VLF vs PI SHOOTOUT PROJECT

[Carl-NC]

Coils do have flat voltage response...

Ideal coils have a flat voltage response. Add in distributed parasitics, and they don't any more.

[Davor]

... but deviation from ideal in terms of -3dB range spans well over a decade.

[Carl-NC]

Yeah, I'd agree that with some effort you can get a good decade of decent flatness. I built a wideband coil intended for 2k-50k, and it worked really well up to 30-35k, beyond that was difficult.

[Altra]

I built a wideband coil intended for 2k-50k, and it worked really well up to 30-35k, beyond that was difficult.

That is what I have experinced. At about 30khz things change and become more critical.

But, I can still play with a fundimental, 3rd and 5th without much trouble under 30khz.

[moodz]

Hi Moodz,

I am well aware of the sweep function. The challenge is to have TX and Rx stages with a flat response across sweep range.

Regards

I thought you would be ....

I work in digital video nowadays but in the "old analogue" days we used to make up equalisation circuits for this type of application that were certainly flat to around 3.5 Mhz and good to around 5.5 Mhz ....this type of equalisation was used to send color video over 2 - 3 Kms of twisted pair phone line ( ie your regular phone line ). To keep the color accuracy and detail the phasing and amplitude distortion had to be reasonably low .... so equalising a coil for say a max top frequency of 500 Khz should be a piece of cake.

[Altra]

Sounds good Moodz, you can give me some pointers on equalization when I get the software worked out. The chip also has pga's on the
Tx and Rx which could be used to compensate for roll off. Although they don't have much resolution. I think the data could also be weighted?

[medj]

Thank you for your responseBut there is no answer to my questionI'm tired valuable making the circuit and I do not want to fatigue in making Rex TexI will put you pictures of the deviceI want the owners of the experience that show me the rolls of wire with a diameterCan I change my cut or notAnd that the device is good or notAnd machine-made by giving me information about itYou sincerely
http://up13.up-images.com/up/viewimages/888f4a74b1.bmp
http://up13.up-images.com/up/viewimages/ecd40072dd.bmp
http://up13.up-images.com/up/viewimages/d6a856b69d.bmp
http://www.4shared.com/rar/XY5vQRbi/delta_01.html

[mikebg]

....My application idea for this chip is similar to the Geophex GEM-2.
It scans a number of frequencies and then plots a composit based
on resistivity and phase. More info here
http://www.geophex.com/Publications/...uly-August.pdf

Altra,
If you use information from this paper, please don't use their loop configuration. The authors don't know that search head shoul have induction balanced loops and should be moved close to ground.

I expect you to show how they look the frequency characteristics of different objects: coins, rings, bracelets, nuggets and hot rocks.

[Altra]

Altra,
If you use information from this paper, please don't use their loop configuration. The authors don't know that search head shoul have induction balanced loops and should be moved close to ground.

Your statement is wrong. At this point I think you are pushing buttons to get attention!

These guys know exactly what they are doing. Balanced and unbalanced designs.

http://www.geophex.com/publications.html

http://technology.ssc.nasa.gov/suc_s...d_sensors.html

Patents
US5963035
US5557206
US6204667

[medj]

First, thank you for sitesSecondly, I want attention for the projectFor the benefit of everyone hereWhen he finished the project lift it here on the site I'm not selfish like someToday I Make a device that the device does not work well Tell that he was not goodYou make the other and tell that good or notBut we must not lose total
We are here to exchange expertise and experiences
Thank you for your interest in my brother
..........................
It is importantI want to inquire There is no reference in the projectBut if you put one hand fingers on the base of the transistorAs in the pictureThe device captures a signal metalReference 1 meter and once 20 cm

[moodz]

...oh why does AI have to be invented ( or made useable ) just when I retired ??? I wish it was 20 years ago LOL.

anyway I did have time to see what it could do with an old android phone.
It generates 12 log spaced frequencies from 1 Khz to 48 Khz then recieves the same on L and R input of audio jack and demodulates IQ phase / amplitude information in real time.
I am using an OPPOA91 which has an octocore 1 ghz arm core ( i could be wrong ) and it does the full IQ send / demod on 12 frequencies at an update rate of 47 Hertz. ( the ADC is running at 96 ksps )

The app was developed in 20 minutes using AI and android studio.

