Abstract:
A modulated input signal is filtered to reduce noise. The state of the input signal controls the state of a switch. Depending upon its state, the switch connects one of two circuits to a gate. The gate receives the input signal and it passes or blocks the signal depending upon the state of the signal received from the switched circuits. When it is passing the input signal, the output of the gate is equivalent to the input signal. When it is blocking the input signal, the output of the gate is set to a default state. The default state is independent of the input signal. One of the switched circuits gradually modifies the signal provided by the gate to allow the input signal to pass through. The other switched circuit gradually modifies the signal provided to the gate to block the input signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to information transmission systems and, more particularly, to the structure and operation of a modulated input signal filter.  
         DESCRIPTION OF THE RELATED TECHNOLOGY  
         [0002]    Many new commercial applications require electronic devices that are able to communicate while maintaining low power consumption. Mobile devices are less convenient if large battery requirements increase their size or weight. Devices mounted on or in automobiles that operate when the engine is inactive must also be able to operate with only low power in order, for example, to avoid draining a car battery.  
           [0003]    Electronic devices can communicate using modulated signals. A characteristic of an electronic signal, e.g., amplitude, frequency, phase, etc., is modified in a pattern that corresponds to information. Such modification is referred to as modulation. One type of modulation is pulse width modulation. Pulse width modulation uses a pattern defined by duty cycle to correspond to zeros and ones. Duty cycle is the ratio of the time that a signal is in a first state to the time that the signal is in a second state during a specific time interval. For example, a signal with a high state and a low state is used for digital communication. An interpretation of 25% of time in the high state as a one and 75% of the time in the high state as a zero is one implementation of pulse code modulation.  
           [0004]    The modulated electronic signal is then transmitted and received. The receiver compares the modifications to the pattern to determine the information that was intended to be transmitted. The receiver, however, does not analyze an exact copy of the modulated signal. Electronic devices near the receiver as well as physical imperfections in the transmission and reception equipment and the transmission medium combine to add a random signal referred to as noise.  
           [0005]    The noise added to the modulated signal, noise can be negative as well as positive, can complicate or create errors in the communication where the noise is significant in relation to the amount of modification corresponding to information. In situations where low power is desirable, noise can also cause a receiver to operate at a higher power level as it attempts to determine what information was intended to be sent by the noise signal. A receiver may successfully determine that the noise does not represent information, but spend an undesirably significant amount of power in producing that output. In situations where the microcontroller in the receiver is performing a background task, processing time is undesirably diverted to analyze noise.  
           [0006]    Some receivers for devices such as garage doors pass the digital signal corresponding to the modulation on to a microcontroller that determines whether the signal is authorized to actuate the device. Increased levels of security can correspond to greater power use in the microcontroller. A sleep mode can be used to reduce the power consumption of the microcontroller when no signal is present.  
           [0007]    U.S. Pat. No. 5,463,662 discloses an apparatus for reducing errors in data caused by noise. A signal whose amplitude is modified in accordance with information is sampled by an analog-to-digital converter to provide digital signals representative of the sample. A low frequency offset is then subtracted from the digital signals. The resulting signal is compared to its average level and the comparison output is used as a control signal to determine when a blanking circuit will blank the digital signal. The selective removal of noise effects can reduce errors, but does not significantly reduce the amount of power used by the receiver in detecting valid signals.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to a modulated input signal filter, a method filtering, and systems employing the apparatus or method.  
           [0009]    In an embodiment of the modulated input signal filter of the present invention, a switch responds to the input signal by coupling one of two circuits to a gate. The input signal is also coupled to the gate. The gate operates to either pass through the input signal as an output or to output a default signal regardless of the input signal state. One of the circuits coupled by the switch influences the gate over time to pass through the input, while the other circuit influences the gate over time to output the default signal.  
           [0010]    In a more specific embodiment of the present invention, the input signal is a voltage and has two states corresponding to lower and higher voltage ranges. Each of the two circuits includes a capacitor, but one circuit drains the capacitor with a current source  
           [0011]    while the other charges the capactor with a current source. The gate receives the voltage currently on the capacitor as an input. When that voltage is in a high state, the gate passes the input signal through to its output. When, the capacitor voltage is in a low state, the gate outputs a default signal. A high voltage input signal couples the charging current source to the capacitor and results in the input signal being passed through the gate in a specific period of time that depends on the initial voltage of the capacitor. In one embodiment, the gate passes through the input signal within  3  milliseconds of being connected to the charging circuit.  
