Android Phone Based Metal Detector - FCMD

[moodz]

Nice Paul.

But my sound card is adding noise. And yes, the EMI noise is also present. With Q-factor beyond 100 I am working with with mono coil VLF, there is EMI noise. We can't ignore it. Even if my demodulator bandwidth is very very low (< 1 Hz).

This isn't an issue, if we use standard IB coil configuration with higher TX current however. This is very easy to get there.
But I am still working on mono coil VLF design.
Aziz

...hmm well you should know there is no physical system to achieve EMI cancellation using external coils or fairy dust within a 1 hz bandwidth.
The other problem is a that with a 1hz bandwidth the detector would be unusable due to lag.
I use 50hz as the token ouput rate on the FCMD detector ... so the bandwith is 25 hz.
The noise floor is around -110db to -120db ... the noise you do measure there is coming from the ADC reference ... not EMI.
You cant fix that with a coil ... it needs better components.
... also remember that EMI is not synchronous ... synchronous EMI can only come from your own system. No coils involved.
If you are picking up non synchrounous EMI then your code is broken.

[Aziz]

Nice Paul.

But my sound card is adding noise. And yes, the EMI noise is also present. With Q-factor beyond 100 I am working with with mono coil VLF, there is EMI noise. We can't ignore it. Even if my demodulator bandwidth is very very low (< 1 Hz).

This isn't an issue, if we use standard IB coil configuration with higher TX current however. This is very easy to get there.
But I am still working on mono coil VLF design.
Aziz

[moodz]

..so we have achieved good sensitivity for target detection in FCMD.
We are now getting the corresponding accuracy in discrimination ( without phase rotations or reference phases I might add ! )

The phase accuracy is not impacted by GB either. ( though amplitude will drop a bit for some targets aligned in the phase plane with the ground. )

Below we have copper, silver, gold, al foil, ferrite.
The orange ring is the threshold setting ... anything exceed the threshold gets a phase tag.

Theres no EMI ... because EMI is not a big issue in narrowband systems ( Aziz )

[moodz]

Yes, I also use these STM32H7 boards and they are really great, the STM32WB board is good for wireless applications for transmission via Bluetooth or the LLD method which works similarly to the NRF24L01, the only disadvantage of these chips is that they do not have a 16bit ADC like the STM32H7 has.

That is the point I was trying to make.
Yes there are boards that have great bluetooth or whatever ... .but are you trying to build a metal detector or a great wireless box ?

A good and cost effective method of capturing the target signal ( ie ADC in this case ) ...is way higher priority in the design criteria than a wireless system.

[Marchel]

I would not bother with an external ADC unless it was 24 bit or more. The ADC implemented on the H7 series is a "proper" 16 bit SAR ADC.
On the H743 there are 3 of these onchip ADCs ..
Below is the demodulated ( 0 - 25 hertz ) noise floor of the current firmware running on the chip with the coil connected and the TX running in a noisy lab environment.

we are interested in around the 5 hertz point ( target sweep speeds ) ... the adc is better than -120 db in noise performance. This rivals alot of so called 24 bit converters.
The real benefit is the tight integration of the ADC to the CPU ... and the H743 is a cheap chip ..
you will be hard pressed to build a better combo unit using an external ADC that is anywhere near this price point and performance.

Yes, I also use these STM32H7 boards and they are really great, the STM32WB board is good for wireless applications for transmission via Bluetooth or the LLD method which works similarly to the NRF24L01, the only disadvantage of these chips is that they do not have a 16bit ADC like the STM32H7 has.

[Aziz]

Hi Paul,

yes, the STM32 platform will allow true real-time DSP processing. Some very special real-time algorithms can be made possible (latency much lesser than 1 ms). For instance a real-time software PLL implementation for demodulating the RX channel via the TXref channel.

As you have 3 ADC channels, you could use one for TX-ref and one for RX channel. With a relative High-Q LC tank in IB-configuration (Q around 10 is enough), you can process the target response on both channels. And a big plus: Active EMI noise cancellation as the TX and RX coils will pick up EMI too, which can be detected and eliminated.