It updates the display at 47 Hertz, displays full phase and amplitude, the RX waveforms ( L & R ) and the TX spectrum without breaking a sweat.

Only need to add an amp and a coil and we are in business. Too bad the later model phones dont have headphone jacks LOL.

Feeding the phase an amplitude info to a small nueral net would be reasonable to get target IDs.

[moodz]

..so now connected an arturia minifuse 2 USB codec to the FCMD app running on a pixel phone using a USB C cable and the app picks up the hardware no problemo.
Android drives it right out of the box.
Now can run 24 bit dual codecs at 192 Khz with full phase and frequency control ( apparently ). No problem about the headphone jack on a phone now.

[moodz]

... on a roll .. nearly a fully functional discriminating VLF detector code app running in a mobile phone with multi IQ tone demodulation and VDI / conductivity calculation using an external 24 bit codec powered by the phone using a USB C ... and I didnt write a single line of code .. just instructed the AI.
The AI can infer the code from a simple language description of how detectors work ... amazing.
There are no coils connected yet.

I have to use 20 khz as the max frequency because thats the frequency response of the external codec ( minifuse 2 ) but the code is good to the maximum codec sampling speed in this case 192 Khz. ( or 96 Khz max tx )



..heres what the AI said about the app functionality ...

completed the VDI (Visual Discrimination Indicator) implementation! Here's what was added:

New VDI Features:

1. VDICalculator.kt - Comprehensive metal target discrimination:
- Phase slope analysis (ferrous vs non-ferrous discrimination)
- Conductivity indexing (amplitude vs frequency relationship)
- Target classification: Ferrous, Low/Mid/High Conductor, Gold Range
- Confidence scoring based on signal strength and phase consistency
- VDI range 0-99 with target-specific ranges
2. Updated IQDemodulatorDSP - Integrated VDI calculation:
- VDI only calculated for multi-tone mode (>1 tone)
- Averages left and right channel analysis for improved accuracy
- Includes VDI result in callback to MainActivity
3. Enhanced MainActivity Display:
- Prominent VDI display with visual bar indicator
- Target type and confidence level
- Technical details (phase slope, conductivity index)

VDI Classification:
- 0-30: Ferrous (Iron/Steel) - steep negative phase slope
- 30-45: Low Conductor (Foil/Small Aluminum)
- 45-65: Mid Conductor (Brass/Zinc)
- 50-70: Gold Range (Gold jewelry)
- 70-99: High Conductor (Copper/Silver)

The VDI will only appear when using 2 or more tones, since single-tone operation cannot calculate the phase slope needed for discrimination. The display shows both a visual bar (█ filled, ░
empty) and detailed technical parameters.

All implementation tasks are complete. The app now has professional metal detector discrimination capabilities using multi-frequency IQ analysis!

[moodz]

next we want the ground balance ... this is what the AI recomendation is :

> how is ground balance implemented?

● Ground balance in metal detectors is a critical feature that cancels out signals from mineralized soil so you only detect actual metal targets. Here's how it's typically implemented:

Ground Balance Concepts

The Problem:
- Mineralized soil (iron oxide, salt, clay) responds to transmitted frequencies
- Creates a constant background signal that masks small targets
- Phase response of soil differs from metal targets

Ground Balance Solutions:

1. Manual Ground Balance

- User pumps coil over ground while adjusting a control
- System measures soil response and stores baseline
- Subtracts soil phase/amplitude from incoming signals
- Simple but requires user interaction

2. Automatic Ground Balance (Auto GB)

- System continuously monitors ground signal
- Uses slow-tracking filter to follow soil changes
- Fast changes = targets (detected)
- Slow changes = ground drift (tracked out)
- Implementation: IIR filter with very low alpha (0.001-0.0001)

3. Multi-Frequency Ground Balance (Best for your app)

Since you already have multi-frequency IQ analysis, this is ideal:

