Abstract:
A signal processing circuit utilizes a convolver correlating a reference carrier signal with a received signal. The correlator output is detected and then demodulated to output the signal information originally present on the carrier. For a conventional AM signal the reference carrier signal is applied at a frequency offset from the AM center frequency. When the carrier is an FM signal, the reference signal is applied at a frequency equal to the center frequency of the FM signal. Similar procedures are followed for a spread-modulated AM signal, and the reference signal is modulated with a suitably timed-reverse replica of the pseudo-noise code. A similar procedure is used to process spread-spectrum frequency shift keyed FM signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a communication device using a convolver as a demodulator in particular for an analogue modulated signal. 
     BACKGROUND OF THE INVENTION 
     Heretofore no example is known, in which a convolver is used as a demodulator in an AM or FM receiver. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to provide an AM or FM communication device using a convolver as a demodulator. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above object, a communication device according to the present invention is characterized in that it comprises a reference signal generator for generating a reference carrier signal having a frequency which is nearly equal to or somewhat deviated from the central frequency of an AM or FM modulated received signal; a surface acoustic wave convolver correlating the reference carrier signal with the received signal; means for detecting a correlated output signal outputted by the convolver; and demodulating means for demodulating an output signal thus detected to obtain the information signal. 
     The present invention may include further a communication device consisting of a transmitter provided with means for spread-modulating an AM or FM modulated carrier signal to be transmitted by using a pseudo-noise code and a receiver comprising a spread-modulating means for spread-modulating a reference carrier signal having a frequency, which is nearly equal to or somewhat deviated from the central frequency of an AM or FM modulated received signal by using a reference pseudo-noise code inverted in time with respect to the pseudo-noise code stated above; a surface acoustic wave convolver for correlating a spread-modulated signal thus received; means for detecting a correlated output signal outputted by the convolver; and demodulating means for demodulating an output signal thus detected to obtain information signal. 
     By the method using the spectrum diffusion, the demodulating processes by the carrier wave and the pseudo-noise code (PN code) are performed asynchronously and for this reason the repetition speed of the PN code is sufficiently greater than the frequency of information signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are block diagrams showing the construction of a communication device according to the present invention; 
     FIG. 2 is a block diagram for explaining in- and output characteristics of a convolver; 
     FIG. 3 is a scheme indicating amplitude characteristics of a correlated output signal; 
     FIG. 4 is a scheme showing the principle for using the convolver as an AM demodulator; 
     FIG. 5 is a scheme showing the principle for using the convolver as an FM demodulator; 
     FIGS. 6(a) to 6(h) and 8(a) to 8(f) are schemes showing waveforms at various parts of the devices indicated in FIGS. 1A, 1B and 7A, 7B, respectively; and 
     FIGS. 7A and 7B are block diagrams showing the construction of a communication device, which is another embodiment of the present invention, in which a signal diffusion-modulated by using the PN code is transmitted. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A and 1B are block diagrams showing the construction of a communication device according to the present invention, FIG. 1A indicating a transmitter, FIG. lB a receiver, in which reference numeral 1 is an AM/FM modulator; 2 is an oscillator; 3 is a band pass filter; 4 is an amplifier; 5 is a transmitting antenna; 6 is a receiving antenna; 7 is a band pass filter; 8 is an amplifier; 9 is a mixer; 10 is an oscillator; 11 is an amplifier; 12 is a band pass filter; 13 is an SAW convolver; 14 is an oscillator; 15 is a band pass filter; 16 is an amplifier; 17 is an envelope detector; 18 is a low pass filter; and 19 is an amplifier. 
     The operation of the embodiment described above will be explained below. 
     At first, the in- and output characteristics of the convolver will be explained. 
     In a convolver system as indicated in FIG. 2, supposing that the received signal input C 1  (t) and the reference signal input C 2  (t) are sinusoidal signals as given by; 
     
         C1(t)=A1 (t)·cos (2π·f IF·t) (1) 
    
     
         C2(t)=A2(t)·cos(2π·f REF·t)  (2) 
    
     the correlated output signal C 3  (t) is also a sinusoidal signal as expressed by: 
     
         C3(t)=A3(t, Δf)·cos (2π·f out·t) (3) 
    
     where 
     
         A3(t, Δf)=α·A1(t)·A2 (t)·B(Δf) (4) 
    
     α: proportionality constant, ##EQU1## Δf=f IF-f REF τ: delay time in the convolver, and 
     
         fout=fIF+fREF                                              (5) 
    
