VLF MD with digital signal processing : Bee-Buzz 1

[moodz]

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!

Some designs seem to have focussed on low noise preamps however there is point at which the noise contribution from the amplifier will not be the significant factor in the signal chain. With VLF detectors we are generally talking about a demodulation system that is trying to extract magnitude and phase changes in one or more frequencies and this will involve the use of a mixer to demodulate the target caused amplitude change and phase change. Also we could note that most of the noise contribution in opamps is below 1 khz generally .. so if we have a transmit / recieve frequency of 20 khz and we only desire a bandwidth of +/- 20 Hz ( target modulation BW ) then the noise for most opamps is going to be fairly low and its going to be random ( in this bandwidth ). This can be proved by measuring the noise in a 20 hertz bandwidth @ 20 Khz on a spectrum analyser that can do this ( eg high spec audio bode analyser ).

So the question is "why did they use these low noise frontends ?" ... (and I was not in the lab looking over the designers shoulder when they made these decisions ) but I will put my money on "mixer leakage" and where they have used a simple "chopper" style demodulator which most designs use ... the presence of low frequency noise ( ie down to DC ) which is within the target demodulated BW at the input to the mixer will pass through the mixer due to low "port isolation" and will appear in the target signal. Of course if you have ever worked with radio designs that is one of the reasons why they invented double and triple balanced mixers to reduce this effect amongst other things.

With an ideal mixer where the input desired frequency is Fc and there is some undesired out of band low frequency noise Fn and we are using a local oscillator of Fo then the ouput of the mixer will be (Fc - Fo) + (Fc + Fo) + (Fo + Fn) + (Fo - Fn). From this we see that the low frequency noise is now translated onto the Local Oscillator carrier frequency as Fo +/- Fn . Eg if our noise frequency is 60 hertz and our Lo is 20 Khz then the ouput of the mixer will be 19.940 khz and 20.060 khz.
Using an "ideal" mixer these two noise frequencies are easily removed with a low pass filter. If the port isolation of the mixer is low then the 60 hertz will appear at the op of the mixer and this is a problem.

Where can you get an "ideal" mixer ... well you can come pretty close in a direct sampling scheme where you sample with an ADC then do the mixing "digitally". You can do it in the analog world but you would need more sophistication than a simple chopper switch tyep demod.

[dbanner]

CubeMX CubeIDE is c?, c++?
MX generates dot C file for chain?

[Aziz]

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!

You have misunderstood me. The discrete RX front end is a standard discrete Op-Amp design. With the differential input stage, buffer, and driver output stage at the end. This is a differential amplifier design such as all standard op-amps are.
Aziz

[dbanner]

Maybe Fisher approach was " we've got surplus bags of thousands of transistors in the stock room, what pray can we do with them?"
Opamps were pricey in those days?
Although matching transistors to a level of precision would be a little time consuming.

[Aziz]

Hi all,

Android systems with USB-C interface support seems to be very interesting now. USB Audio Class 1 (UAC1) is already supported. Maybe USB Audio Class 2 (UAC2) too or very soon. Newer Smartphones and Tablets support now USB-C.

This is becoming very very interesting. Android systems are cheap. Damn cheap. You can use a smartphone or 6 inch Android tablet. You have a lot of processing power. An external USB sound card will be required for signal processing. The internal sound of the smartphone or tablet should be used for beeping. You have incredible possibilities. This is it.
This is very very interesting now.
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!

[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]

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.

[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.

[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/1005006266345748.html?spm=a2g0o.productlist.main.1.192aWteQWteQAH&algo_pvid=4ad6decc-20c3-46d9-b348-e39a25daf301&algo_exp_id=4ad6decc-20c3-46d9-b348-e39a25daf301-0&pdp_npi=4%40dis%21EUR%212.02%211.82%21%21%212.03%211.83%21%40211b61d017370215603661125e2e1d%2112000036532239367%21sea%21SK%21178235430%21X&curPageLogUid=MxKsW0xY0fwQ&utparam-url=scene%3Asearch%7Cquery_from%3A

https://vi.aliexpress.com/item/1005007253095259.html?spm=a2g0o.productlist.main.5.38927e9099jNFT&algo_pvid=323ca5b0-687c-462e-a7da-50a697441821&algo_exp_id=323ca5b0-687c-462e-a7da-50a697441821-2&pdp_npi=4%40dis%21EUR%213.92%213.92%21%21%213.94%213.94%21%40210385a817369492820046626eb6f8%2112000039955386736%21sea%21SK%21178235430%21X&curPageLogUid=ehyYZnX6yKcB&utparam-url=scene%3Asearch%7Cquery_from%3A

[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

[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

[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?

[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.

[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