a power amp, a receiver front-end, this filter and a sophisticated DSP software.

You can also drive it with digital clock feed (containing the two fundamental frequencies) instead of sine waves. Should give more power to the coil.

Cheers,
Aziz

Hi Aziz

what of part is V5 0 (or maybe Vs 0)?

Voltage source? What of voltage source is then on left.

[Aziz]

Hi Aziz

what of part is V5 0 (or maybe Vs 0)?

Voltage source? What of voltage source is then on left.

V5 is only a voltage source of 0 volt with very low resistance (ideally 0 but LTspice makes a very small value to be able to calculate). It is measuring the current without causing a power loss. It's voltage drop is ideally 0 Volt. You can replace it with a very low resistor (1 mOhm or less) if you like.

The voltage source on the left (not shown) is the signal source containing two superimposed sine waves of two different frequencies (f1, f2). I have put the two left and right sound card lines together to lower the output impedance of the output line (more current drive). You can also feed a single line voltage source if you have only one channel (mono channel).

Aziz

[Aziz]

Dual Frequency VLF - High Power Version

Hi all,

BTW, there are half a dozen other possible configurations possible. Next step is to make the power amplifier simple & efficient. And the two frequencies should be driven with pure digital clocks (makes easy interfacing to µC/DSP).

So we need a digital mixer. A simple way is doing it with two resistors. But this solution is not efficient and has power limitation.
Well, we can use complex resistors instead. Indeed, a center-tapped coil and two capacitors can be used as a good mixer.

Power amplifier? Why not using a mosfet-driver. Simple & cheap.

The main filter can be made more simple with two capacitors (Note: L1, LTX no tap). Below is a version for you to play with. Spice file included.
Cheers,

Aziz