VLF MD with digital signal processing : Bee-Buzz 1

[Carl-NC]

I asked some of the engineering students to build some of the oscillators of vlf schematics, power them in the air and close to the ground, also analyse the circuits through simulators on the maximum possible power consumptions.

I'd be interested in an example. In general, the power consumption of the TX circuit has little to do with the transmitted energy except in a total-loss PI design. And the amount of power consumed by ground mineralization depends heavily on the loss angle and susceptibility of the soil. Most mundane soils have a near-0° loss angle so the power loss probably isn't measurable. What is the loss angle of the local soil? In the absence of ground losses or in a sim, how do you measure TX power?

[Detectorist#1]

Thanks Carl,

I understand that your answer supports my opinion that 0.6nV/sqrt(Hz) noise at front-end stage is not useful in VLF MD.

[Carl-NC]

I would not think so. Post 83 has an example calculation; if you instead use -120dBfs noise floor and NBW = 100Hz then you need a preamp with 0.75nV/rtHz. You can get close to this with an off-the-shelf opamp but, personally, I've never seen any metal detector with such an aggressively low preamp noise.

[moodz]

Guys ... the noise contributed from the preamp is the least of your concerns ... the environmental noise from the coil etc is orders of magnitude higher.
Below is a real pic of the noise off the coil ( on the CRO ) and the raw sampled signal from the ADC on a VLF frontend. The actual signal in this case is 19 khz and the magnitude is about 10 mV peak.
The noisy input is around 600 mV peak.( so the 10 mV signal is buried in there )

Yes you can see some noise on the ADC sampled signal o/p ( red trace VGA screen ) but let me spell it out ..
Its S-Y-N-C-H-R-O-N-O-U-S noise ... which is completely different from asynchronous noise ( AKA white / pink / shot / etc etc whatever noise etc ) which is the type of R-A-N-D-O-M noise you get in an opamp.

Previous threads have bogged down in who can find the lowest noise opamp .... move on!

[Atul Asthana]

I'd be interested in an example. In general, the power consumption of the TX circuit has little to do with the transmitted energy except in a total-loss PI design. And the amount of power consumed by ground mineralization depends heavily on the loss angle and susceptibility of the soil. Most mundane soils have a near-0° loss angle so the power loss probably isn't measurable. What is the loss angle of the local soil? In the absence of ground losses or in a sim, how do you measure TX power?

Carl,

I think, I'll ask the profs of the students to verify, however, the figeures here are for various kinds of soils, including high iron and other metals content soils, and the measurements are the differences in current consumption before and after immersion.

importantly, in my case, power is controllable and any errors will be corrected by saturation analysis.

I do expect that core loss in the soil is both reactive and resistive to the extent that it is measurable, and for a free running oscillator, this will cause frequency shift/detuning/change in inductance. (thats how bfo mds work).

[Aziz]

Hi all,

I also will provide you with some real measurements on my USB sound card soon. I haven't found my coil, cables and connectors yet.
Noise floor spectrum of the ADC system, RX coil connected EMI spectrum, preamp noise spectrum, etc.
My soldering iron is still cold.
Aziz

[Atul Asthana]

Hi all,

I also will provide you with some real measurements on my USB sound card soon. I haven't found my coil, cables and connectors yet.
Noise floor spectrum of the ADC system, RX coil connected EMI spectrum, preamp noise spectrum, etc.
My soldering iron is still cold.
Aziz

excellent. Aziz

that should give us some real data to design with, and help in creating a better detector.

[Atul Asthana]

Guys ... the noise contributed from the preamp is the least of your concerns ... the environmental noise from the coil etc is orders of magnitude higher.
Below is a real pic of the noise off the coil ( on the CRO ) and the raw sampled signal from the ADC on a VLF frontend. The actual signal in this case is 19 khz and the magnitude is about 10 mV peak.
The noisy input is around 600 mV peak.( so the 10 mV signal is buried in there )

Yes you can see some noise on the ADC sampled signal o/p ( red trace VGA screen ) but let me spell it out ..
Its S-Y-N-C-H-R-O-N-O-U-S noise ... which is completely different from asynchronous noise ( AKA white / pink / shot / etc etc whatever noise etc ) which is the type of R-A-N-D-O-M noise you get in an opamp.

Previous threads have bogged down in who can find the lowest noise opamp .... move on!

great analysis,

this helps a lot in finding strategies to minimise environmentsl noise. I belive, you've found the most important design point, that will require me to redesign some parts of Bee-Buzz 1.

if you can provide more information for me to guess the sources of this noise :
was your coil shielded or one end connected to the ground plane?
what was the testing environment ? lots of high voltage ac lines running around? tube lights/electronic powersupply for LEDs? some motors running? power supply isolated? any electromagnetic / rf transmission stations around? your mobile phone around? even an electronic watch on your wrist? your music system powered up? electronic fan/motor regulators? unfiltered power lines?

