VLF MD with digital signal processing : Bee-Buzz 1

[moodz]

yes, good amount of information in the project document.

Have you made it? hows the performance?

In our case, we are still discussing an exploratory project, where everything is under scrutiny and discussion, and nothing is concretised yet. once we are clear about the hardware, methodology etc, it will be converted into a project document.

I was looking for explicit references to the noise figures, what noise figures have these students quoted in this project?

btw, I read that esp32's adc is not very linear and this reduces senstivity, injects harmonic noise.

No I have not made it ... but Ivica might like it because it uses ESP32 .
IMHO Noise specs are not relevant till the noise is specified with respect to bandwidth ... and also if the noise is synchronous with the wanted signal elements. Generally speaking non synchronous wideband noise has little effect on the wanted synchronous components. - in oversampling schemes .. noise at the input actually helps.
A wideband software defined radio has no preselectors at its wideband input but can resolve less than 1 Hertz of bandwith signal in an input bandwidth spanning 10s of Mhz. Lock in amplifiers are an extreme example of this and can recover signals below the wideband noise floor at the input ... in fact the lock in amp will recover the phase and amplitude of a signal well below the input noise floor ( if the TX phase and frequency is known --- CRITICAL POINT 1 ). If the signal is very weak then you just integrate the phase and amplitude for longer. Sampling ADCs use a modulator to do the sampling .... the sampling frequency adds convolution noise to the samples .. almost impossible ( cant be filtered ) to remove once its in the data so care in choice of sampling frequency for high performance systems..CRITICAL POINT 2. The sweep speed of the coil determines the target detection passband ( for most handheld detectors this will be from DC to 10 hertz for example ) this is the demodulation target bandwith CRITICAL POINT 3. Has to be DC so phase and amplitude polarity can be resolved without reference to an artificial tracking level... most detectors cant do this because integrator baselines drift all over the place. ( hence motion and non motion ).
moodz

[Atul Asthana]

great, good that you joined in the discussions

[ivconic]

No I have not made it ... but Ivica might like it because it uses ESP32 .
IMHO Noise specs are not relevant till the noise is specified with respect to bandwidth ... and also if the noise is synchronous with the wanted signal elements. Generally speaking non synchronous wideband noise has little effect on the wanted synchronous components. - in oversampling schemes .. noise at the input actually helps.
A wideband software defined radio has no preselectors at its wideband input but can resolve less than 1 Hertz of bandwith signal in an input bandwidth spanning 10s of Mhz. Lock in amplifiers are an extreme example of this and can recover signals below the wideband noise floor at the input ... in fact the lock in amp will recover the phase and amplitude of a signal well below the input noise floor ( if the TX phase and frequency is known --- CRITICAL POINT 1 ). If the signal is very weak then you just integrate the phase and amplitude for longer. Sampling ADCs use a modulator to do the sampling .... the sampling frequency adds convolution noise to the samples .. almost impossible ( cant be filtered ) to remove once its in the data so care in choice of sampling frequency for high performance systems..CRITICAL POINT 2. The sweep speed of the coil determines the target detection passband ( for most handheld detectors this will be from DC to 10 hertz for example ) this is the demodulation target bandwith CRITICAL POINT 3. Has to be DC so phase and amplitude polarity can be resolved without reference to an artificial tracking level... most detectors cant do this because integrator baselines drift all over the place. ( hence motion and non motion ).
moodz

Don't think I overlooked that!
After returning from downtown this morning, I immediately researched that project.
Interesting because it is simple to make. I have a surplus of ESP32 modules...
BTW Thanks for posting the link!

Happy New Year to You and ALL other good people!
...
But at the same time, this also caught my attention: https://x.com/Alexand36635968/status...ja-foto%2Fvest
But this comment knocked me off my chair:

[Atul Asthana]

Here are some details I managed to gather about the Garrett AT Pro (current internet price of around USD 600/-) which appears to be similar or more advanced version of what we are aiming for. However, I am uncertain about the accuracy of this information below. I also came across mentions of attempts to reverse engineer the Garrett AT Pro.

I couldn't find specifics about the weakest target it can detect or the total gain in its signal chain. That said, it might be possible for us to achieve similar performance with our detector.

While I am unsure about the overall performance of this detector compared to current detectors in the same price range, we could attempt to compare its technical specifications with our expected performance.

The Garrett AT Pro employs advanced digital signal processing (DSP) to detect and discriminate metallic targets. It begins with the RX coil capturing weak electromagnetic signals, which are amplified and digitized using a 12-16 bit ADC, sampling at ~100 kHz. A microcontroller or DSP (likely an ARM Cortex-M4/M7) processes this data in real-time.

Digital filtering isolates the detector's operating frequency (15 kHz) while removing ground noise and interference using adaptive algorithms. Phase and amplitude analysis, often involving cross-correlation and Hilbert transforms, determine the target's conductivity. Proprietary lookup tables and pattern recognition algorithms identify specific metals by comparing signal characteristics to pre-stored profiles.

Depth estimation is achieved by analyzing signal attenuation, while proportional audio tones reflect the target’s depth and conductivity. Advanced features include Iron Audio, which uses separate signal paths to detect ferrous materials, and Ground Balance, which compensates for mineralized environments by dynamically adjusting ground phase response.

The hardware includes a crystal oscillator for the TX coil, low-noise amplifiers for signal conditioning (e.g., TL072/OPA2134), and efficient power management for portable operation. Digital signal pipelines involve ADC, band-pass filtering, and discrimination, culminating in real-time visual and audio feedback. The exact chipsets and algorithms are not in the open.

