This invention relates to cordless digitizer systems and, in particular, in an electro-magnetic, cordless digitizer system wherein a transducer emits an AC magnetic field which is detected by a first grid of scanned parallel first wires in a tablet, to the improvement for obtaining a phase reference signal to be used in a demodulator demodulating data signals from the first grid of scanned parallel first wires comprising, a first multiplexer connected to the first grid of scanned parallel first wires for connecting individual ones of the first wires to an output thereof, the output of the first multiplexer being connected to provide data signals therefrom; a second multiplexer connected to the first grid of scanned parallel first wires for connecting individual ones of the first wires to an output thereof, the output of the second multiplexer being connected to provide a phase reference signal to the demodulator; and, logic means connected to the first multiplexer and the second multiplexer for first scanning the first wires to determine ones of the first wires close to a present location of the transducer and for then using the second multiplexer to connect a one of the first wires displaced from the ones of the first wires close to a present location of the transducer a sufficient distance to provide a valid and usable voltage for use in generating a phase reference signal to the demodulator. 
In a digitizer system, a cursor is moved over the surface of a tablet by a user to effect data input.  In many digitizer systems, the cursor is physically connected to the tablet by a connecting cable.  This allows the position determination logic associated with the tablet to know what is happening at both the cursor and tablet and makes the job of positional determination much easier.  In an electro-magnetic digitizer system with a driven coil, an AC electrical signal is connected to a coil in the cursor through the cable.  The signal causes the coil to emit a changing magnetic field that induces signals into the wires of a grid system in the tablet.  The wires of the grid system are sequentially and alternatively scanned in the X and Y directions of an X-Y coordinate system associated with the tablet.  The size of the signal in the respective wires of the X and Y grids changes with proximity to the position of the cursor (i.e. the coil).  Most important, the phase of the signal changes from one side of the coil to the other since the magnetic field from the coil cuts the wires in opposite directions from one side of the coil to the other.  Using this data knowledge, the positional logic can then use interpolation techniques well known in the art to find the point where the signal passes through zero (i.e. changes phase) and, thereby, the exact position of the cursor. 
Elimination of the cord between the cursor and the tablet can provide many benefits to a user in the way of convenience of use.  This is particularly true in large tablet systems where the cord can become long and cumbersome.  Thus, more recently, cordless digitizer systems have been made available commercially.  In a cordless digitizer system, there are many technical advantages to employing an electro-magnetic approach such as that described above for a cord-connected system.  Because very low signal levels are employed in digitizer systems in general, they are susceptible to outside interference from various sources of radiation that can be present in the environment of use such as cathode ray tube displays, and the like.  An electro-magnetic system allows the designer and builder to maximize the valid data from the signal of interest while minimizing the effects of undesired interference such as electrostatic fields. 
In a cordless digitizer system as developed by the assignee of this application, a battery operated circuit within the cordless cursor outputs bursts of a square wave drive signal which causes an oscillating circuit response output in and from the transducer coil also located within the cordless cursor.  As the circuit including the transducer coil begins to oscillate, the initial induced response is so low that the detecting circuitry and logic in the tablet are unable to detect and determine the phase of the signal.  As a consequence, a third, seven-wire loop has been disposed in the tablet in combination with the two X and Y grid loops from which to obtain a phase reference signal.  While the extra looping does provide the required phase reference signal, it also adds to the manufacturing costs and complexity of the digitizer.  Additionally, flexible cordless looping digitizers will not work if an aluminum shield is employed as in other digitizers and system errors can occur due to added resistance and the capacitance of the phase reference loop. 
As mentioned above, in a typical corded electro-magnetic digitizing system, a grid of wires is disposed for each of the two axes, i.e. one grid for the X-axis and one grid for the Y-axis.  The grids are alternately scanned to sense and determine the X and Y positions, respectively, of the cursor.  The coil 10 within the cursor is activated by an alternating current so as to develop a magnetic field 12 as depicted in FIG.  1.  The wires 14 of the grid 16 thereby have a voltage induced therein which is sensed and employed to determine the exact position of the coil employing interpolative techniques well known to those skilled in the art.  The typical output wave form of an entire grid scan appears as in FIG.  2 wherein as the electromagnetic field 12 moves from one side of a wire 14 to the opposite side of the wire 14, the polarity of the signal changes according to well known characteristics of such induced voltages.  The exact position of the coil 10 is, therefore, where the signal passes through zero or changes phase. 
In a cordless digitizer system, there is no electrical connection between the cursor and the tablet whereby the logic can know the phasing of the signals at the cursor and in the tablet.  Since the phase of the detected signal with respect to the signal at the cursor which produced it is unknown because of this lack of common connection (i.e. the cursor signal-producing circuitry is asynchronous to the scanning in the tablet) the only data provided by the grid wires in an electro-magnetic, cordless digitizer system is the magnitude-only response depicted in FIG.  3.  The determination of phase is important, however, as can be appreciated from an inspection of the magnitude-only data curve of FIG.  3.  As can be seen, the data represent points on two curves meeting in the middle in a sharp notch and tailing off more slowly on their outer ends.  As data are gathered, the position determination logic does not have the advantage of of the curve FIG.  3 to view so that the position of the curve relative to the data points can be seen and established.  Assuming data points "A", "B", "C", "D", "E", and "F" as depicted, we can see from FIG.  3 that the cursor is located between data points C and D. With only the magnitude data to work from, however, the position determination logic would not know whether the cursor is located (i.e. the minimum or notch of the two curves is located) between data points C and D or between data points D and E. Since precision is desired and expected, this decision is critical.  It is the phase information which provides the necessary indication thereof. 
Wherefore, it is the object of the present invention to provide a method and apparatus for providing a phase reference in a cordless digitizer system without the need of extra phase-detecting loops in the tablet. 
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.