[Aziz]

Here is some information about a PC-based metal detector and there should also be a link to the source code if you're good at it, you just have to register.

http://md4u.ru/viewtopic.php?f=11&t=3772

Hi Marchel,

this topic was an interesting reading.

Apart from having a bad google translator, the synchronous decoding is made via Lockin-Amplifier. And a lot of discussion about synchronisation problems on a non real-time operating system.
At the end, the developer is not happy about the ergonomy of the system. Oh well, that's really true.

6 inch Tablet PC or 6 inch mini PC with display and touch screen could be perhaps better.
Something like HEIGAOLA Mini-PC Win 11 Pro. Android systems are way cheaper. But I don't know, whether they support external USB sound cards. Anyway. Thanks for the link.
Aziz

[Marchel]

Hi Marchel,

this topic was an interesting reading.

Apart from having a bad google translator, the synchronous decoding is made via Lockin-Amplifier. And a lot of discussion about synchronisation problems on a non real-time operating system.
At the end, the developer is not happy about the ergonomy of the system. Oh well, that's really true.

6 inch Tablet PC or 6 inch mini PC with display and touch screen could be perhaps better.
Something like HEIGAOLA Mini-PC Win 11 Pro. Android systems are way cheaper. But I don't know, whether they support external USB sound cards. Anyway. Thanks for the link.
Aziz

I'm glad you liked my link

[Marchel]

Hello everyone

I started working on the PCB design that I wrote about in previous posts. On the top PCB where the buttons will be, I added a small I2C 1.3" OLED display which will be quite nice and I'm still thinking about adding this audio amplifier which can process both analog and digital signals which I think would be useful too. I'm attaching the file for STM32CubeMX so you can see the output connections and write your objections what would be good to change. The board on which all this will run has one minor problem because it doesn't have direct ADC pins PC2_C and PC3_C connected which is a bit of a shame. The top PCB is designed completely and I'm still working on the main bottom PCB.

https://www.ti.com/product/TAS2521

https://vi.aliexpress.com/item/10050...Cquery_from%3A

https://vi.aliexpress.com/item/10050...Cquery_from%3A

[moodz]

great analysis,

this helps a lot in finding strategies to minimise environmentsl noise. I belive, you've found the most important design point, that will require me to redesign some parts of Bee-Buzz 1.

if you can provide more information for me to guess the sources of this noise :
was your coil shielded or one end connected to the ground plane?
what was the testing environment ? lots of high voltage ac lines running around? tube lights/electronic powersupply for LEDs? some motors running? power supply isolated? any electromagnetic / rf transmission stations around? your mobile phone around? even an electronic watch on your wrist? your music system powered up? electronic fan/motor regulators? unfiltered power lines?

Yes the coil is a commercial type with faraday shielding ... there are plenty of magnetic noise sources within 1 - 10 meters of the coil. LED light is the closest at around 1.5 meter and solar inverter at 10 meters. But many other sources like AC switchmode plugpacks , wiring in the walls etc. However the point here is that all these sources are non synchronous to the 19 Khz transmit frequency and if the FFT is taken you can see the 19 Khz carrier with about 30 db signal to noise @ 19 khz amongst all the other noise sources ( noting that some of the noise carriers at other frequencies are 10 db above 19 khz ) . So for preamp selection one of the main criteria is the noise contribution at 19 Khz ( or whatever your TX frequency is ) and the linearity of the amp as non linear amplification can cause intermods to affect your frequencies of interest. Most op-amps are OK in this regard so there is no need for expensive ultra low noise opamps in this application.

[Detectorist#1]

In this case, how to explain that Fisher Gold Bug working at 19.2KHz use as first RX stage very complex OpAmp completely designed at separate transistors? I think, the aim of this is very low noise.

[Aziz]

In this case, how to explain that Fisher Gold Bug working at 19.2KHz use as first RX stage very complex OpAmp completely designed at separate transistors? I think, the aim of this is very low noise.

Hi Detectorist,

in Fisher Gold Bug RX front end it is a more or less complex differential amplifier. Gain is defined by G=1+ R14/R8, with inherent band pass filtering using capacitors C5, C4. At the input there is a high pass filter (C1, R1). So it is AC-coupled.
This isn't an ultra low noise design. But quite low noise.
The preamp I have shown outperforms it. By simplicity and noise specs.
Aziz

[Detectorist#1]

Hi Aziz,

I wrote "very low noise", not "ultra low noise". Explanations for R8,C4,R14,C6,C1 and R1 are more than obvious. This part of schematic is connected as noninverting OpAmp, not as differential amplifier. For me will be interesting answer from Moodz at my question, sorry!