Abstract:
Amplitude Shift Keying (ASK) modulation system and method with an ASK demodulator that is implemented with an analog-emulating digital bandpass filter. The bandpass filter also generates a carrier detect signal when it detects a carrier frequency that passes the filter pass band.

Description:
CROSS-REFERENCE 
     The present application and invention are related in subject matter to that of U.S. patent application Ser. No. 08/323,282 filed concurrently herewith and having the same inventorship and assigned to the same assignee as this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to modulated electronic communications and more particularly to a system and method using Amplitude Shift Keying (ASK) modulation for such communications with an analog-emulating digital bandpass filter for demodulation and having application to infrared communication links. 
     2. Problem to be Solved 
     Amplitude Shift Keying (ASK) Modulation is one of the commonly used modulation methods for electronic communication. In communication systems using ASK encoding, demodulation is typically performed using an analog circuit while the controller which decodes the serial communication is designed using digital logic. The requirement for both analog and digital circuits tends to raise the cost of such communication systems. Furthermore, the analog bandpass filter used in the analog circuit usually requires many components, especially if it is to have a programmable cutoff frequency. In contrast, a digital bandpass filter lacks accuracy and still requires some minimal analog circuit to amplify the incoming signal. Although both types of filters have drawbacks, a system that requires various demodulators or different bandpass filters for multi-channel communication, will usually find a digital bandpass filter to be advantageous over an analog bandpass filter. 
     3. Objects 
     It is therefore an object of the present invention to provide an analog-emulating digital bandpass filter for use in demodulating ASK encoded signals. 
     It is another object of the present invention to provide an ASK MODEM including an improved digital bandpass filter for use in demodulating ASK modulated signals. 
     It is a further object of the present invention to provide an ASK MODEM including an improved digital bandpass filter for use in demodulating ASK modulated signals used in infrared (IR) communications. 
     SUMMARY OF THE INVENTION 
     The present invention involves an ASK MODEM with a demodulator that is implemented with a digital bandpass filter which is an analog-emulating filter that also generates a carrier detect signal when it detects a carrier frequency that is within the pass band. 
     The ASK demodulator takes an ASK modulated incoming signal, that is amplified by an analog circuit, and runs it through a Schmitt triggered input buffer to recondition the signal as well as to digitize it. The analog circuit just amplifies the input signals, and is AC coupled and DC offset so that it has a minimum at zero volts and a maximum at the supply voltage of the digital logic. The reconditioned signal is fed to the digital bandpass filter after synchronization with the system clock. 
     The analog-emulating digital bandpass filter consists of a carrier frequency qualifier, carrier detect logic, and a delay block that emulates the discharge characteristics of a capacitor in an analog band pass filter implementation. 
     The carrier frequency qualifier has a window that accepts a range of frequencies. The time between the receipt of the leading edge of one pulse to the leading edge of the next pulse is measured with a very high frequency clock. If this time is within the acceptable tolerance or within the desired band frequency, the carrier frequency qualifier activates a CARRIER --  GOOD signal which follows the pattern of the ASK signal. Since this is done in the digital logic the width of the band as well as the cut off frequency are readily programmable. The carrier detect logic then counts the number of consecutive incoming pulses while the CARRIER --  GOOD signal is active. If this logic counts a predefined number of pulses, it activates an ASK carrier detect circuit, which outputs a CARRIER --  DETECT signal. This number may also be programmed so that the accuracy of the carrier detect logic may be adjusted depending on the application. At the same time, the delay block logic prolongs the CARRIER-GOOD signal for a predefined amount of time, which also can be designed to be programmable. The BANDPASS-OUTPUT signal output of this delay block logic is the output of the bandpass filter and the logic emulates the discharge period of an analog filter. 
     The modulator portion of the MODEM consists of a carrier frequency generator and adder circuit. The system clock is divided down to generate a clock with the desired frequency and an almost 50 percent duty cycle. In some applications it may be desirable to make both the frequency and duty cycle programmable. The output of the carrier frequency generator is ANDed with inverted raw transmit data to create the required ASK transmit signal. 
