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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
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