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A VCO BASED DIGITAL MICROPHONE UTILIZING A FIR SIGMA DELTA CONVERTER

Agenda. Condenser microphones problemProposed circuitDelta-sigma modulator equivalentPerformance and non-idealitiesFIR microphone. Condenser Problem. Thermal noise of high impedance resistor dominates at low frequencies. (Low-Pass)High-Pass RC network prevents measuring low frequency a

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A VCO BASED DIGITAL MICROPHONE UTILIZING A FIR SIGMA DELTA CONVERTER

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    1. A VCO BASED DIGITAL MICROPHONE UTILIZING A FIR SIGMA DELTA CONVERTER Sven Soell Dr. Bernd Porr Glasgow University, UK

    2. Agenda Condenser microphones + problem Proposed circuit Delta-sigma modulator equivalent Performance and non-idealities FIR microphone

    3. Condenser Problem Thermal noise of high impedance resistor dominates at low frequencies. (Low-Pass) High-Pass RC network prevents measuring low frequency audio signals.

    4. Main Problems Thermal noise of resistor High impedance output High-pass for voice

    5. Condenser Solution Use capsule directly for frequency modulation in a Hartley oscillator.

    6. Condenser Solution If we can do this then: Effective tank resistance less than high impedance resistor. Capable of measuring low frequency signals. Digital output can be sent over long distances. High dynamic range w/o use of attenuation switch.

    7. De-Modulation How can we directly de-modulate and digitize with as little circuitry as possible? We can use an asynchronous D-FF.

    8. ?S-Equivalent w/o Feedback Input signal gets integrated, quantized and differentiated. Quantization noise gets differentiated only.

    9. Integrator with VCO VCO acts as an integrator

    10. Integrator with VCO VCO acts as an integrator

    11. Integrator with VCO VCO acts as an integrator

    12. A/D + Differentiator with D-FF D-FF acts as quantizer and differentiator

    13. A/D + Differentiator with D-FF D-FF acts as quantizer and differentiator

    14. A/D + Differentiator with D-FF D-FF acts as quantizer and differentiator

    15. Overall System VCO with D-FF acts as 1st order delta-sigma modulator without feedback.

    16. Performance Q-Noise same as in conventional SDM. SNR improvement of 3dB per 2xOSR whereas 9dB in conventional modulators.

    17. Limitations Make sure won’t overload D-FF. System will only be as linear as the VCO, thus non-linearities, phase-noise and jitter will directly add to the output.

    18. Simulation Example LC-Oscillator with asynchronous D-FF:

    19. Simulation Example LC-Oscillator performance

    20. Simulation Example Resulting Output

    21. Digital Condenser Microphone Condenser Capsule changes capacitance with applied sound pressure, resulting in a FM signal which is then digitized.

    22. Digital Condenser Microphone Behringer B1 microphone: 33pF±1pF Oscillator Phase Noise: VerilogA model (relates phase noise to jitter) Clock Jitter (D-FF): VerilogA model

    23. Digital Condenser Microphone

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