           [0012]    In one system embodiment of the present invention, the input signal is received at an antenna. The output signal of the gate is coupled to a microcontroller. The microcontroller is coupled to an automobile door lock mechanism to unlock the automobile door when a specific signal is received. In a more specific embodiment, the microcontroller has a sleep mode that is entered after receiving the default signal for a specified time from the gate output. The sleep mode uses less power than than mode is which the microcontroller analyzes received input signals for the specific signal.  
           [0013]    In another system embodiment of the present invention, the input signal is received at an antenna. The output signal of the gate is coupled to a microcontroller. The microcontroller is coupled to a pet door lock mechanism to unlock the pet door when a specific signal is received. In a more specific embodiment, the microcontroller has a sleep mode that is entered after receiving the default signal for a specified time from the gate output. The sleep mode uses less power than than mode is which the microcontroller analyzes received input signals for the specific signal.  
           [0014]    In a method embodiment of the present invention, an input signal is filtered by the application of specific steps. The input signal is received by an antenna. The ratio of time that the input signal is high relative to the time that the input signal is low is measured. The input signal is blocked from reaching an output if the input signal has not been in a high state for a specified length of time. The input signal is blocked if the measured ratio is less than a specified ratio. If the input signal is not blocked it is transmitted to the output. The output is connected to a circuit with sleep functionality.  
           [0015]    A feature of the invention is filtering an input signal.  
           [0016]    Another feature is operating communications systems at low power.  
           [0017]    An advantage of the present invention is setting a receiver to ignore noise based on length characteristics of the noise.  
           [0018]    Another advantage is ignoring noise based on the duty cycle of the noise.  
           [0019]    Still another advantage is decreasing the power used by a signal analyzing microcontroller or circuit by blocking noise signals.  
           [0020]    Another advantage is allowing an override to selectively impose a time delay on signal receipt.  
           [0021]    Other and further features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings. Not all embodiments of the invention will include all the specified advantages. For example, one embodiment may only ignore noise based on the duty cycle of the noise, while another only decreases the power used by the signal analyzing microcontroller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a schematic block diagram of a microcontroller system with an input signal receiver and a filter;  
         [0023]    [0023]FIG. 2 is a circuit diagram of one embodiment of a modulated input signal filter;  
         [0024]    [0024]FIG. 3 is a circuit diagram of one embodiment of a modulated input signal filter;  
         [0025]    [0025]FIG. 4 is a chart of signals in time for the input and output of a modulated input signal filter; and  
         [0026]    [0026]FIG. 5 is an illustration of a system employing the input signal filter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    The invention is a filter for an input signal in which the transmittance of the signal depends on the historical state of the signal. The invention may be used to block noise that would bring a microcontroller out of a low power sleep state. The receiver with filter may be fabricated on the same or different die as the microcontroller. The invention may also include input for modifying the required input signal history for transmittance.  
         [0028]    Referring now to the drawings, the details of preferred embodiments of the invention are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix.  
         [0029]    Referring to FIG. 1, a schematic block diagram of a microcontroller system with an input signal receiver and a filter is illustrated. The microcontroller system is generally represented by the numeral  10 . The microcontroller system  10  may be fabricated on one or more integrated circuit dice and enclosed in an integrated circuit package. The integrated circuit package may be, for example, but not limited to, plastic dual in-line package (PDIP), small outline (SO), shrink small outline package (SSOP), thin shrink small outline package (TSSOP), windowed ceramic dual in-line package (CERDIP), leadless chip carrier (LCC), plastic leaded chip carrier (PLCC), plastic quad flatpack package (PQFP), thin quad flatpack package (TQFP), pin grid array (PGA), ball grid array (BGA), TO-220, T0-247 and T0-263.  