     By using the digital bandpass filter in the ASK MODEM, a less expensive and yet effective demodulator is provided, which, for example, may facilitate infrared (IR) communications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating an ASK MODEM digital bandpass filter in accordance with the present invention. 
     FIG. 2 is a schematic diagram illustrating an ASK MODEM carrier detect circuit in accordance with the invention. 
     FIG. 3 is a schematic diagram illustrating an analog filter emulation in the ASK MODEM of the invention. 
     FIG. 4 is a schematic diagram illustrating a transmit carrier modulation circuit in the ASK MODEM of the invention. 
     FIG. 5 is a diagram illustrating the form of input signal and of the modulated signal produced by the ASK MODEM of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An ASK MODEM in accordance with the present invention may be implemented as part of a custom ASIC designed to support multiple IR modulation formats and protocols. The ASIC may include a standard 16550 UART (obtainable from VLSI Technology, Inc., Burlington, Mass.) which is used to produce and receive serial data in an asynchronous format. The present ASK modulator and demodulator are designed to be compatible with other ASK systems using a 500 kHz carrier. 
     Referring first to FIG. 4, a modulator circuit for producing an ASK transmit signal in accordance with the invention is shown with a carrier frequency generator and adder circuit wherein a seven bit counter 10 may be used to divide the input of about 36.864 MHz master clock source counts by 74, with 36 of those counts producing a high output signal, in order to generate a square wave output of about 498.16 kHz from latch 13. A latch 14 is set on the clock edge after the seven bit counter 10 reaches the count of 72. The output of the latch 14 is fed back on line 15 to reset the counter 10 so that the counter cycle always consists of 74 clock counts. The value of the counter 10 output is sampled by latch 13, the output of which goes HIGH if the count value is in the range from 28 to 63. The output of latch 13 is a square wave of 498.16 kHz and is used to modulate the signal from the transmit line 12 of a standard UART by logically gating it through AND gate 16. 
     Accordingly, a square wave is generated on line 11 whenever the UART output transmit signal on line 12 is LOW. The forms of the transmit signal and square wave are shown in FIG. 5. Thus, an approximately 500 kHz carrier signal is produced during the start bit of each character in the input signal and during each zero bit period. The 500 kHz output signal is then fed on line 11 to a suitable output device, such as an IR source 17, in order to produce the desired ASK modulated IR light signal. The clock to the seven bit counter 10 is disabled whenever the UART transmit line 12 is HIGH in order to conserve power. The system clock is divided down to generate a clock with the desired frequency and an almost 50 percent duty cycle. In some applications it may be desirable to make both the frequency and duty cycle programmable. The output of the carrier frequency generator is ANDed with inverted raw transmit data to create the required ASK modulated transmit signal. 
     On the receiver side, as seen in FIG. 1, the ASK MODEM of the invention, has a demodulator comprising an IR transceiver 18, acting as a receiver, and an analog-emulating digital bandpass filter. The IR receiver 18 amplifies the incoming signal with an analog circuit and takes the amplified incoming signal and runs it through a Schmitt-triggered input buffer to recondition the signal as well as to digitize it. The analog circuit just amplifies the input signals and is AC coupled and DC offset so that it has a minimum at zero volts and a maximum at the supply voltage of the digital logic. The reconditioned signal is fed to the digital bandpass filter after synchronization with the system clock. 
