Low power cordless magnetic field digitizer with differential grid sensing and synchronous position demodulation

In a digitizing system including a tablet and a pointing device, a carrier recovery circuit comprising in combination:

• (a) a first controllable gain amplifier having an input coupled to a first carrier recovery conductor in the tablet, respectively, and a second controllable gain amplifier having an input coupled to a second carrier recovery conductor in the tablet, respectively;
• (b) a first frequency doubler circuit having an input responsive to an output of the first controllable gain amplifier and a second frequency doubler circuit having an input responsive to an output of the second controllable gain amplifier;
• (c) signal combining circuitry having first and second inputs coupled to outputs of the first and second frequency doubler circuits respectively;
• (d) a threshold generating circuit having an input coupled to the output of the signal combining circuit and an output coupled by an error amplifier circuit to gain control inputs of the first and second controllable gain amplifiers;
• (e) a signal slicing, limiting circuit having a first input coupled to an output of the signal combining circuit and a second input coupled to an output of the threshold generating circuit;
• and
• (f) a phase-locked-loop circuit having an input coupled to the output of the comparator that is in a precise phase relationship to a carrier signal induced in the first and second carrier recovery conductors.

In a digitizing system, including a tablet and a pointing device, oscillator circuitry in the pointing device, comprising in combination:

• (a) a power source having first and second terminals;
• (b) an inductor having a center tap connected to a first terminal of the power source;
• (c) first and second transistors, an emitter of the first transistor being coupled to an emitter of the second transistor and to a first terminal of a current source having a second terminal connected to the second terminal of the power source, a collector of the first transistor being coupled to a first terminal of the inductor, a collector of the second transistor being coupled to a second terminal of the inductor, a base of the first transistor being coupled by a first capacitor to the collector of the second transistor, a base of the second transistor being coupled by a second capacitor to the collector of the first transistor;
• (d) a third capacitor coupled between the collectors of the first and second transistors;
• and
• (e) a bias voltage source coupled to the bases of the first and second transistors by first and second resistors, the inductor and the third capacitor forming a tank circuit, the current source producing a predetermined current, the first and second transistors forming a current switch that alternately switches the predetermined current into different parts of the tank circuit,
• whereby the power level of a magnetic signal transmitted from the inductor is substantially independent of the voltage of the power source and the inductor radiates maximum power at a self-resonant frequency of the tank circuit regardless of the capacitance coupled to the inductor.

In a digitizing system including a tablet and a pointing device, an improvement in the pointing device comprising a self-resonant, current switched LC oscillator including:

• (a) an inductor and a capacitive element coupled to form a self-resonant tank circuit, wherein the inductor radiates a magnetic signal to the tablet;
• (b) current source means for supplying a measured amount of current;
• and
• (c) current switching means coupled to the current source means and the tank circuit for alternately switching the measured amount of current into different sections of the tank circuit,
• whereby the inductor radiates maximum power at the self-resonant frequency of the oscillator regardless of the capacitance of the capacitive element.

A digitizing system including a tablet and a cordless pointing device, comprising in combination:

• (a) a plurality of pairs of position resolving conductors each of which forms one or more differential coil elements in the tablet, receiving a magnetic signal transmitted by the pointing device;
• (b) first and second pairs of carrier recovery conductors each including at least one differential coil element;
• (c) synchronous demodulating circuitry having a first input responsive to selected signals induced onto the differential coil elements and a second input responsive to a recovered pointing device carrier signal 51 for producing position resolving information;
• (d) carrier recovery circuitry including
• i. a first controllable gain amplifier having a pair of inputs coupled to the first pair of carrier recovery conductors, respectively, and a second controllable gain amplifier having a pair of inputs coupled to the second pair of carrier recovery conductors,
• ii. a first frequency doubler circuit having an input responsive to an output of the first controllable gain amplifier and a second frequency doubler circuit having an input responsive to an output of the second controllable gain amplifier,
• iii. signal combining circuitry having first and second inputs coupled to outputs of the first and second frequency doubler circuits, respectively,
• iv. a threshold generating circuit having an input coupled to the output of the signal combining circuit and an output coupled by an automatic gain control circuit to gain control inputs of the first and second controllable gain amplifiers,
• v. a signal slicing circuit having a first input coupled to an output of the signal combining circuit and a second input coupled to an output of the threshold generating circuit,
• vi. a phase-locked-loop circuit having an input coupled to the output of the signal slicing circuit and producing a double frequency output signal;
• vii. a divide-by-two circuit having an input coupled to receive the double frequency output signal from the phase-locked-loop circuit to produce the recovered pointing device carrier signal;
• and
• (e) means for resolving the position of the pointing device in response to digital output signals produced by the analog-to-digital converter.

A method of operating the digitizing system including a tablet and a pointing device, comprising the steps of:

• (a) transmitting a magnetic signal from the pointing device to a plurality of pairs of position resolving conductors, each pair of position resolving conductors including one or more differential coil elements, and also transmitting the magnetic signal to first and second carrier recovery conductors in the tablet;
• (b) differentially amplifying selected signals induced onto the position resolving conductors;
• (c) applying the differentially amplified signals to a first input of a synchronous demodulating circuit having a second input responsive to a recovered pointing device carrier signal;
• (d) essentially simultaneously with steps (b) and (c), differentially amplifying signals induced by the magnetic signal onto the carrier recovery conductors;
• (e) doubling the frequency differentially amplified signals of Step (d);
• (f) operating a phase-locked-loop circuit in response to the combination of the frequency doubled signals to produce the regenerated pointing device carrier signal 51 which bears one or more precise phase relationships with the magnetic signal;
• (g) operating the synchronous demodulating circuitry in response to the differentially amplified signals of Step (b) and the regenerated pointing device carrier signal to produce position resolving information;
• and
• (h) operating on the information resolving position to produce a coordinate representing the location of the pointing device.

In a digitizing system including a tablet and a pointing device, a method of regenerating a carrier signal transmitted by the pointing device for use in the tablet, comprising the steps of:

• (a) transmitting a signal from the pointing device to first and second sets of carrier recovery conductors in the tablet;
• (b) amplifying signals induced onto the carrier recovery conductors by the carrier signal;
• (c) doubling the frequency of the amplified signals of Step (b);
• (d) operating a phase locked-loop-circuit in response to the frequency doubled signals to produce a regenerated pointing device carrier signal which has a precise phase relationship with the magnetic signal.

In a digitizing system including a tablet and a pointing device, a method of regenerating a carrier signal transmitted by the pointing device for use in the tablet, comprising the steps of:

• (a) transmitting a signal from the pointing device to first and second sets of carrier recovery conductors in the tablet;
• (b) amplifying signals induced onto the carrier recovery conductors by the carrier signal;
• (c) doubling the frequency of the amplified signals of Step (b);
• and
• (d) producing a regenerated pointing device carrier signal in response to the frequency doubled signals, which regenerated pointing device carrier signal has a precise relationship with the magnetic signal.