VLF MD with digital signal processing : Bee-Buzz 1

[Aziz]

Hi all,

the fastest way of implementing a fully digital direct sampling VLF is using the PC (Tablet) and an external USB sound card. At least, you can make the proof-of-concept easy and fast.
You have:
- an operating system (multi-core)
- GUI-API
- drivers (only API interfacing the signal data)
- display, keyboard, touchpad, Wi-Fi, blue-thooth, etc.
- lots of DSP-code written in C/C++ (open source)
- debugger (within Visual Studio for instance)
- no external HW required (no soldering iron) -> you can twist the input/output cables with a resonant capacitor and the coils if you like
- possibility to start immediately with coding

Think of it. This is a good advise.
Aziz

[Marchel]

Hi all,

the fastest way of implementing a fully digital direct sampling VLF is using the PC (Tablet) and an external USB sound card. At least, you can make the proof-of-concept easy and fast.
You have:
- an operating system (multi-core)
- GUI-API
- drivers (only API interfacing the signal data)
- display, keyboard, touchpad, Wi-Fi, blue-thooth, etc.
- lots of DSP-code written in C/C++ (open source)
- debugger (within Visual Studio for instance)
- no external HW required (no soldering iron) -> you can twist the input/output cables with a resonant capacitor and the coils if you like
- possibility to start immediately with coding

Think of it. This is a good advise.
Aziz

Yes, it's a simple solution, but the results will be very poor because without input preamplifiers and output mosfets on the TX it probably won't work. And the second thing is that the device would be difficult to carry outside and do ground tests with. I want my prototype PCB to contain 2 TX outputs for a sinusoidal signal and also for a square wave signal so that both types of coils can be connected there.

[Detectorist#1]

Hi Marchel,

What is the necessary output power of a VLF detector is interesting question for me. Too low power ( in case of using of USB sound card) maybe will be not OK. I'm sure, Carl knows the right answer.

[Marchel]

Hi Marchel,

What is the necessary output power of a VLF detector is interesting question for me. Too low power ( in case of using of USB sound card) maybe will be not OK. I'm sure, Carl knows the right answer.

It depends more or less on the TX winding and frequency. I use an XP Deus X35 coil and the total electronics draw is about 40mA. Other detectors I have seen draw something between 80-120mA from the battery.

[Detectorist#1]

Is it this current is draw from Tx circuit or from total detector with processor and all other electronics?

[Marchel]

Is it this current is draw from Tx circuit or from total detector with processor and all other electronics?

In my case, the total current is the same with the processor and all the electronics. This figure varies depending on the frequency used, I think the maximum current I measured was 60-80mA.

[Marchel]

Here are current measurements on the XP Deus X35 at two different frequencies.

[Marchel]

Now I did a current measurement because I wasn't completely sure of the current draw so the current at a frequency of 16.6kHz is 35mA in my electronics which I replaced in the X35 coil.

[Aziz]

The Tx power is enough. You can drive the Tx coil either differential or parallel output using the 2-ch stereo output. One can also take Earphone output instead of the line output, to increase the Tx-power.

Rx side can be rised with the capacitor (resonant Rx-tank) by at least 20 dB. No amplifier required.
Not producing any noise.

Oh well, we can add amplifiers too. But later.
On the Rx-side an ultra-low-noise preamp with 0.6 nV/sqrt(Hz) noise voltage density.

On the Tx-side, where the Tx voltage goes up to 1000 V if you want.
Aziz

[Aziz]

Hi all,

just checked my old USB sound card band width:
24 Bit at 96 kHz SR, input and output signals up to 45 kHz. The sound card is Sound Blaster X-fi from Creative Labs (USB Audio Class 1 only).

Effective number of bits (ENOB) is 20 bit, -120 dB noise floor (average). 1 uV detection possible.

VLF up to 45 kHz, direct sampling digital fancy stuff! Who has any doubts?
Aziz

[Marchel]

Hi all,

just checked my old USB sound card band width:
24 Bit at 96 kHz SR, input and output signals up to 45 kHz. The sound card is Sound Blaster X-fi from Creative Labs (USB Audio Class 1 only).

Effective number of bits (ENOB) is 20 bit, -120 dB noise floor (average). 1 uV detection possible.

VLF up to 45 kHz, direct sampling digital fancy stuff! Who has any doubts?
Aziz

The problem is that it is useless in the field, no one will carry a computer with them into the forest. In my opinion, the device must be light and compact.

[Detectorist#1]

This sounds too good to be true. What is the type of OpAmp with 0.6uV/sqrt(Hz) at 5 to 45 KHz? 1000V Tx voltage at VLF MD is not serious! But the hope dies last.

[Aziz]

Hi Detectorist#1,

This sounds too good to be true. What is the type of OpAmp with 0.6uV/sqrt(Hz) at 5 to 45 KHz? 1000V Tx voltage at VLF MD is not serious! But the hope dies last.

no joke. Seriously.
The preamp is a discrete AC amplifier. Allready realised and tested here somewhere.

I have built Tx-Oscillators or drivers, which were capable of driving huge coil current. The Tx voltage rises according to L*di/dt. No joke.

But you dont really need much Tx current. You are dealing with more noise and other problems then.

It is better to go without anything in the first stage. A preamp can be added easily later.

The simple "naked" system (without amplifiers) would outperform most VLF detectors. With ease! No joke.
Aziz

[Aziz]

The problem is that it is useless in the field, no one will carry a computer with them into the forest. In my opinion, the device must be light and compact.

Ok, I agree.

But you can make a detector, which is way beyond the scope of the current technology.

Once you have seen the possibilities, you won't going to do it the other way.

Aziz

[Marchel]

Ok, I agree.

But you can make a detector, which is way beyond the scope of the current technology.

Once you have seen the possibilities, you won't going to do it the other way.

Aziz

Yes, I agree, it's definitely about something else, but I could imagine it on a Raspberry PI.