     The bandpass filter is variously shown in FIGS. 1-3 and consists of a carrier frequency qualifier (FIG. 1), carrier detect logic (FIG. 2), and a delay block (FIG. 3) that emulates the discharge characteristics of an analog band pass filter implementation. Briefly the filter operates as follows. The carrier frequency qualifier of FIG. 1 has a window that accepts a range of pulse frequencies. The time between the receipt of the leading edge of one pulse to the leading edge of the next pulse is measured with a very high frequency clock. If this time is within the acceptable tolerance or within the desired band frequency, the carrier frequency qualifier activates a CARRIER --  GOOD signal. Since this is done in the digital logic, the width of the band and the cut off frequency may readily be made programmable. The carrier detect logic then counts the number of consecutive incoming pulses while CARRIER --  GOOD is active. If it counts a predefined number of pulses, it activates an ASK carrier detect circuit, shown in FIG. 2, which outputs a CARRIER --  DETECT signal. This predefined number may also be programmed so that the accuracy of the carrier detect logic may be adjusted depending on the application. At the same time, the delay block logic of FIG. 3 prolongs the CARRIER --  GOOD signal for a predefined amount of time, which also can be designed to be programmable. The output of this delay logic is the output of the bandpass filter. 
     More particularly, referring to FIG. 1, the digital bandpass filter for emulating an ASK analog bandpass filter, and centered around a carrier frequency of 500 kHz, consists of a seven bit counter 19 which is clocked by a master 36.864 MHz clock. Counter 19 is reset on each edge of an incoming IR pulse, from IR transceiver 18, which is detected by latches 20 and 21. The current count value is compared each time the counter 19 is reset and a latch 22 is set if the count value is in the range of 68 to 79. The output of latch 22 is gated by an AND gate with the inverted output of latch 26 which becomes set when counter 19 reaches the count of 80. The CARRIER --  GOOD signal is therefore shut off if no subsequent IR pulse is received within 80 clock pulse cycles of the previous IR pulse. 
     By setting latch 22 in the 68 to 79 count range, latch 22 is therefore set when the period between two incoming IR pulses corresponds to a frequency of anywhere from 467 kHz to 542 kHz and its ANDed output produces the CARRIER --  GOOD signal. Since such signal sensing is an overly strict bandpass indicator, an additional stage (FIG. 3) may be added to allow the CARRIER --  GOOD output of the filter to stay set for up to three carrier pulse periods after latch 22 becomes set, regardless of the incoming IR signal. This additional stage functions to simulate the time constant of discharging which occurs in an analog bandpass circuit. 
     Specifically, as seen in FIG. 3, a four bit counter 23 is reset, in response to the CARRIER --  GOOD signal output which results whenever a valid IR signal is detected by the band pass filter of FIG. 1. The valid CARRIER --  GOOD IR signal is then prolonged by a latch 24 which is only reset when the output of the counter 23 reaches a count of 15. Latch 24 accordingly controls the output of the digital band pass filter, which output is then fed to a UART for implementing. Thus this additional stage simulates the time constant of discharging of an analog band pass filter. 
     An additional circuit shown in FIG. 2 is included which sets a flag, the CARRIER --  DETECT SIGNAL, when a minimum of eight consecutive IR pulses satisfy the frequency range requirement. This circuit uses a four bit counter 25 to count the number of valid IR pulses of the CARRIER --  GOOD signal. The CARRIER --  DETECT SIGNAL flag is used to provide an indication to the controlling software that a valid ASK modulated signal is being received. 
     By using the digital bandpass filter in the ASK MODEM, a less expensive and yet effective demodulator is provided which may be used for handling and facilitating infrared (IR) communications. 
     The ASK MODEM described herein is particularly suitable for use in the Interoperable Multiprotocol Directed Infrared Wireless Communication Controller disclosed in our U.S. patent application Ser. No. 08/323,282 filed concurrently herewith and incorporated by reference. This MODEM also can be used in any communication controller that uses ASK modulation. The programmable digital bandpass filter of the invention can be used in any application that requires a digital bandpass filter. 
     This MODEM has been found to be compatible, for example, with the Sharp WIZARD personal organizer unit which employs a 500 kHz analog IR transceiver. Additionally, the ASK modulation has been tested between two custom ASIC units up to data rates of 38.4 kbps. 
     While the invention has been particularly shown and described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.