Browse Prior Art Database

Demodulator and Frequency Identifier for an Electronic Pen

IP.com Disclosure Number: IPCOM000113299D
Original Publication Date: 1994-Aug-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 100K

Publishing Venue

IBM

Related People

Judice, DE: AUTHOR [+4]

Abstract

Disclosed is a method for discriminating between two different radio frequencies emitted by an electronic pen. This feature allows the system user to test the radio frequency emitted by the pen, indicating a "pen-up" condition separately from the radio frequency emitted by the pen, indicating a "pen-down" condition. This feature also allows the system to be shut-off automatically, after a period of time, when the pen is turned off or moved away from the system.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Demodulator and Frequency Identifier for an Electronic Pen

      Disclosed is a method for discriminating between two different
radio frequencies emitted by an electronic pen.  This feature allows
the system user to test the radio frequency emitted by the pen,
indicating a "pen-up" condition separately from the radio frequency
emitted by the pen, indicating a "pen-down" condition.  This feature
also allows the system to be shut-off automatically, after a period
of time, when the pen is turned off or moved away from the system.

      The Figure is a schematic view of the circuit performing this
function.  As an untethered magnetic pen 10 is moved or held adjacent
to a magnetically sensitive grid 12, which is part of a tablet
computer, electronic circuits (not shown) within the computer receive
and demodulate a modulated radio-frequency signal emitted by the pen
10.  Information detected in this way is used by the system software
to determine the location of pen 10 relative to the magnetic grid 12,
allowing the user to enter data into the computer using the pen.

      The radio-frequency signal emitted by pen 10 is detected by XY
magnetic grid 12, or by a pick-up coil (not shown), and is amplified
by a difference amplifier 14 to be injected into a phase-locked loop
16, which produces a demodulated DC output voltage proportional to
the frequency of the signal from pen 10.  A reference voltage VR1,
developed by a voltage regulator 18, is equal to the voltage produced
as an output of phase-locked loop 16 in response to an input
frequency mid-way between the two frequencies which can be emitted by
pen 10.  The output of phase-locked loop 16 is provided as a positive
input to a non-inverting operational amplifier 20, while the
reference voltage VR1 is provided through a resistor 22 as a negative
input to this amplifier 20.  Thus, a signal representing the
difference between the demodulated output voltage of phase-locked
loop 16 and VR1 is amplified to be placed at the inputs of hysteresis
comparators 24 and 26.

      Phase-locked loop 16 also provides a lock-detect signal on line
28 whenever a signal of either frequency is emitted by pen 10.  This
lock-detect signal and the output of comparator 24 are provided as
inputs to an AND gate 30, which in turn provides an input signal to
Exclusive OR (XOR) gate 32.  The other input to XOR gate 32 is
provided by the output of AND gate 34, which has, as inputs, the
lock-detect signal on line 28 and the output of second hysteresis
comparator 26.  The outputs of XOR gate 32 and AND gate...