Demodulation apparatus and communication system using the same

A demodulation apparatus and a communication system using the same are disclosed. The apparatus comprises code generation means for generating a reference spread code corresponding to a first signal modulated by a spread code, and an elastic surface wave element for receiving the first signal and a second signal output from the code generation means, and outputting a demodulated information signal. The elastic surface wave element comprises a piezoelectric substrate, a first excitation electrode, formed on the piezoelectric substrate, for generating a first elastic surface wave which propagates in a predetermined direction in accordance with the first signal, a second excitation electrode, formed on the piezoelectric substrate, for generating a second elastic surface wave which propagates in a direction opposite to the predetermined direction in accordance with the second signal, and an acousto-electric converter, formed between the first and second excitation electrodes on the piezoelectric substrate, for outputting a convolution signal of the first and second signals on the basis of the first and second elastic surface waves, wherein the acousto-electric converter selectively converts an elastic surface wave having a wave number twice a wave number of the first and second elastic surface waves into an electrical signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention releases to an apparatus for demodulating an 
information signal and, more particularly, to a demodulation apparatus 
suitable for demodulating a signal modulated by spread spectrum modulation 
and a communication system using the same. 
2. Related Background Art 
A spread spectrum communication is a communication for transmitting an 
information signal by spreading the information signal to a sufficiently 
wide bandwidth, and has the following features. That is, the spread 
spectrum communication allows code divided multiplexing, discriminates 
strongly against a disturbance, has a high privacy function, and so on. 
In reception processing of the spread spectrum communication, an 
information signal is demodulated by executing so-called inverse spreading 
processing for correlating spread codes assigned to respective channels 
with a received signal. Conventionally, a receiver of the spread spectrum 
communication is divided into a synchronization unit and a demodulation 
unit. The synchronization unit achieves synchronization by a sliding 
correlation method for detecting a correlation between received spread 
codes and reference spread codes having a bit rate slightly different from 
that of the received spread codes. 
As a system capable of high-speed synchronous acquisition, a system using a 
convolver is known. The convolver is a convolution arithmetic element, and 
serves as a correlator when one of two input signals is set to be a 
temporally inverted signal. More specifically, when a received signal is 
input as one of input signals to the convolver, and a signal obtained by 
temporally inverting the received signal is input as the other input 
signal, the two signals coincide with each other at a certain timing, and 
generate a sharp peak output. In particular, if spread codes used in this 
case are those with good auto-correlation characteristics, a sharp peak 
output is generated only when the two signals coincide with each other; 
otherwise, almost no output appears. As one of convolvers, an elastic 
surface wave convolver is known. The elastic surface wave convolver is 
effective for high-speed transmission since it is an analog arithmetic 
element, and can execute signal processing in real time. 
FIG. 1 shows a conventional spread spectrum receiver using an elastic 
surface wave convolver. Referring to FIG. 1, the receiver comprises first 
and second frequency converters 2 and 4, an elastic surface wave element 
(convolver) 5, a filter (F) 6, a detector (D) 10, a peak detection circuit 
(PD) 8, and code generators (CG) 3 and 9. A received signal 1 is converted 
into an intermediate frequency by the frequency converter 2, and is input 
to the elastic surface wave element 5. The code generator 3 generates 
codes obtained by temporally inverting spread codes of the received signal 
as reference spread codes, and inputs these codes to the elastic surface 
wave element 5 via the frequency converter 4. 
The elastic surface wave element 5 comprises first and second excitation 
electrodes 102 and 103 for exciting an elastic surface wave on a 
piezoelectric substrate 101, and a rectangular output electrode 105 
arranged between the first and second excitation electrodes 102 and 103. 
Each of the first and second frequency converters 2 and 4 comprises an 
oscillator 15, a multiplier 13, and a filter (F) 14. 
When the received signal 1 is input to the first excitation electrode 102, 
and the output from the code generator 3 is input to the second excitation 
electrode 103, a first elastic surface wave excited by the first 
excitation electrode 102 and a second elastic surface wave excited by the 
second excitation electrode 103 overlap each other on the output electrode 
105 while propagating in opposite directions. Since the displacement and 
potential of a product of the two elastic surface waves are generated on 
the substrate 101 to have a frequency twice that of the input signal and a 
wave number=0 by the parametric mixing phenomenon of the two overlapping 
elastic surface waves, the output electrode 105 can extract the 
overlapping elastic surface waves as an electrical signal by integrating 
them within the range of the output electrode. Therefore, the elastic 
surface wave element 5 generates a sharp peak output at a center frequency 
2f (where f is the center frequency of the input signal) when the two 
elastic surface waves coincide with each other. This output is extracted 
via the filter 6, and is envelope-detected by the detector 10. Thereafter, 
the output from the detector 10 is subjected to peak detection by the peak 
detection circuit 8. On the basis of peak information obtained by the peak 
detection circuit 8, the generation timing of the reference spread codes 
to be generated by the code generator 3 is adjusted, so that the reference 
spread codes generated by the code generator 3 coincide with the received 
signal on the elastic surface wave element 5 in a desired state, thus 
synchronizing the codes. 