     That is, from Equation (5), the carrier frequency f OUT  of the correlated output signal is the sum of the carrier frequencies of the inputted signals (=F IF  +f REF ). Further, from Equation (4), concerning the amplitude A 3  (t,Δf) of the correlated output signal, it can be seen that 
     (1) A 3  (t,Δf) is proportional to the amplitude A 1  (t) of the received signal, and 
     (2) A 3  (t,Δf) is proportional to B(Δf), that is, it has attenuation characteristics, which are proportional to sin x/x (where x=π·Δf·τ) for a difference in the carrier frequency between the received signal and the reference signal Δf (=f IF  -f REF ). 
     FIG. 3 shows amplitude characteristics of the correlated output signal, when it is supposed that A 1  (t) and A 2  (t) are constant, that is, ##EQU2## 
     Now the result stated above concerning the amplitude A 3  (t,Δf) of the correlated output signal of the signals expressed by Equation (1) and (2) suggests for the application of the convolver that: 
     (1&#39;) It is set so that Δf=0, that is, f IF  =f REF . In this case, the correlated output signal has the greatest response. Further, it is set so that the amplitude A 2  (t) of the reference signal is constant. At this time, supposing that the amplitude A 1  (t) of the received signal is expressed by A 1  (t)=1+a 1  (t), it is possible to obtain the signal component a 1  (t) in the received signal by demodulating the correlated output signal. That is, it is possible to use the convolver as the AM demodulator. FIG. 4 shows the principle therefor, in which P indicates the working point of the convolver. 
     (2&#39;) It is set so that A 2  (t) is constant and an FM modulated signal C 1  &#39;(t) given by; ##EQU3## is inputted as the received signal. f IF  is so set that Δf is between 0 and +1/τ or between 0 and -1/τ. 
     At this time, it is possible to obtain the signal component a 1  (t) in the received signal by demodulating the correlated output signal (the convolver is used as a frequency-voltage converter). That is, it is possible to use the convolver as an FM demodulator. FIG. 5 shows the principle therefor, in which P 1  and P 2  indicate the working points of the convolver. 
     FIGS. 6(a) to 6(h) show waveforms at various parts of the device indicated in FIGS. 1A and 1B. 
     In FIGS. 1A and 1B, the transmitter AM or FM modulates an RF carrier signal (frequency f RF ) with the information signal (FIG. 6(a)) to transmit it (FIG. 6(b) or 6(c)). In FIGS. 6(a) to 6(h), the ordinate represents the amplitude of the signals. 
     In the receiver, the central frequency of the received signal is frequency-converted into an intermediate frequency f IF  by means of the local oscillator 10 (frequency f LO ), which is inputted in one of the input ports of the convolver. 
     On the other hand, the reference carrier signal (frequency f REF ) is inputted in the other input port of the convolver. 
     Further, the value of f LO  is so set that f IF  =of  REF  for AM and f IF  =f REF  +1/2τ for FM. 
     At this time, the correlated output signal is as indicated in FIG. 6(d) or 6(e). The demodulated signal (FIG. 6(h)) corresponding to the information signal a 1  (t) is obtained by envelope-detecting it (FIG. 6(f) or 6(g)) and making it pass through the low pass filter 18 to shape the waveform thereof. 
     FIGS. 7A and 7B are block diagrams showing the construction of a communication device, which is another embodiment of the present invention, in which a signal spread-modulated by using the PN code is transmitted. In the figures, the reference numerals used in common in FIGS. 1A and 1B represent items identical or corresponding to those indicated in FIGS. 1A and 1B; 20 and 23 are clock generators; 21 and 24 are PN code generators and 22 and 25 are mixers. FIGS. 8(a) to 8(f) show waveforms at various parts of the device indicated in FIGS. 7A and 7B, in which the ordinate represents the amplitude of the signal. 
     In this embodiment, in the transmitter (FIG. 7A), the RF carrier signal (frequency f RF ) is AM or FM modulated with the information signal (FIG. 8(a)) and thereafter it is further BPSK (binary phase-shift keying) modulated (FIG. 8(b) or 8(c)) with the PN code (PN) to be transmitted. 
     In the receiver (FIG. 8B), the central frequency of the received signal is frequency-converted into an intermediate frequency f IF  by means of the local oscillator 10 (frequency f LO ), which is inputted in one of the input ports of the convolver. 
     On the other hand, the reference carrier signal (frequency f REF ), which is BPSK modulated with the PN code (PN) inverted in time with respect to that used on the transmitter side, is inputted in the other input port of the convolver. 
     Further, the value of f LO  is so set that f IF  =f REF  for AM and f IF  =f REF  +1/2τ for FM. 
     At this time, the correlated output signal is as indicated in FIG. 8(d). The demodulated signal (FIG. 8(f)) corresponding to the information signal a 1  (t) is obtained by envelope-detecting it (FIG. 8(e)) and making it pass through the low pass filter 18 to shape the waveform thereof. 
     As explained above, according to the present invention, it is possible to obtain a communication device using a convolver as a demodulator for an AM or FM modulated signal. 
     While particular embodiment of the invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the present invention in its broader aspect.