Technical Details:

• Submersible: The AT Pro is submersible to 10 feet, making it suitable for underwater use in lakes, rivers, and even saltwater beaches.
• Frequency: The AT Pro operates at 15 kHz, which is generally considered a good frequency for finding both gold and silver coins.
• Coil: The AT Pro comes standard with an 11-inch DD coil, but other coil options are available, including an 8-inch DD coil and a larger concentric coil.
• Ground Balance: The AT Pro has an automatic ground balance feature that makes it easy to adjust to different soil conditions.
• Discrimination: The AT Pro has adjustable discrimination settings that allow you to filter out unwanted targets like iron.
• Notch Feature: The AT Pro has a notch feature that allows you to eliminate specific targets, such as pull tabs, from your detection.
• Iron Audio: The AT Pro has an iron audio feature that allows you to hear the sound of iron targets, even when they are being discriminated out.
• Pinpoint: The AT Pro has a pinpoint feature that helps you locate the exact position of a target.
• Depth Indication: The AT Pro provides an estimated depth reading for detected targets.

Performance Details:

• Depth: The AT Pro demonstrated good depth performance in the video, detecting a quarter at 10 inches and a dime at 8 inches in challenging soil conditions.
• Sensitivity: The AT Pro is a sensitive detector, capable of picking up small and deeply buried targets.
• Recovery Speed: The AT Pro has a fast recovery speed, allowing you to quickly detect multiple targets in close proximity.
• Stability: The AT Pro is a stable detector, even in high-mineralization areas.
• Ease of Use: The AT Pro is relatively easy to use, with simple controls and a user-friendly interface.

points to be noted (differences and similarities) :
1. tx frequency is 15 khz as against 6 khz of ours, if recommended, we can also change to some higher frequency.
2. rx sampling is around 100 ksps (I guess, must be around 120 khz : 8 times per sinewave.
3. tx sine is generated by xtal.
4. adc could be 12 to 16 bits.
5. an ARM processor is used.
6. uses adaptive filtering.
6. uses hilbert transform/FFT for phase+amplitude detection.
7. uses pattern recohnition and lookup tables for target identification.
8. dynamic ground balancing.

please comment on our design, and changes that we should carry out, without overwhelming the processor.

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[Detectorist#1]

Hi Atul,

It is interesting how low noise OpAmp needs VLF MD for first Rx stage. You mention using of TL072 ( 18nV sqr Hz at 1KHz) and OPA2134 (8nV sqr Hz at 1KHz) is possible. How to calculate the real value of the voltage noise of OpAmp allowed to use for first Rx stage in VLF MD?

[Atul Asthana]

Hi Atul,

It is interesting how low noise OpAmp needs VLF MD for first Rx stage. You mention using of TL072 ( 18nV sqr Hz at 1KHz) and OPA2134 (8nV sqr Hz at 1KHz) is possible. How to calculate the real value of the voltage noise of OpAmp allowed to use for first Rx stage in VLF MD?

The RX signal in the coil already contains some noise, which gets amplified in the first op-amp, along with the addition of the op-amp's own noise. The critical factor is the required SNR at the signal extraction stage to recover a reliable signal. To determine the allowable op-amp noise, you must work backward, accounting for all noise sources in the chain, ensuring the first op-amp's noise is low enough to maintain the target SNR.

Thumbrule : get the opamp with the lowest possible noise, at your frequency of interest, that you can afford.

The opamps you mentioned, are likely to have been used in this Garret detector.

[Carl-NC]

The AT Pro is not a direct sampling design, it uses analog demods. Micro is likely a Kinetis Cortex M0 @ 48MHz. ADC is 16b. The TX is driven by a relaxation oscillator using an LM386.

[Carl-NC]

How to calculate the real value of the voltage noise of OpAmp allowed to use for first Rx stage in VLF MD?

Suppose you want to direct-sample with an ADC with 16 ENOBs and a 3V range. The SNR is 98dB; the max signal is 3vpp or 1.06v rms so the noise floor is at 13.7uv rms. Let's say you have a preamp gain of 100; now the input-referred noise is 137nv rms. Let's also say that the eventual signal BW is 50Hz; this makes the input-referred noise density 19.3 nV/rtHz. But you want the preamp noise to be lower than this, by at least 3dB, so that becomes 13.7 nV/rtHz. This will be dominated by the opamp voltage noise and the input resistor noise.

[ivconic]

The AT Pro is not a direct sampling design, it uses analog demods. Micro is likely a Kinetis Cortex M0 @ 48MHz. ADC is 16b. The TX is driven by a relaxation oscillator using an LM386.

At ACE series too.


I always wanted to try it on some of my diys, yet never did it.





I also have .txt on my disk containing the inscription, can't remeber what's this and why am i keeping it.
Could it be the inscription on the mcu?

GARRETT
8155300 E OM48Z
CTYHBK15320
GARRETT 8155300 E 0M48Z

[Carl-NC]

Yes, Garrett has the NXP factory custom-label their micros.

[Detectorist#1]

Thank you Carl. Also the opinion of Atul for this calculation will be interesting.

[Altra]

While you are on the AT and Ace series. They use parallel adc inputs to increase the dynamic range. Not sure if this idea could be applied to direct sampling?

[Detectorist#1]

Maybe Carl knows the true in this situation.

[Carl-NC]

I didn't think either one used paralleled ADC inputs. That's most useful with a true multi-channel simultaneous-sampling ADC, which the micro ADC is not.

[Detectorist#1]

Altra also not knows the final solution of the proposed project. This task is more complex than AMX project where no requirements for the final price. Proposed aim from Altra is for good and low price solution at the same time. Not easy task!