The above-mentioned peak information is also input to the code generator 9 
for generating the same spread codes as those of the received signal. When 
the code generator 9 generates spread codes in synchronism with the 
received signal, and a multiplier 11 multiplies the generated codes with 
the received signal, the received signal modulated by spread spectrum 
modulation is inversely spread. Since the inversely spread signal is a 
modulated signal modulated by frequency modulation, phase modulation, or 
the like, which is normally used, the signal is demodulated via a 
conventional demodulator (DM) 12. 
However, in the prior art described above, the output from the elastic 
surface wave element is used only for synchronizing codes, and the 
demodulation unit must be arranged in addition to the synchronization 
unit, resulting in a large circuit scale. 
On the other hand, Nakagawa et al. "dc effects in elastic surface waves", 
Applied Physics Letters, Vol. 24, No. 4, pp. 160-162, 15 Feb. 1974 
discloses an elastic surface wave element shown in FIG. 2. Referring to 
FIG. 2, input interdigital transducers 17 and 18, and an output 
interdigital transducer 19 are formed on a piezoelectric substrate 16. 
When a pulse signal having a wave number=k and a frequency=.omega. is 
input to the transducer 17, and a pulse signal having a wave number=-k and 
a frequency=.omega. is input to the transducer 18, the two transducers 17 
and 18 generate elastic surface waves which propagate in opposite 
directions. These elastic surface waves cause an interaction on a region 
between the transducers 17 and 18, and a signal having a wave number=2k 
and a frequency=0 is extracted from the output transducer 19 by a 
nonlinear effect. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a demodulation 
apparatus capable of demodulating a signal demodulated by spread codes by 
a simple arrangement, and a communication system using the demodulation 
apparatus. More specifically, the present invention facilitates data 
demodulation by utilizing the element disclosed in the reference of 
Nakagawa et al. in the demodulation apparatus. 
In order to achieve the above object, a demodulation apparatus according to 
an aspect of the present invention comprises: 
code generation means for generating a reference spread code corresponding 
to a first signal modulated by a spread code; and 
an elastic surface wave element for receiving the first signal and a second 
signal output from the code generation means, and outputting a demodulated 
information signal, 
the elastic surface wave element comprising: 
a piezoelectric substrate; 
a first excitation electrode, formed on the piezoelectric substrate, for 
generating a first elastic surface wave which propagates in a 
predetermined direction in accordance with the first signal; 
a second excitation electrode, formed on the piezoelectric substrate, for 
generating a second elastic surface wave which propagates in a direction 
opposite to the predetermined direction in accordance with the second 
signal; and 
an acousto-electric converter, formed between the first and second 
excitation electrodes on the piezoelectric substrate, for outputting a 
convolution signal of the first and second signals on the basis of the 
first and second elastic surface waves, 
wherein the acousto-electric converter selectively converts an elastic 
surface wave having a wave number twice a wave number of the first and 
second elastic surface waves into an electrical signal. 
A demodulation apparatus according to another aspect of the present 
invention comprises: 
code generation means for generating a reference spread code corresponding 
to a first signal modulated by a spread code; 
a first elastic surface wave element for receiving the first signal and a 
second signal output from the code generation means, and outputting a 
demodulated information signal; and 
a second elastic surface wave element for receiving the first signal and 
the second signal output from the code generation means, and outputting a 
demodulated information signal, 
each of the first and second elastic surface wave elements comprising: 
a piezoelectric substrate; 
a first excitation electrode, formed on the piezoelectric substrate, for 
generating a first elastic surface wave which propagates in a 
predetermined direction in accordance with the first signal; 
a second excitation electrode, formed on the piezoelectric substrate, for 
generating a second elastic surface wave which propagates in a direction 
opposite to the predetermined direction in accordance with the second 
signal; and 
an acousto-electric converter, formed between the first and second 
excitation electrodes on the piezoelectric substrate, for outputting a 
convolution signal of the first and second signals on the basis of the 
first and second elastic surface waves, 
wherein the acousto-electric converter selectively converts an elastic 
surface wave having a wave number twice a wave number of the first and 
second elastic surface waves into an electrical signal. 
