Abstract:
A frequency ramp modulation technique that encodes information using frequency ramps in a transmit signal. Frequency ramp modulation according to the present teachings may be employed in a communication system that includes a narrow band receiver to maintain communication even as the pass band of the narrow band receiver drifts with temperature or other factors.

Description:
BACKGROUND  
       [0001]     A variety of modulation techniques exist that may be used in a communication system to encode information into a transmit signal. Examples of prior modulation techniques include amplitude modulation (AM) and frequency modulation (FM). Other prior modulation techniques include techniques that may combine amplitude and phase modulation, e.g. quadrature amplitude modulation (QAM), GPSK, etc.  
         [0002]     It may be desirable in a communication system to employ a receiver having a relatively narrow receive band. For example, a narrow receive band may reduce the power consumption of a receiver. In addition, a narrow receive band may enable an increase in the sensitivity of a receiver.  
         [0003]     Unfortunately, prior modulation techniques may have limited applicability to a communication system that includes a narrow band receiver. For example, the center frequency of the pass band of a narrow band receiver may drift with temperature. In addition, manufacturing variation in the components of a narrow band receiver may cause variation in the center frequency of its pass band. As a consequence, the pass band of a narrow band receiver may drift outside of the transmit band of a transmitter that employs a prior modulation technique, thereby breaking the communication link between the transmitter and the narrow band receiver.  
       SUMMARY OF THE INVENTION  
       [0004]     A frequency ramp modulation technique is disclosed that encodes information using frequency ramps in a transmit signal. Frequency ramp modulation according to the present teachings may be employed in a communication system that includes a narrow band receiver to maintain communication even as the pass band of the narrow band receiver drifts with temperature or other factors.  
         [0005]     Other features and advantages of the present invention will be apparent from the detailed description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:  
         [0007]      FIG. 1  shows a communication system according to the present teachings;  
         [0008]      FIG. 2  illustrates an example series of continuously varying frequency ramps in the transmit signal;  
         [0009]      FIG. 3  illustrates an example series of step-wise frequency ramps in the transmit signal;  
         [0010]      FIG. 4  illustrates a transmitter according to the present teachings;  
         [0011]      FIG. 5  illustrates a receiver according to the present teachings.  
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows a communication system  10  according to the present teachings. The communication system  10  includes a transmitter  12  and a receiver  14 . The transmitter  12  generates a transmits signal  20 . The transmit signal  20  carries information that is indicated by a frequency ramp in the transmit signal  20 . The receiver  14  receives the transmit signal  20  and obtains the information carried in the transmit signal  20  by detecting the frequency ramp in the transmit signal  20 .  
         [0013]     The frequency ramp in the transmit signal  20  may be an increase in a frequency of the transmit signal over time. The frequency ramp in the transmit signal  20  may be a decrease in a frequency of the transmit signal  20  over time. The frequency ramp in the transmit signal  20  may be a step-wise increase or decrease in a frequency of the transmit signal over time.  
         [0014]      FIG. 2  illustrates an example series of continuously varying frequency ramps in the transmit signal  20 . In this example, a falling ramp in a frequency of the transmit signal  20  indicates a “0” of information and a rising ramp in a frequency of the transmit signal  20  indicates a “1” of information. The series of ramps shown encodes the information “0011000” in the transmit signal  20 .  
         [0015]      FIG. 3  illustrates an example series of step-wise frequency ramps in the transmit signal  20 . In this example, a step-wise decrease in a frequency of the transmit signal  20  indicates a “0” of information and a step-wise increase in a frequency of the transmit signal  20  indicates a “1” of information. The series of step-wise ramps shown encodes the information “0011000” in the transmit signal  20 .  
         [0016]      FIG. 4  illustrates the transmitter  12  in one embodiment. The frequency ramp modulator in the transmitter  12  in this embodiment includes a ramp generator  30  and a voltage controlled oscillator  32 .  
         [0017]     The ramp generator  30  generates a ramp signal  42  in response to an information signal  40 . For example, the information signal  40  may carry digital information including a series of 0&#39;s and 1&#39;s to be carried via the transmit signal  20 . In an embodiment corresponding to the information coding illustrated in  FIG. 2 , the ramp generator  40  generates a rising ramp in the amplitude of the ramp signal  42  in response to a “1” in the information signal  40  and a falling ramp in the amplitude of the ramp signal  42  in response to a “0” in the information signal  40 . The ramp generator  30  may generate triangle waveform ramps or some other form of linear ramp or a non-linear monotonic ramp.  
         [0018]     The voltage controlled oscillator  32  generates the transmit signal  20  at a frequency that is determined by a magnitude of the ramp signal  42 . A rising ramp in the magnitude of the ramp signal  42  causes a rising ramp in the frequency of the transmit signal  20  and a falling ramp in the magnitude of the ramp signal  42  causes a falling ramp in the frequency of the transmit signal  20 .  
         [0019]      FIG. 5  illustrates the receiver  14  in one embodiment. The receiver  14  in this embodiment includes a band-pass filter  60  and a frequency ramp demodulator  62 . The band-pass filter  60  controls the receive band of the receiver  14 . In some embodiments, the receiver  14  may not include the band-pass filter  60  or may include another type of filter.  
         [0020]     The band-pass filter  60  generates a filter output signal  70  by filtering out frequency components of the transmit signal  20  that do not fall within a pass band of the band-pass filter  60 . The range of frequencies of the frequency ramps in the transmit signal  20 , between f 1  and f 2 , is selected in response to the pass band of the band-pass filter  60  and the likely drift in the center frequency of the pass band of the band-pass filter  60 . The range between f 1  and f 2  is selected so that some part of the frequency ramps between f 1  and f 2  will overlap the pass band of the band-pass filter  60  even after variation in the center frequency of the band-pass filter  60 , thereby maintaining communication between the transmitter  12  and the receiver  14 .  
         [0021]     The frequency ramp demodulator  62  recovers information from the filter output signal  70  by detecting rising and falling frequency ramps in the filter output signal  70 . In an embodiment corresponding to the information coding illustrated in  FIG. 2 , the frequency ramp demodulator  62  generates a “1” in an information signal  72  in response to a rising ramp in a frequency of the filter output signal  70  and generates a “0” in the information signal  72  in response to a falling ramp in a frequency of the filter output signal  70 .  
         [0022]     The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.