VLF MD with digital signal processing : Bee-Buzz 1

[dbanner]

Try Octave it's prob closer in capability.

Octave compares favorably with MatLab. It's also free and open source and has good support. Capability seems to be on par with most of the functionality.

[Aziz]

https://www.analog.com/en/lp/001/cus...ad-cortex.html
Unrelated but impressive.

Hi dbanner,

529-ball grid BGA package
Neural Cash Cow -oops- DSP.
Typical marketing B$.

Not enough pins to start with!
I wouldn't touch a cpu/dsp under LGA1155 pin count.

Don't dream of such machines. You can have much better machines without using soldering iron.
Cheers
Aziz

[Detectorist#1]

Noise, noise, noise .... of the first stage in VLF and PI - now it is not problem in serious designs with now days OpAmps and new schematics. We have other problems in improving of our MDs.

[dbanner]

https://www.analog.com/en/lp/001/cus...ad-cortex.html

Unrelated but impressive.

[dbanner]

"Generation-recombination noise" in a transistor, particularly due to the base current, refers to a type of electrical noise that occurs when electrons and holes randomly generate or recombine within the semiconductor material of the transistor's base region, causing fluctuations in the base current and subsequently impacting the collector current, leading to a noisy output signal; this noise is most prominent at low frequencies and often exhibits a "1/f" (pink noise) characteristic."

"Compared to transistors on integrated circuits, discrete transistors generally exhibit higher levels of flicker noise due to the less controlled manufacturing process, resulting in more imperfections and variations in the semiconductor material, which are the primary source of flicker noise (also known as 1/f noise) in transistors; whereas, modern integrated circuits with advanced fabrication techniques tend to have significantly lower flicker noise levels due to tighter control over the manufacturing process and smaller feature sizes."

-Google AI overview

[Aziz]

Hi all,

in my special USB high latency case (Windows isn't an RTOS),
I have to decode even twice more to get the correct absolute phase lag between TX and RX. So I have to process the TX signal as well (besides the RX signal). Btw, any TX energy loss and frequency shift will be detected too. This gives more info for further processing.
For a true dual frequency VLF/LF detector with three narrow frequencies around the resonant frequencies, I need 12 decoders. This is no problem with CPU power on Tablet PC. But can be critical on embedded systems with micro controllers.

BTW, it is important, that the narrow band width should not be large, as we don't want to to operate the coils in the high Z region (Z is impedance of the LC-tank). We need some current flow through the TX coil of course. We are operating the TX coil in the low Z region around the resonant frequency fr.

Heavy mineralisation will increase the TX/RX inductance (and thus lower the resonant frequency) and targets will lower the inductance slightly (hence increases the resonant frequency). Magnetic field conduction TX -> RX occurs on ferro magnetic materials nearby the coil. Eddy current induction on metal targets too. All possible effects can be processed at the same time.

Aziz

[Atul Asthana]

Hi all,

just want to tell you, if you are going to make the best digital VLF detector with an embedded project, be prepared to transmit and decode at least 3 frequencies on a driven LC-tank (note: not free running LC-oscillator!). If the resonant frequency of the LC-tank is fr and you have a quite high Q coil, define a short narrow band frequency step df. df could be in range 50 Hz - 300 Hz for instance depending on your Q of your LC-tank.

fr = resonant frequency of the LC-tank

Transmit:
Transmit frequency 1: f1 = fr-df
Transmit frequency 2: f2 = fr
Transmit frequency 3: f3 = fr+df
The transmit frequencies can be driven via narrow band chirp wave form going from f1 to f3 or you could transmit them sequentially in a burst f1, f2, f3.

Decode:
Decoding frequency 1: f1 = fr-df
Decoding frequency 2: f2 = fr
Decoding frequency 3: f3 = fr+df

Then you have 3 magnitude and 3 phase information for single frequency LC resonant tank. Enough for good ground balancing and discrimination (resistive R and reactive X response extraction). A simplified version would process the magnitudes only.

Now, let's make it more:
If you make your LC-tank to resonate on two different frequencies (for instance: 12 kHz & 40 kHz), then you have 6 magnitudes and 6 phase informations. This should give even better results. But you need definitelly more processing power on your embedded project.

Oh well, I have really enough number crunching power on my Tablet PC. I could implement much much more.

Cheers,
Aziz

great,
even narrowband / high Q tank, excited by 3 or more frequencies, should be able to provide enough info, for you to calculate about more of target's properties.

my design has considered this point, despite the lower computational power stm32f103c8t6, which should be able to handle the three closely spaced frequency issue with moderate Q of the coil.

the resultant data can be processed with material science / electrical / magnetic properties to guess the material/metal.

[Aziz]

Hi all,

just want to tell you, if you are going to make the best digital VLF detector with an embedded project, be prepared to transmit and decode at least 3 frequencies on a driven LC-tank (note: not free running LC-oscillator!). If the resonant frequency of the LC-tank is fr and you have a quite high Q coil, define a short narrow band frequency step df. df could be in range 50 Hz - 300 Hz for instance depending on your Q of your LC-tank.

fr = resonant frequency of the LC-tank

Transmit:
Transmit frequency 1: f1 = fr-df
Transmit frequency 2: f2 = fr
Transmit frequency 3: f3 = fr+df
The transmit frequencies can be driven via narrow band chirp wave form going from f1 to f3 or you could transmit them sequentially in a burst f1, f2, f3.

