Proximity sensor and mobile wireless device
A capacitive proximity sensor for use in mobile devices such as smartphones and connected tables, in which it is used to switch off a display ( 70 ) when the device is brought to the ear, and to reduce selectively the RF power when the device is in close proximity to a body part of a user, in order to fulfil regulatory SAR limits. The capacitive sensor uses two electrodes ( 60, 30 ), the first of which may also serve as RF antenna, and the other is preferably on the back of the phone and is opposite the display.
1. A proximity sensor for a portable device, the sensor being arranged for detecting a body portion of a user in proximity of the portable device; the sensor comprising: a first electrode capacitively couplable with a body part of a user of the portable outside of the portable device; a second electrode capacitively couplable with a body part of a user of the portable outside of the portable device; a readout circuit operatively arranged for acquiring a first capacity seen by the first electrode and a second capacity seen by the second electrode, and a to generate a directional proximity signal based on said first capacity and second capacity as well as a second proximity signal based on said first capacity and second capacity.
2. The proximity sensor of claim 1 in a portable device having a display and/or a touch sensitive panel, a radio transmitter, and a logic circuit operatively arranged to inhibit the display and/or the touch sensitive panel based on the directional proximity signal, and to reduce a transmission power of the radio transmitter based on the second proximity signal.
3. The proximity sensor of claim 2 , the first electrode being part of a radiofrequency antenna of the portable device, and being connected to the radio transmitter and to the readout circuit through decoupling elements.
4. The proximity circuit of claim 1 , wherein the readout circuit is arranged to generate digital values indicative of the raw capacities seen by the first electrode and/or the second electrode and to subtract a noise component and/or a pedestal component and/or a drift component from said digital values to obtain processed digital values, and to generate said directional proximity signal and second proximity signal based on said processed digital values.
5. The proximity circuit of claim 1 , wherein the second proximity signal is generated when the first capacity of the first electrode fulfils predetermined criteria, and the directional proximity signal is generated when a linear combination of the first and second capacity exceeds a predetermined threshold, or a ratio of the first capacity on the second capacity exceeds a predetermined threshold.
6. The proximity circuit of claim 1 , wherein the second proximity signal is generated when the first capacity of the first electrode fulfils predetermined criteria, and the directional proximity signal is generated when a vector having as coordinates the first and second capacity falls in a predetermined region in a cartesian plane.
7. The proximity circuit of claim 1 , comprising one guard electrode in proximity the second electrode.
8. The proximity circuit of claim 1 , in a portable phone, wherein the first electrode is on an upper part of the phone, relative to the position of normal use, and the second electrode is on a back side of the phone opposite to a touch-sensitive display.
9. A proximity sensor for a portable device, the sensor being arranged for detecting a body portion of a user in proximity of the portable device; the sensor comprising: a first electrode capacitively couplable with a body part of a user of the portable outside of the portable device; a second electrode capacitively couplable with a body part of a user of the portable outside of the portable device; a readout circuit operatively arranged for acquiring a first capacity seen by the first electrode and a second capacity seen by the second electrode, and a to generate a directional proximity signal based on said first capacity and second capacity as well as a second proximity signal based on said first capacity and second capacity, wherein the second proximity signal is generated when the first capacity of the first electrode fulfils predetermined criteria, and the directional proximity signal is generated when a linear combination of the first and second capacity exceeds a predetermined threshold, or a ratio of the first capacity on the second capacity exceeds a predetermined threshold, the portable device having a display and/or a touch sensitive panel, a radio transmitter, and a logic circuit operatively arranged to inhibit the display and/or the touch sensitive panel based on the directional proximity signal, and to reduce a transmission power of the radio transmitter based on the second proximity signal.
10. A proximity sensor for a portable device, the sensor being arranged for detecting a body portion of a user in proximity of the portable device; the sensor comprising: a first electrode capacitively couplable with a body part of a user of the portable outside of the portable device; a second electrode capacitively couplable with a body part of a user of the portable outside of the portable device; a readout circuit operatively arranged for acquiring a first capacity seen by the first electrode and a second capacity seen by the second electrode, and a to generate a directional proximity signal based on said first capacity and second capacity as well as a second proximity signal based on said first capacity and second capacity, wherein the second proximity signal is generated when the first capacity of the first electrode fulfils predetermined criteria, and the directional proximity signal is generated when a vector having as coordinates the first and second capacity falls in a predetermined region in a cartesian plane, the portable device having a display and/or a touch sensitive panel, a radio transmitter, and a logic circuit operatively arranged to inhibit the display and/or the touch sensitive panel based on the directional proximity signal, and to reduce a transmission power of the radio transmitter based on the second proximity signal.