Quadrature amplitude modulation circuitry for ultrasonic transducer
An ultrasonic transmitter system includes a digital controller, bandpass pulse-width modulator (BP-PWM) unit, a digital to analog converter (DAC), and an ultrasound transducer. The controller generates pulse width and phase reference signals. The BP-PWM configured receives these signals generates a pulse width modulation (PWM) output characterized by a pulse width and a phase based on the pulse width and phase reference signals. The DAC) receives the PWM output from the BP-PWM unit and generates an output characterized by the pulse width and phase. The ultrasonic transducer receives the output from the DAC and generates an output sound pressure in response to the output from the DAC. An amplitude of the RMS sound pressure depends on the pulse width of the output from the DAC.
1. An ultrasonic transmitter system comprising a digital controller configured to generate pulse width and phase reference signals wherein the phase reference signal is in the form of a phase ramp signal, a bandpass pulse-width modulator (BP-PWM) unit configured to receive the pulse width and phase reference signals from the digital controller and generate a pulse width modulation (PWM) output based on the pulse width and phase reference signals, wherein the PWM output is characterized by a pulse width and a phase wherein the BP-PWM is configured to map the phase ramp signal to PWM output and the period of the phase ramp signal is equal to the period of the PWM output, a digital to analog converter (DAC) configured to receive the PWM output from the BP-PWM unit and generate an output characterized by the pulse width and phase, and an ultrasonic transducer configured to receive the output from the DAC and generate an output sound pressure in response to the output from the DAC, wherein an amplitude of the RMS sound pressure depends on the pulse width of the output from the DAC.
2. The system of claim 1 , wherein the BP-PWM unit has a programmable pulse width.
3. The system of claim 1 , wherein an RMS sound pressure level of the output sound pressure from the ultrasound transducer is determined by an amplitude of the output from the DAC as well as a pulse width setting of PWM output.
4. The system of claim 1 , wherein the digital controller is configured to generate a phase offset signal and wherein the BP-PWM unit is configured to receive the phase offset signal and generate the PWM output based on the pulse width, phase reference and phase offset signals, wherein a phase of the output sound pressure from the ultrasound transducer is determined by the phase offset signal.
5. The system of claim 1 , wherein the digital controller generates the phase reference signal in the form of a digital phase ramp.
6. The system of claim 1 , wherein the ultrasonic transducer is a micromachined piezoelectric ultrasound transducer (PMUT) or a capacitive ultrasonic transducer (CMUT).
7. The system of claim 1 , wherein the BP-PWM unit uses quadrature amplitude modulation (QAM) to encode the phase and amplitude of the output sound pressure transmitted by the ultrasonic transducer.
8. The system of claim 7 , wherein the QAM encoding of the output sound pressure is a code used to uniquely identify an ultrasound transmitter.
9. The system of claim 7 , wherein the QAM encoding of the ultrasound output sound pressure is a message to be transmitted to a second ultrasound transducer that is used as a receiver.
10. The system of claim 1 , wherein the BP-PWM unit is used to control the amplitude and phase of the voltage applied to an ultrasonic transducer in order to reduce the ring-down time of the transducer following the transmission of an ultrasonic pulse.