Decode:
Decoding frequency 1: f1 = fr-df
Decoding frequency 2: f2 = fr
Decoding frequency 3: f3 = fr+df

Then you have 3 magnitude and 3 phase information for single frequency LC resonant tank. Enough for good ground balancing and discrimination (resistive R and reactive X response extraction). A simplified version would process the magnitudes only.

Now, let's make it more:
If you make your LC-tank to resonate on two different frequencies (for instance: 12 kHz & 40 kHz), then you have 6 magnitudes and 6 phase informations. This should give even better results. But you need definitelly more processing power on your embedded project.

Oh well, I have really enough number crunching power on my Tablet PC. I could implement much much more.

Cheers,
Aziz

[Atul Asthana]

I think, the task scheduler for the Bee-Buzz 1 is working
I will test its logic next week, when I have the time.
It will then be tested with the coils and opamps

This amounts to solving major portion of the process complexity.
Building rest of the hardware and software is much less complicated and more straight forward, with lower chances of errors.

[Aziz]

I think they may have limited the input BW to 20 khz .... this would be as for anti aliasing purposes at the lowest sample rate 44.1 Khz with 20 Khz being just below the nyquist or 22.05 khz.
I use 20 Khz in the FPGA detector ( it is VLF not PI ) and it detects .05 gram gold no problemo.
Users have reported some problems with the ASIO driver for this unit ... but the specs are excellent in the audio performance department.

Hi Moodz,

I have seen a frequency response of the Creative Sound BlasterX G6 somehere in the internet, which is obviously capable to detect signals close up to the nyquist frequency of 96 kHz (@192 kHz SR). It implements a wall-brick anti-aliasing filter. So I should be able to sample up to 80 - 90 kHz true analog signals. Right now, my Creative Sound Blaster X-Fi Surround can go up to 48 kHz bandwith with wall-brick anti-aliasing filter. So I'm able to operate up to 45-47 kHz signals with ease.

I don't use the ASIO interface. ASIO, WDM, DirectSound, DirectPlay is quite dead. WASAPI is the new cash cow. WASAPI is complex to use. M$ developers don't know the KISS-Principle. (M$: Keep It Super Strange! Keep It Super Struggle!)

My detector software is at least 20 years old and I don't really need low latency interfaces as I use two dedicated sound cards: One for signal processing (external USB sound card) and another one for beeping using the internal sound card with low sampling rates and shorter buffers for low latency. It isn't much time critical and I can use the high level API (MME API, WaveIn/WaveOut..). It works so far from XP to Win11. The use of external USB sound cards benefit from high SNR as they don't pickup much noise from the main board.

Your FPGA detector is really very sensitive.
It will be a challenge to outperform your's.

Aziz

[moodz]

Hi all,

I have purchased a new external USB sound card just to test new possibilities. Its the Creative Sound BlasterX G6.
I am going to test the 192 kHz SR. So we are leaving the VLF and entering the LF region (>30 kHz). I hope, they haven't band limitted the output and input lines to the standard audio range (up to 20 kHz). Bad companies and bad sound cards do this just to hide their bad performance.

The sound card has a discreate headphone amplifiers in it and is capable to drive upto 600 Ohm impedances. This gives the TX coil enough bang.
Cheers
Aziz

I think they may have limited the input BW to 20 khz .... this would be as for anti aliasing purposes at the lowest sample rate 44.1 Khz with 20 Khz being just below the nyquist or 22.05 khz.
I use 20 Khz in the FPGA detector ( it is VLF not PI ) and it detects .05 gram gold no problemo.
Users have reported some problems with the ASIO driver for this unit ... but the specs are excellent in the audio performance department.

[idhafa]

[Aziz]

Hi all,

I have purchased a new external USB sound card just to test new possibilities. Its the Creative Sound BlasterX G6.
I am going to test the 192 kHz SR. So we are leaving the VLF and entering the LF region (>30 kHz). I hope, they haven't band limitted the output and input lines to the standard audio range (up to 20 kHz). Bad companies and bad sound cards do this just to hide their bad performance.

The sound card has a discreate headphone amplifiers in it and is capable to drive upto 600 Ohm impedances. This gives the TX coil enough bang.
Cheers
Aziz

[Carl-NC]

The Gold Bug was designed by David Johnson. Most likely he thought he could design a better discrete opamp than what was available. The same thing was used in the CZ designs. David was insistent that the preamp needs to have ultra-low flicker noise even though a balanced demodulator cancels flicker noise. I suspect you can replace the discrete amp with any decent opamp and not see any difference.

Look up Gilbert Cell for ( you can get a chip NE602 NE612 I think ) ... they can achieve balanced mixing ...

The Bounty Hunter Red Baron (George Payne, 1977) used LM1496 analog mixers (Gilbert cells). Barry Gilbert worked at Analog Devices when I was there, chatted with him a few times. He was a master of bipolar design.

[Aziz]

Thanks Paul.
If I find my other preamp bread board with the NE5534, the comparison would be interesting too.
I'm sure, there isn't any remarkable difference.

I tend to upgrade my experimental detector software, just to show the simulated mixer outputs (time domain and frequency domain).
One can see, how the synchronous demodulator works in digital wourld.
Aziz