A communication system according to an aspect of the present invention 
comprises: 
a transmitter for transmitting a first signal modulated by a spread code; 
a receiver for receiving the first signal transmitted from the transmitter; 
code generation means for generating a reference spread code corresponding 
to the first signal; and 
an elastic surface wave element for receiving the first signal received by 
the receiver, and a second signal output from the code generation means, 
and outputting a demodulated information signal, 
the elastic surface wave element comprising: 
a piezoelectric substrate; 
a first excitation electrode, formed on the piezoelectric substrate, for 
generating a first elastic surface wave which propagates in a 
predetermined direction in accordance with the first signal; 
a second excitation electrode, formed on the piezoelectric substrate, for 
generating a second elastic surface wave which propagates in a direction 
opposite to the predetermined direction in accordance with the second 
signal; and 
an acousto-electric converter, formed between the first and second 
excitation electrodes on the piezoelectric substrate, for outputting a 
convolution signal of the first and second signals on the basis of the 
first and second elastic surface waves, 
wherein the acousto-electric converter selectively converts an elastic 
surface wave having a wave number twice a wave number of the first and 
second elastic surface waves into an electrical signal. 
A communication system according to another aspect of the present invention 
comprises: 
a transmitter for transmitting a first signal modulated by a spread code; 
a receiver for receiving the first signal transmitted from the transmitter; 
code generation means for generating a reference spread code corresponding 
to the first signal; 
a first elastic surface wave element for receiving the first signal 
received by the receiver, and a second signal output from the code 
generation means, and outputting a demodulated information signal; and 
a second elastic surface wave element for receiving the first signal 
received by the receiver, and the second signal output from the code 
generation means, and outputting a demodulated information signal, 
each of the first and second elastic surface wave elements comprising: 
a piezoelectric substrate; 
a first excitation electrode, formed on the piezoelectric substrate, for 
generating a first elastic surface wave which propagates in a 
predetermined direction in accordance with the first signal; 
a second excitation electrode, formed on the piezoelectric substrate, for 
generating a second elastic surface wave which propagates in a direction 
opposite to the predetermined direction in accordance with the second 
signal; and 
an acousto-electric converter, formed between the first and second 
excitation electrodes on the piezoelectric substrate, for outputting a 
convolution signal of the first and second signals on the basis of the 
first and second elastic surface waves, 
wherein the acousto-electric converter selectively converts an elastic 
surface wave having a wave number twice a wave number of the first and 
second elastic surface waves into an electrical signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
[First Embodiment] 
FIG. 3 is a diagram showing the first embodiment of a demodulation 
apparatus according to the present invention. Referring to FIG. 3, the 
demodulation apparatus comprises a first frequency converter 2 for 
converting the frequency of a received signal 1, a reference signal 
generator (RG) 3 for generating a reference signal including reference 
codes, and a second frequency converter 4 for converting the frequency of 
an output signal from the reference signal generator 3. Each of the first 
and second frequency converters 2 and 4 comprises an oscillator 15, a 
multiplier 13, and a filter (F) 14. 
The apparatus also comprises an elastic surface wave element (convolver) 5 
for receiving the output signals from the first and second frequency 
converters 2 and 4, and outputting a base-band convolution signal of the 
two input signals. The elastic surface wave element 5 comprises a 
piezoelectric substrate 101, first and second excitation electrodes 102 
and 103, formed on the piezoelectric substrate 101, for respectively 
exciting first and second elastic surface waves, and an acousto-electric 
converter 104 formed on a transmission path along which the first and 
second elastic surface waves propagate in opposite directions. 
The first and second excitation electrodes 102 and 103 comprise so-called 
interdigital transducers, and are formed on the piezoelectric substrate 
101 using a conductive film of aluminum, silver, gold, or the like. An 
arranging pitch P.sub.i of electrode fingers of the interdigital 
transducer (to be abbreviated as IDT hereinafter) is set to be 
substantially equal to v/2f where f is the center frequency of an input 
signal, and v is the propagation speed of an elastic surface wave. 
The acousto-electric converter 104 comprises a so-called IDT (interdigital 
transducer), and is formed on the piezoelectric substrate 101 using a 
conductive film of aluminum, silver, gold, or the like. An arranging pitch 
p.sub.o of electrode fingers of the IDT is set to be substantially equal 
to v/4f, i.e., 1/2 the pitch p.sub.i of the first and second excitation 
electrodes 102 and 103. 
The apparatus further comprises a filter (F) 6 for extracting a desired 
signal from the output from the elastic surface wave element 5. 
