Multifrequency VLF IB Detector using Dual-Resonance Foster Network (6 18 kHz, 1st 3rd Harmonic)

[Byko]

20260923 #2 Multifrequency VLF IB Detector using Dual-Resonance Foster Network (6 &18 kHz, 1st & 3rd Harmonic)

The hardware base is the Quasar ARM: https://fandy.ucoz.org/QuasarARM/Quasar_ARM_SCH.pdf

Multifrequency firmware is written, but the TX driver output is a 0/5 V square wave, and the boost to roughly 35 Vp for a single frequency is obtained from the TX series resonance with the quality factor controlled to about 6.
With this hardware, non-resonant TX drive yields insufficient coil current. If two frequencies are acceptable and sufficient to be called as multifrequency - a dual-resonance Foster network should work well and enable multifrequency operation.
Below is the LTspice simulation of the dual-resonance Foster network at 6 / 18 kHz (1st / 3rd harmonics): Q(6k / 18k) = 5.42 / 17.30 — both within the 5…20 band; Itx(6k / 18k) = 134.9 / 134.8 mA.
5-th harmonic doesn't make sense doe to high about 41 quality factor, the stability of 5-th harmonic will be poor especially on wet conductive soils. So decision to use 6 and18kHz seams reasonable.
Tx stage: Lp=0.68mH, Cp=200nF, Cs=353nF for my Ltx=1.153mH (Rdc=2.5R), Rdump=9.1R.
Rx stage: Lrx=13.19mH, Rdc=24.7R, Cp=18nF.

The first plot is the binary 0/5 V driver signal, which contains mostly the 1st and 3rd harmonics. The second plot shows the TX coil voltage (not sinusoidal) together with the coil current, which is a clean sum of the 6 kHz and 18 kHz components. The third plot is the RX coil voltage.

The RX front end of the Quasar ARM requires only a minimal change for multifrequency operation:

Alternatively, a more thorough modification gives approximately three times lower noise and an expected detection depth gain of +23…31 %.

Real HW shows similar to simulation waveforms: YELLOW: TX DRIVE, GREEN: TX COIL current, INDIGO: RX Amp output to ADC.


Previous post 20260922 #1: DD Coil Electronic Balancing with Compensation Coil and Trimpot

https://www.geotech1.com/forums/forum/technology/coils/452974-dd-coil-electronic-balancing-with-compensation-coil-and-trimpot

[Byko]

Today I tested a 5" AliExpress coil with the Quasar ARM hardware and the firmware I’m developing.

The coil has no resonance components inside. It’s sold for the Chinese TX-850 metal detector.

The resonant networks are tuned for a TX frequency of about 10 kHz. I tuned the RX resonance lower than the TX frequency for better VDI stability:
- TX, series resonance: Rs = 18 Ω, Cs = 330 nF (polypropylene)
- RX, parallel resonance: Cp = 47 nF (polyester)

Air test results at 9.615 kHz:


The RX coil's residual imbalance is 30 mV peak-to-peak, so I had to reduce the compensation network resistor R5 from 91k to 22k.



Here’s the adapter with the resonant networks:


With the adapter, any coil with any pinout can be tuned for any single or dual-frequency operation.

[alex.drozdov]

I used another method in quasar arm, for tx I used custom half bridge with RCD driver adopted for high voltage (like whites spectra v3i) , powering made by dcdc step up mt3608 converter (that has high working frequency more than 1mhz and low inteference noise on quasar arm) , tx coil connected series with big cap (some uf , 470uf for example) , so in tx coil I have high p-p amplitude on any frequencys.
For rx I connected rx coil in 1/2vcc virtual ground and positive input of opamp without any caps that made resonance with rx coil, I corrected parts around opamp that made bandpass filter and I cut out output antialiasing filter at all.
so rx has working at any frequensys too.

I replace quartz to 16mhz so quasar arm now working from 8 to 41khz!

and as a result I used any frequency for working , tested sensors is garrett ace dd for ace euro/ace 350 1.74mh/2.6mh, karma 13 for fisher f75 6.8mh/0.68mh that hasnt caps so it has only rx tx coils like sensors for tx850.

Multifrequency firmware for quasar arm is very interesting for testing.

[Byko]

Notes on the Quasar ARM hardware

1. No real-time TX current sensing. The board measures average TX current only, which is enough for resonance search. What it lacks is high-resolution, real-time TX current sensing on a second ADC channel sampled synchronously with the RX voltage, at the same sample rate. That would give the I/Q demodulators a true phase reference locked to the primary magnetic field of the TX coil, and would make the ferrite-calibration workarounds unnecessary.

2. RX preamp. The current op-amp is adequate but not optimal. An AD8656 should give lower RX noise. The 100 kΩ feedback resistor is also a significant noise contributor, because resistor thermal noise voltage scales as √(4kTRB), so it rises with resistance and temperature.

3. TX driver. The discrete 0/5 V TX driver is overly complex. An integrated half-bridge gate driver or a full-bridge DC motor driver IC would simplify the schematic considerably.

4. Power supply. An LDO-based supply is inefficient for a lithium-battery system.

Multifrequency-specific

5. The 2×16 character LCD can show only two frequency signagraphs at once. The display options are therefore F1 (4–11 kHz), F1+F3 (6+18 kHz), and F1+F5 (6+30 kHz).

6. Free choice of operating frequency (as opposed to optimizing sensitivity) requires non-resonant TX drive. At 0/5 V levels, though, non-resonant TX current would be very low. As a workaround I used a dual-resonant Foster network. The firmware has F1, F3 and F5 I/Q demodulators and processes all three harmonics in real time, but on this hardware only two frequencies are usable. A redesign would use a half- or full-bridge TX stage powered from a boost DC-DC converter (12–48 V). That would enable non-resonant drive and simultaneous operation on more than two frequencies.

7. A differential RX preamp would reject common-mode crosstalk coupled from the TX conductors to the RX conductors in the coil cable.

8. The TX drive could also be differential. The open question is TX/RX compatibility with the range of existing coils.

9. I would replace the MCU with an STM32G4 series part.

For now I accept the hardware as it is and focus on firmware only.