Current Sensing Analog PLL Oscillator for digital theremin

Current Sensing Theremint Oscillator With PLL and Differential Full Quadrant Sine Output

Yet another attempt to design the best theremin oscillator ever, with PLL and current sensing.

- VCO is based on State Variable Filter with fully differential integrators.

- Full quadrant 0.5Vpp 1V differential voltage VCO outputs, allows to synthesize any other phases.

- Pure sine waveform 3Vpp drive signal for LC tank (less than -70dB harmonics)

- 4.5V power supply (draws about 20mA when driving LC tank with 10mA current)

- Stable amplitude for wide range of output frequencies (100KHz .. 2MHz)

I believe, it should be practically usable as a front-end for digital theremins.

LTSpice model to play with is available on GitHub (clone repository and open theremin_oscillator_bjt_pll_v01.asc)

The schematic is expected to be cheap, with PCB manufacturing and assembly using JLCPCB.

* about 60 BJTs, most of them should be matching pairs

* LMH6642 is used as LC tank drive buffer, can provide up to 75mA current, with close to R/R output voltage.

* MCP6121 is used as Control Voltage buffer

Simulation screenshots:

Antenna voltage and inductor current

Control voltage

Antenna voltage and inductor current when locked

Drive signal voltage and current when locked

https://github.com/fpga-theremin/sensors/raw/main/models/ltspice/theremin_oscillator_bjt_analog_pll/images/theremin_oscillator_bjt_pll_sim_drive_voltage_and_current_locked.png

Drive signal FFT in locked state

VCO differential output

A theremin designer’s nightmare.

Happy New Year, everyone!
May your circuits be simple!”

A theremin designer’s nightmare.
-- ILYA

Agree. Probably, it's an over-engineering.
I'm trying to simplify VCO, to use just a single LM13700 + 2 opamps + a few BJTs.

Simplified LM13700 based SVF VCO

* Simplified LM13700 based voltage controlled oscillator.

* Still provides full quadrant output (two sine signals shifted by 90 degrees), but not differential as in previous case.

* Opamps used in VCO may drive LC tank directly with close to rails amplitude (0.2V minimal offset from rails is recommended).

LTSpice model: ota_svf_vco_v01.asc - clone from GitHub for experiments

- Stable amplitude at frequency range 200KHz-2MHz

- Single LM13700, two LMH6642 (or single LMH6643), 4 BJTs.

- Working range may be adjusted by R10, R20 to cover the desired frequency range

- R18 controls amplitude of the output signals, while R17 is responsible for keeping the amplitude constant for the range of frequencies

- R14, R12 provide fine-tuning for input bias, should be tuned to keep the same zero point of both VCO outputs.

Simulation results

Output signal amplitude in wide frequency range (sweep 400KHz to 1600KHz)

Zoom in of VCO output signals

Resonance visible in antenna voltage and inductor current while drive frequency is crossing LC resonant frequency

Consumed current from power lines