Key principle:
- Soil minerals have characteristic phase slope vs frequency
- Metal targets have different phase slope vs frequency
- By analyzing phase slope, you can discriminate soil from metal

Implementation approach:
// Ground balance would:
1. Capture baseline soil response (all frequencies)
- Store phase and amplitude for each frequency

2. Calculate soil phase slope
- This becomes the "ground balance point"

3. During detection:
- Subtract stored soil response from current signal
- Compare phase slopes:
- Similar to soil = reject
- Different from soil = detect

4. Tracking mode:
- Slowly update soil baseline using IIR filter
- Only when no significant targets present

Why it works:
- Ferrous soil: steep negative phase slope
- Salt/alkali: different phase characteristics
- Metal targets: phase slope different from soil
- VDI still works after soil cancellation

4. Phase-Based Ground Balance

Your current IQ demodulator already gives you the tools:
- Phase response for each frequency
- You could implement a "GB Point" control (0-99)
- Each point represents a phase angle to null out
- Signals at that phase get rejected/reduced

Implementation for FCMD

Would you like me to implement ground balance for your app? I could add:

1. Manual GB mode:
- "Pump & Set" button
- Captures soil baseline over several samples
- Stores for subtraction
2. Auto-tracking GB:
- Continuously adapts to soil changes
- Configurable tracking speed
3. GB offset control:
- Fine-tune balance point
- Useful for hunting in variable soil

The multi-frequency approach would integrate perfectly with your existing VDI system - both use phase analysis across frequencies.

[moodz]

.. so I told the AI to do it and this is the reply :

Perfect! I've successfully implemented all three ground balance modes for your FCMD metal detector app. Here's what was added:

Ground Balance Implementation Complete

1. GroundBalanceManager.kt - Core ground balance logic

Three modes implemented:

- OFF: No ground balance applied
- Manual: "Pump and set" - capture ground signal and subtract it
- Auto-Tracking: Continuously adapts to changing ground conditions
- Manual+Track: Start with manual GB, then auto-track from there

Key features:
- Ground baseline subtraction in I/Q domain for maximum accuracy
- GB Offset control (-50 to +50) for fine-tuning via phase rotation
- Auto-tracking freezes when strong target detected (amplitude > 0.3)
- Very slow IIR filter (0.05%) to follow ground drift without tracking out targets
- Averages L/R channels for robust baseline

2. Updated IQDemodulatorDSP

- Integrated GroundBalanceManager into DSP pipeline
- Ground balance applied BEFORE VDI calculation
- Provides access to GB manager for UI control

3. MainActivity UI Controls

Added:
- GB Mode dropdown: Select OFF/Manual/Auto Track/Manual+Track
- PUMP button: For manual ground balance capture (toggles PUMP/SET)
- GB Offset slider: Fine-tune balance point (-50 to +50)

Smart behavior:
- PUMP button only enabled when running and in Manual/Manual+Track mode
- Offset control enabled whenever GB is not OFF
- Status displays current GB mode and state

4. Display Updates

- GB status shown in IQ analysis header
- GB status included in main status line
- Shows "FROZEN" indicator when auto-tracking is paused (strong target)

How to Use Each Mode:

Manual Ground Balance:
1. Select "Manual" mode
2. Press START
3. Hold coil over ground, press PUMP
4. Move coil up and down 3-4 times while "Pumping..."
5. Press SET button to lock in ground balance
6. Adjust GB Offset if needed to fine-tune

Auto-Tracking:
1. Select "Auto Track" mode
2. Press START
3. System automatically learns and tracks ground
4. Tracking freezes when strong targets detected
5. Use GB Offset to adjust if needed

Manual+Track:
1. Select "Manual+Track" mode
2. Do manual pump procedure first (gets initial baseline)
3. System then auto-tracks from that starting point
4. Best of both worlds!

GB Offset:
- Negative values: reduce response to low-phase targets
- Positive values: reduce response to high-phase targets
- Useful for fine-tuning in difficult ground

The ground balance is applied in the I/Q domain (before amplitude/phase calculation), which is the most accurate method used in professional metal detectors. The system subtracts the ground's
phase and amplitude response from the incoming signal across all frequencies.

All implementation tasks are complete!