Abstract:
In a feedback peak detector fitted wireless handwriting tablet, the feedback peak detector comprises a feedback comparator buffer unit, a peak detection switch unit, a fast charge path unit, a peak signal holding unit, and a fast discharge unit. The feedback peak detector is designed with a light load capable of stabilizing peak signals and fast charging/discharging a capacitor to ensure the correct calculation of coordinates for keeping handwriting at a normal and stable speed.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a feedback peak detector fitted wireless handwriting tablet that stands at the key position in analog-digital conversion for enhancing correctness of analog signals by peak sampling and holding to facilitate detection of absolute coordinates of a wireless stylus on a handwriting tablet.  
         BACKGROUND OF THE INVENTION  
         [0002]    Referring to FIG. 1—a circuit block diagram of a conventional wireless handwriting tablet—a plurality of antennas is distributed in array equidistantly in X-axis and Y-axis direction on both faces of a detector  1  for receiving electromagnetic waves emitted by a wireless stylus  2 . A microprocessor  3  is provided to scan all the antennas one after another sequentially and analyze intensity of the signal received to find out to which antenna the stylus  2  is adjacent, then compare the gradients and slopes of the signals of the immediate neighboring antennas to calculate and obtain the absolute coordinates of the wireless stylus  2 .  
           [0003]    A peak detector  5  plays a key role in the hardware design of the wireless handwriting tablet for receiving analog voltage signals  40  come from a precedent OP amp  4  and sending out peak signals  50  to an Analog/Digital Converter (ADC)  6 . Hence, errors may be incurred in A/D conversion and coordinates accordingly in the case of a defective peak detector.  
           [0004]    The circuitry of a conventional peak detector may be classified in two categories: one is shown in FIG. 2 and the other in FIG. 3, which are described below.  
           [0005]    The one shown in FIG. 2 is defective in a small capacitor C 1 , which is liable to be charged to an undesirable peak by an instantaneous surge or noise, however, if the capacitor C 1  is enlarged improperly, the charging speed will be slowed down to result in amplification deficiency and waveform distortion. The other shown in FIG. 3 is defective that when a MOSFET Q 2  is biased by a control pulse signal  30  and turned on or off, a surge is generated to apply upon a capacitor C 2 , which then holds voltage at a level quite different from that of the precedent circuits to therefore incur jittering and erroneous calculation of the absolute coordinates of the stylus. Besides, a problem of charge/discharge speed is introduced in such a circuit.  
         SUMMARY OF THE INVENTION  
         [0006]    The primary object of this invention is to provide a feedback peak detector fitted wireless handwriting tablet, wherein the peak detector is capable of stabilizing peak signals and performing fast charge/discharge to make a stable and correct coordinate calculation and maintain a normal handwriting speed of a wireless tablet.  
           [0007]    In order to realize abovesaid object, the peak detector of this invention is composed of: a feedback comparator buffer unit, a peak detection switch unit, a fast charge path unit, a peak signal holding unit, and a fast discharge unit.  
           [0008]    For more detailed information regarding advantages or features of this invention, at least an example of preferred embodiment will be elucidated below with reference to the annexed drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The related drawings in connection with the detailed description of this invention, which is to be made later, are described briefly as follows, in which:  
         [0010]    [0010]FIG. 1 is a circuit block diagram of a conventional wireless handwriting tablet;  
         [0011]    [0011]FIG. 2 is a circuit diagram of a conventional peak detector;  
         [0012]    [0012]FIG. 3 is another circuit diagram of a conventional peak detector;  
         [0013]    [0013]FIG. 4 is a circuit diagram of a peak detector of this invention; and  
         [0014]    [0014]FIG. 5 shows a substantial embodiment of the peak detector of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    As illustrated in FIG. 4, a peak detector  100  of this invention comprises a feedback comparator buffer unit  101 , a peak detection switch unit  102 , a fast charge path unit  103 , a peak signal holding unit  104 , and a fast discharge unit  105 , wherein a plurality of output ends of the peak detection switch unit  102  is coupled to the feedback comparator buffer unit  101  and the fast charge path unit  103  respectively; the peak signal holding unit  104  is connected respectively to the peak detection switch unit  102 , the fast discharge unit  105 , and an analog/digital converter (ADC); a train of analog voltage signals  40  is applied to an input end of the feedback comparator buffer unit  101 ; an output end of the feedback comparator buffer unit  101  is coupled to the peak detection switch unit  102 ; an output end of the fast charge path unit  103  is connected to the peak signal holding unit  104 ; a control pulse signal  30  is applied to the peak detection switch unit  102 , the fast charge path unit  103 , and the fast discharge unit  105  respectively. After being treated in the peak detector  100  of this invention, the train of analog voltage signals  40  becomes pure peak signals  50  providing to the ADC.  
         [0016]    In an embodiment of this invention shown in FIG. 5, the feedback comparator buffer unit  101  is an OP amp U 3 A for comparison of an analog voltage signal  40  and a feedback capacitor&#39;s voltage signal  20 , in which a non-inversion input end is applied with the analog voltage signal  40  come from a precedent OP amp while the other end, an inversion input end, is arranged to receive the feedback capacitor&#39;s voltage signal  20 . When the analog voltage signal  40  is greater than the feedback capacitor&#39;s voltage signal  20 , a voltage comparison signal  10  of the OP amp U 3 A is positively saturated, or negatively saturated vice versa. Meanwhile, the feedback comparator buffer unit  101  serves as a segregation buffer between the precedent analog circuit and a rear digital circuit so that the analog voltage signal  40  won&#39;t be distorted due to a heavy load or interfered by noise of the rear digital circuit.  