         [0030]    The microcontroller system includes two antennas  12   a ,  12   b . The antennas receive signals propagating through the atmosphere. Other embodiments of the invention can include one or three antennas. Multiple antennas can be arranged orthogonally to maximize the probability of receiving a signal. Each antenna  12   a ,  12   b  is coupled to provide any signals it receives to one of the amplifiers  16   a ,  16   b . The resulting input signals are combined in a summer  20 . In another embodiment of the invention, a filter is placed between each antenna  12   a ,  12   b  and the summer  20  rather than between the summer  20  and the rest of the microcontroller system  10 .  
         [0031]    The output of the summer  20  is coupled as an input to the AND gate  24  and the control circuit  22 . The AND gate  24  and control circuit  22  act together to filter the signal. In another embodiment of the invention, another type of gate such as a NAND, OR, or NOR gate is used instead of the AND gate  24 . An alternative embodiment includes a gate that acts as a combination of an OR gate stage and an AND gate stage. The control circuit  22  output is coupled to an OR gate stage input along with an override signal. The OR gate stage output is then provided as an input to the AND gate stage along with the input signal from the summer  20 . In this manner, a high override signal causes the AND gate to pass the input signal regardless out of the output of the control circuit  22  without affecting the output of the control circuit  22 . A low override signal then allows the control circuit  22  to determine from its present state whether the input signal will be transmitted or blocked. Those of ordinary skill in the art would realize that there are many circuits and gates that could implement the alternative embodiment described above.  
         [0032]    The filter components of FIG. 1 also include an reset input  25 . The reset input  25  can force the output of the control circuit  22  high. Unlike the override input of the alternative embodiment, the control circuit  22  state is not preserved when the reset input  25  is driven high. The reset input  25  can be driven high temporarily to allow the filter to receive and transmit input signals without a delay, but maintaining the duty cycle requirement for continued transmission of the input signal.  
         [0033]    The output of the AND gate  24  is coupled to a microcontroller  28  and a wake up circuit  26 . In one embodiment, the wake up circuit  26  is a component of the microcontroller  28 . The microcontroller  28  has a low power sleep or standby mode and a higher power active mode. When the AND gate  24  outputs a high signal while the microcontroller is in sleep mode, the wake up circuit  26  activates the microcontroller  28 . When the AND gate has output low signals for a specified time, the microcontroller  28  enters sleep mode. In active mode the microcontroller  28  analyzes the signal received from the AND gate  24 . If the signal matches a code signal, the microcontroller  28  activates a controlled device  30 . In one embodiment, the controlled device is an automobile door lock that unlocks in response to the signal from the microcontroller  28 .  
         [0034]    Referring now to FIG. 2, a circuit diagram of one embodiment of a control circuit  22  is illustrated. The switch  32  is driven by the input signal from the summer  20 . When the input signal is high, the switch  32  connects a current source  34  to the capacitor  38 , forming a circuit that is connected to the AND gate  24  input. The current source is also connected to a high voltage point  40 . When the input signal is low, the switch  32  connects current source  36   a  to the capacitor  38 , forming a circuit that is connected to the AND gate  24  input. Both the capacitor  38  and the current source  36   a  are connected to ground  42 .  
         [0035]    When the switch  32  connects current source  34  to the capacitor  38 , the voltage on the capacitor  38  is increased until it reaches high level. When the switch connects current source  36   a  to the capacitor  38 , the voltage on the capacitor is decreased until it reaches ground. The ratio of magnitudes between the current source  34  and current source  36   a  determines the duty cycle of the input signal that will allow the voltage on the capacitor  38  to be maintained. In one embodiment, a minimum duty cycle of 25% is attained by current source  34  having approximately three times the magnitude of current source  36   a . A magnitude factor of between 2.9 and 3.1 is sufficient in one embodiment. The 75% of the time that the input signal is low results in an equal and opposite current from the capacitor  38  as the 25% of the time that the input signal is high. With the same amount of charge being removed and then added (or the inverse) the voltage over the capacitor  38  returns to the same level.  
         [0036]    In addition to controlling the required duty cycle, the attributes of the current source  34  and capacitor  38  determine the hold time or duration of high level input signal guaranteed to pass the input signal through the AND gate  24 . The voltage on the capacitor  38  corresponds to the charge by the following equation V=q/C where V is the voltage, q is the charge, and C is the capacitance. When the capacitor  38  has no voltage, is at ground, the current source  34  must be connected long enough to provide an amount of charge that corresponds to a voltage level of high. That duration is determined by the capacitance, because the ratio of charge to voltage equals the capacitance. The magnitude of the current sources  34 ,  36   a  and the capacitance of the capacitor  38  can be modified to independently control the duty cycle requirement and the hold time.  