The first embodiment will be described in more detail below. 
The received signal 1 received from an antenna or a cable is input to the 
first frequency converter 2 via a filter, an amplifier, or the like, as 
needed, and is converted into an operation frequency (input center 
frequency f) of the elastic surface wave element 5, and the converted 
signal is input to the first excitation electrode 102 of the elastic 
surface wave element 5. On the other hand, the reference signal generator 
3 generates a code string obtained by temporally inverting a code string 
of the signal to be received, and the generated code string is converted 
into the operation frequency (input center frequency f) of the elastic 
surface wave element 5. The converted signal is input to the second 
excitation electrode 103 of the elastic surface wave element 5. 
In the elastic surface wave element 5, a first elastic surface wave having 
components of the received signal 1 is excited by the first excitation 
electrode 102, and a second elastic surface wave having reference signal 
components is excited by the second excitation electrode 103. The first 
and second elastic surface waves propagate in opposing direction on a 
region between the first and second excitation electrodes 102 and 103. A 
product component of the two elastic surface wave signals is generated on 
an overlapping region of the two elastic surface waves to have a wave 
number twice that of the first and second elastic surface waves, and a 
base-band frequency by the parametric mixing effect. This effect is 
reported in the above-mentioned reference of Nakagawa et al. More 
specifically, if the first elastic surface wave is given by: 
EQU F(t-x/v) exp (2.pi.jf(t-x/v)) 
and, if the second elastic surface wave is given by: 
EQU G(t+x/v) exp (2.pi.jf(t+x/v)) 
the displacement and potential of a component given by: 
EQU F(t-x/v).multidot.G(t+x/v) exp (4.pi.jf x/v) 
are generated on the overlapping region of the first and second elastic 
surface waves. The generated component corresponds to a signal having a 
carrier frequency=0 and a wave number=4.pi.f/v. Thus, when the 
acousto-electric converter 104 is formed so that the pitch P.sub.o of the 
IDT is substantially equal to v/4f on the overlapping region of the first 
and second elastic surface waves, a component given by: 
EQU .intg.F(t-x/v).multidot.G(t+x/v)dx 
that is, a convolution signal of signals F and G, is generated by the 
acousto-electric converter 104 as a base-band signal. This signal is 
extracted through the filter 6. Since the output from the filter 6 is the 
base-band signal, data can be easily demodulated. 
A spread spectrum communication, in particular, a direct spreading type 
spread spectrum communication, will be described below. In the direct 
spreading type spread spectrum communication, a plurality of bits are 
assigned to one bit of data using high-speed spread codes. When a spread 
spectrum signal is input as the received signal 1, if the reference signal 
generator 3 outputs a signal obtained by temporally inverting spread codes 
of the received signal, a code pattern of the received signal 1 and a code 
pattern output from the reference signal generator 3 periodically coincide 
with each other on the elastic surface wave element 5, and a peak output 
is obtained by the acousto-electric converter 104 at that time. The peak 
output is extracted via the filter 6. When the received signal 1 is 
modulated by two-phase phase modulation, an output signal has a positive 
or negative potential depending on data "1" or "0", as shown in FIGS. 4A 
and 4B. Therefore, by only checking if the output signal from the filter 6 
is positive or negative, data "1" or "0" can easily be discriminated. 
[Second Embodiment] 
FIG. 5 is a diagram showing the second embodiment of a demodulation 
apparatus according to the present invention. The same reference numerals 
in FIG. 5 denote the same parts as in FIG. 3. The arrangement of this 
embodiment is substantially the same as that in the first embodiment, 
except that a phase shifter (PS) 404 is arranged between an oscillator 401 
and a multiplier 402 in the second frequency converter 4, the output from 
the filter 6 is also input to a phase control circuit (PC) 7, and the 
phase control circuit 7 supplies a control signal to the phase shifter 
404. 
In this embodiment as well, the same effect as in the first embodiment can 
be obtained. Furthermore, in this embodiment, since the phase of a signal 
to be input to the multiplier 402 can be controlled in accordance with a 
variation in output level of the filter 6, a stable output can be obtained 
by controlling the phase relationship between the first and second elastic 
surface waves propagating on the acousto-electric converter 104 of the 
elastic surface wave element 5. 
In this embodiment, the phase shifter 404 is arranged in the second 
frequency converter 4. However, the same effect as described above can be 
obtained when a phase shifter is arranged in the first frequency converter 
2. 
[Third Embodiment] 
FIG. 6 is a diagram showing the third embodiment of the present invention. 