         [0017]    The peak detection switch unit  102  in Fig. D is composed of a first and a second transmission gate U 4 A. U 4 B. wherein the input end of the first transmission gate U 4 A is coupled to the output end of the feedback comparator buffer unit  101  and the output end of the second transmission gate U 4 B is connected with one end of a resistor R 2 ; a control end of the first and the second transmission gate U 4 A, U 4 B are connected to a source of the control pulse signal  30 . The peak detection switch unit  102  is offered to switch ON/OFF operation of the feedback capacitor&#39;s voltage signal  20  and the voltage comparison signal  10 .  
         [0018]    The fast charge path unit  103  comprises a first and a second MOSFET Q 2 , Q 3 , and an inverter USA, wherein the Gate of the first MOSFET Q 2  and the Drain of the second MOSFET Q 3  are jointed together and coupled to the output end of the first transmission gate U 4 A; the input end of the inverter U 5 A is connected to the source of the control pulse signal  30  and the output end to the Gate of the second MOSFET Q 3 ; the Drain of the first MOSFET Q 2  is coupled with the positive end of a power supply+Vcc while its Source is coupled with one end of the resistor R 2 . By charging through a transistor-switching path with a relatively larger voltage offset and current, the fast charge path unit  103  can charge a capacitor far quicker than the conventional charging process by way of an OP amp, and meanwhile, the fast charge path unit  103  is also designed to stabilize the Gate of the first MOSFET Q 2  to avoid the floating state.  
         [0019]    The peak signal holding unit  104  is composed of a Schottky diode D 3  connected in series with the other end of resistor R 2 , a potential-following capacitor C 3  connected in parallel. and a peak-holding capacitor C 4 , wherein one end of the potential-following capacitor C 3  is jointed with one end of the resistor R 2 ; one end of the peak-holding capacitor C 4  is coupled to the output end of the Schottky diode D 3  and the input end of the ADC. The peak signal holding unit  104  functions to filter any possible noise to ensure a stable voltage of the peak-holding capacitor C 4 , which is then relayed to the ADC.  
         [0020]    The fast discharge unit  105  is composed of a first resistor R 3 , a second resistor R 4 , a bipolar junction transistor Q 4 , and an inverter U 6 A, wherein the control pulse signal  30  is applied to the input end of the inverter U 6 A; one end of the second resistor R 4  is coupled to the output end of the inverter U 6 A; one end of the first resistor R 3  is connected to the other end of the second resistor R 4  and the Base of the bipolar junction transistor Q 4 ; the Collector of the bipolar junction transistor Q 4  is coupled with one end of the potential-following capacitor C 3 ; and the Emitter of the bipolar junction transistor Q 4  is connected with the other end of the first resistor R 3 . The fast discharge unit  105  enables the potential-following capacitor C 3  and the peak-holding capacitor C 4  to discharge fast through a transistor-switching path.  
         [0021]    The detailed operation process is described below basing on abovesaid architecture.  
         [0022]    When the control pulse signal  30  provided by a microprocessor is positive, both the first and the second transmission gates U 4 A, U 4 B are turned on, and in the negative half cycle of the analog voltage signal  40 , the voltage comparison signal  10  of the feedback comparator buffer unit  101  is negatively saturated. At this moment, because a negative output is given by the inverter USA to keep the MOSFET Q 3  “OFF” and the Gate of the MOSFET Q 2  at a low level, namely, the MOSFET Q 2  is kept “OFF” too, so that both the potential-following capacitor C 3  and the peak-holding capacitor C 4  are not yet charged and held in a low voltage level.  
         [0023]    In the positive half cycle of the analog voltage signal  40 , the voltage comparison signal  10  of the feedback comparator buffer unit  101  is turned positively. At this moment, the MOSFET Q 2  is turned ON and the power supply+Vcc starts charging the potential-following capacitor C 3  to have a voltage feedback applied to the inversion input end of the OP amp U 3 A via the second transmission gate U 4 B. When the potential of the potential-following capacitor C 3  climbs to equal the peak of the analog voltage signal  40 , the voltage comparison signal  10  is then turned to negatively saturated to thereby turn the MOSFET Q 2  “OFF” and hinder the potential-following capacitor C 3  from going up any more. However, if the analog voltage signal  40  increase its amplitude further, the voltage comparison signal  10  will become positively saturated to turn the MOSFET Q 2  “ON” again and charge the potential-following capacitor C 3  one more time accordingly.  
         [0024]    As mentioned above, the potential of the capacitor C 3  follows the analog voltage signal  40  to go higher and higher unit the highest amplitude of the latter is reached and held, whereas when the analog voltage signal  40  becomes smaller or negative, the potential of the capacitor C 3  is held at constant instead of descending down. During the low level of the control pulse signal  30 , both the first and the second transmission gates U 4 A, U 4 B are “OFF” while the MOSFET Q 3  is turned “ON” and the MOSFET Q 2  is “OFF” distinctly without floating, and at this time, the power supply+Vcc would stop charging the capacitor C 3 , C 4 . The inverter U 6 A provides now a positive bias to the bipolar junction transistor Q 4 , Q 6  to turn them “ON” for discharge of both the potential-following capacitor C 3  and the peak-holding capacitor C 4 .  
         [0025]    In the above described, at least one preferred embodiment has been described in detail with reference to the drawings annexed, and it is apparent that numerous variations or modifications may be made without departing from the true spirit and scope thereof, as set forth in the claims below.