Proximity sensor

An electronic device having a front surface and including an ultrasonic proximity sensor arrangement comprising:

• an ultrasonic transmitter recessed from the front surface;
• an ultrasonic receiver recessed from the front surface;
• the ultrasonic proximity sensor arrangement being arranged to determine proximity of an object to said front surface based on a signal received by the ultrasonic receiver, wherein the received signal is a reflection from said object of a signal transmitted from the ultrasonic transmitter;
• the ultrasonic proximity sensor arrangement further comprising a structure disposed in front of the receiver defining at least first and second discrete signal paths having different path lengths such that signals travelling from the transmitter without passing the front surface to the receiver via the first and second paths undergo a destructive interference which is greater than a destructive interference undergone by signals travelling from the transmitter to the receiver after reflection from the object.

An electronic device having a front surface and including an ultrasonic proximity sensor arrangement comprising:

• an ultrasonic transmitter recessed from the front surface;
• an ultrasonic receiver recessed from the front surface;
• the ultrasonic proximity sensor arrangement being arranged to determine proximity of an object to said front surface based on a signal received by the ultrasonic receiver, wherein the received signal is a reflection from said object of a signal transmitted from the ultrasonic transmitter;
• the ultrasonic proximity sensor arrangement further comprising a structure disposed in front of the transmitter defining at least first and second discrete signal paths having different path lengths such that signals travelling from the transmitter without passing the front surface to the receiver via the first and second paths undergo a destructive interference which is greater than a destructive interference undergone by signals travelling from the transmitter to the receiver after reflection from the object.

A method of calibrating a proximity sensor for an electronic device comprising an ultrasonic transmitter and an ultrasonic receiver in a common cavity, the method comprising:

• transmitting from the ultrasonic transmitter a plurality of ultrasonic test signals of different frequencies;
• receiving the plurality of ultrasonic test signals at the ultrasonic receiver;
• and
• determining from the received plurality of ultrasonic test signals an attenuation frequency corresponding to a maximum attenuation of the associated test signal between the transmitter and the receiver;
• and
• configuring said ultrasonic transmitter to transmit a proximity signal having at least one dominant frequency substantially equal to said attenuation frequency to determine proximity of an object to a front of said cavity based a signal received by the ultrasonic receiver, wherein the received signal is a reflection of said proximity signal from said object.