         [0037]    Referring now to FIG. 3, a circuit diagram of another embodiment of a control circuit  22  is illustrated. In this embodiment, a current source  36   b  that drains the capacitor  38  is connected regardless of the state of the switch  32 . The current source  34  that charges the capacitor  38  is only connected when the switch  32  receives a high input from the input signal. The hold time and duty cycle can be determined in the same way as discussed with reference to FIG. 2, though the difference between the charging current source  34  and draining current source  36   b  is used as the current source magnitude in the calculations. Many other circuit configurations are possible to implement the control circuit  22 . FIGS. 2 and 3 illustrate some of the possibilities.  
         [0038]    Referring to FIG. 4, a chart of signals in time for the input and output of a modulated input signal filter are illustrated. The input signal  50  is received from the antennas  12   a ,  12   b . The gate  24  outputs a filtered signal  52  to the microcontroller  28  and wake up circuit  26 . The control signal  62  is received by the gate  24  and determines whether the input signal  50  is passed or blocked. Each of the signals is shown with two states, high and low. Different sets of states can be used to control the filtering of the input signal.  
         [0039]    The first portion  54  of the input signal  54  includes noise artifacts though no signal is present. Though the artifacts have some effect on the control signal  62 , none of those effects are sufficient to change the control signal  62  from low to high. The division between low and high is indicated by the dotted line. Because the control signal  62  stays low, the filtered signal  52  does not include the noise artifacts and the microcontroller  28  does not leave its low power sleep mode.  
         [0040]    The second portion  56  of the input signal  54  is a high state of duration equal to the hold time as determined by the component values of the control circuit  22 . At the end of the second portion  56 , the control signal  62  has just exceeded the dotted line and is interpreted as a high state by the gate  24 . As a result, the gate  24  stops blocking the input signal  50  and begins to pass it through as the filtered signal  52 .  
         [0041]    The third portion  58  of the input signal  50  contains duty cycle coded information. The duty cycle of each bit is equal to or greater than the required duty cycle determined by the components of the control circuit  22 . The control signal  62  stays high during this period and gradually rises above the dividing line between low and high. As a result, the gate  24  continues to pass through the input signal  50  as the filtered signal  52 . In an alternative embodiment, certain information states may be less than the required duty cycle so that a large group of those bits or states will cause the gate  24  to block the input signal  50 .  
         [0042]    In fourth portion  60  of the input signal  50  the duty cycle encoded signal has been discontinued, but noise artifacts are still present. The control signal  62  is driven low in the absence of a signal. While some noise is transmitted before the control signal  62  achieves a low state, the noise received thereafter is not allowed into the filtered signal  52 . Because the microcontroller  28  receives only the filtered signal  52  rather than the input signal  50 , it can enter a sleep mode to conserve power.  
         [0043]    Referring to FIG. 5, a consumer system employing the input signal filter is illustrated. An automobile  204  includes a circuit capable of unlocking the doors  110 . The circuit  110  transmits a duty cycle encoded signal. A microcontroller and receiver are included on a passive key  200  that can be mounted on a key ring  202 . When the passive key  200  is distant from the automobile  204 , the receiver filters noise out and the microcontroller stays in a sleep mode that conserves energy. When the passive key  200  approaches the automobile, the signal from the circuit  110  becomes strong enough to be received by the antennas and drives the control signal to high. Once the control signal is high, the input signal is passed on to the microcontroller  28  which returns a signal recognized by the circuit  110 . The circuit  110  then unlocks the automobile  204  doors. In an alternative embodiment, the receiver, filter, and microcontroller may be provided on the automobile  204 . In that embodiment, the key  200  is active and sends a signal when actuated. The signal drives the control signal to high and wakes the microntroller. The microcontroller analyzes the signal and opens the doors in response.  
         [0044]    The invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While the invention has been depicted and described and is defined by reference to particular preferred embodiments of the invention, such references do not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts. The depicted and described preferred embodiments of the invention are exemplary only and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.