I think such a complex IB design processing has not been done yet.
The STM32 platform will make it easily possible with less problems which I am facing with the sound card solution at the moment.
Cheers

[moodz]

I started testing this STM32 wireless board and it is also very good, just need at least 16bit external ADC. For a metal detector that will work with a mobile phone via Bluetooth, it is a good solution.

https://www.aliexpress.com/item/1005...origin_prod%3A

I would not bother with an external ADC unless it was 24 bit or more. The ADC implemented on the H7 series is a "proper" 16 bit SAR ADC.
On the H743 there are 3 of these onchip ADCs ..
Below is the demodulated ( 0 - 25 hertz ) noise floor of the current firmware running on the chip with the coil connected and the TX running in a noisy lab environment.

we are interested in around the 5 hertz point ( target sweep speeds ) ... the adc is better than -120 db in noise performance. This rivals alot of so called 24 bit converters.
The real benefit is the tight integration of the ADC to the CPU ... and the H743 is a cheap chip ..
you will be hard pressed to build a better combo unit using an external ADC that is anywhere near this price point and performance.

[moodz]

Hi Paul,

maybe I should buy a few STM32H743VIT6 boards to be able to use your ultimate detector soon.
The STM32 boards are really quite interesting.
Cheers

The sound card is interesting for playing with the math .. but you need low level access to timings and data to achieve real results and the STM32 boards allow that for low cost.

[Marchel]

I started testing this STM32 wireless board and it is also very good, just need at least 16bit external ADC. For a metal detector that will work with a mobile phone via Bluetooth, it is a good solution.

https://www.aliexpress.com/item/1005007119406784.html?spm=a2g0o.productlist.main.2.5648685ebgbTpw&algo_pvid=536b9731-4d44-42df-bd4e-58cd4b8f422d&algo_exp_id=536b9731-4d44-42df-bd4e-58cd4b8f422d-1&pdp_ext_f=%7B%22order%22%3A%223506%22%2C%22eval%22%3A%221%22%2C%22fromPage%22%3A%22search%22%7D&pdp_npi=6%40dis%21EUR%217.04%217.04%21%21%2155.55%2155.55%21%40211b813f17723853982564008eb10b%2112000039466790734%21sea%21SK%21178235430%21X%211%210%21n_tag%3A-29919%3Bd%3Ae8cbfba2%3Bm03_new_user%3A-29895&curPageLogUid=dTWTCt2UBQN8&utparam-url=scene%3Asearch%7Cquery_from%3A%7Cx_object_id%3A1005007119406784%7C_p_origin_prod%3A

[Aziz]

Hi Paul,

maybe I should buy a few STM32H743VIT6 boards to be able to use your ultimate detector soon.
The STM32 boards are really quite interesting.
Cheers

[moodz]

hi mr moodz.
thank you for your nice & exciting work.
congulation.
i'd like to make it, but unfortunately i cant find your app.
would you please out it here ???????
please help me.
so so thanks.
best regards

... thankyou for your comments .. this is still a work in progress.
I am just reporting where I am up to in the project.
It is not ready for release yet.

moodz

[syavash]

Here is the FCMD app video .... the video was made by the app itself ( no editing ).
This makes detector videos super easy ..

hi mr moodz.
thank you for your nice & exciting work.
congulation.
i'd like to make it, but unfortunately i cant find your app.
would you please out it here ???????
please help me.
so so thanks.
best regards

[moodz]

Here is the FCMD app video .... the video was made by the app itself ( no editing ).
This makes detector videos super easy ..

[moodz]

The HC05 is just a stop gap measure to send some data to the app for testing ... the software is the priority. Wired headphones are fine for now.

[JoyJo]

But in the very idea of using Bluetooth, aren't you confused by the delay in sound transmission? At the moment I know only 1 fairly good chip with support for fast codecs and at a reasonable price: csr8670. By the way, it is installed on equinox. However, there is another chtp CC8530 on the equinox. They work on one antenna and switch with a high-frequency radio switch. But that's not the point. And the essence is the delay of sound transmission that Bluetooth has. Nrf24l01 has a very low delay even in comparison with csr8670.