The same reference numerals in FIG. 6 denote the same parts as in FIGS. 3 
and 5. The arrangement of this embodiment is substantially the same as 
that of the first embodiment, except that first and second excitation 
electrodes 202 and 203 of the elastic surface wave element 5 comprise 
double electrodes (split electrodes), and the pitch p.sub.i of the 
electrodes is substantially equal to v/4f, i.e., the electrode pitch of 
the acousto-electric converter 104. 
In this embodiment, since the first and second excitation electrodes 202 
and 203 of the elastic surface wave element 5 comprise double electrodes, 
and the electrode pitch p.sub.i is set to be v/4f, the input center 
frequency is f as in the first embodiment, and the same effect as in the 
first embodiment can be obtained. Furthermore, in this embodiment, since 
the double electrode structure is adopted, reflection of elastic surface 
waves is suppressed in the first and second excitation electrodes 202 and 
203, and spurious signal components are eliminated. 
The first and second excitation electrodes need only have a structure which 
can efficiently convert an electrical signal of the input center frequency 
f into an elastic surface wave, and may comprise unidirectional 
electrodes. 
[Fourth Embodiment] 
FIG. 7 is a diagram showing the fourth embodiment of the present invention. 
The same reference numerals in FIG. 7 denote the same parts as in FIGS. 3 
to 6. In this embodiment, first and second elastic surface wave elements 
51 and 52 are arranged. An output from the reference signal generator 3 is 
divided into two signals, and these signals are input to a second 
excitation electrode 113 of the first elastic surface wave element 51 and 
a second excitation electrode 123 of the second elastic surface wave 
element 52 via second and third frequency converters 41 and respectively. 
In the third frequency converter 42, a signal from an oscillator 401 
common to the second frequency converter 41 is input to a multiplier 422 
via a 90.degree. phase shifter (PS) 424. A received signal 1 is 
frequency-converted by the first frequency converter 2, and is then 
divided into two signals. These two signals are input to a first 
excitation electrode 112 of the first elastic surface wave element 51 and 
a first excitation electrode 122 of the second elastic surface wave 
element 52. 
In this embodiment, the divided signals of the received signal 1 are input 
to the first excitation electrodes 112 and 122 of the first and second 
elastic surface wave elements 51 and 52 to be in phase with each other, 
while the divided signals of the output from the reference signal 
generator 3 are input to the second excitation electrodes 113 and 123 of 
the first and second elastic surface wave elements 51 and 52 to have a 
90.degree. phase difference therebetween. For this reason, since a signal 
having a carrier frequency=0 and a wave number=4.pi.f/v generated on a 
substrate 111 of the first elastic surface wave element 51 has a 
90.degree. phase difference from a signal having a carrier frequency=0 and 
a wave number=4.pi.f/v generated on a substrate 121 of the second elastic 
surface wave element 52, the outputs from two acousto-electric converters 
114 and 124 have a 90.degree. phase difference therebetween, i.e., have a 
relationship between SIN and COS components. 
Thus, data can be easily demodulated after the outputs from the 
acousto-electric converters 114 and 124 of the first and second elastic 
surface wave elements 51 and 52 are filtered through filters (F) 61 and 
62. 
In this embodiment, the output from the reference signal generator 3 is 
divided into two signals, and the two signals are phase-shifted by 
90.degree.. Alternatively, the received signal 1 may be divided into two 
signals, and the two signals may be phase-shifted by 90.degree.. The first 
and second elastic surface wave elements 51 and 52 may be formed on a 
single substrate, and the first excitation electrodes 112 and 122 or the 
second excitation electrodes 113 and 123 may be integrally formed. 
[Fifth Embodiment] 
FIG. 8 is a diagram showing the fifth embodiment of the present invention. 
The same reference numerals in FIG. 8 denote the same parts as in FIGS. 3 
to 7. In this embodiment, the first and second elastic surface wave 
elements 51 and 52 are arranged, and the arrangement of electrode fingers 
of an IDT constituting the acousto-electric converter 124 of the second 
elastic surface wave element 52 is shifted from that of the 
acousto-electric converter 114 of the first elastic surface wave element 
51 by v/8f in the propagation direction of an elastic surface wave (i.e., 
.vertline.d.sub.1 -d.sub.2 .vertline.=v/8f). 
In this embodiment, the received signal 1 is frequency-converted by the 
first frequency converter 2, and is then divided into two signals. 
Thereafter, the two signals are respectively input to the first excitation 
electrodes 112 and 122 of the first and second elastic surface wave 
elements 51 and 52. The output from the reference signal generator 3 is 
frequency-converted by the second frequency converter 4, and is then 
divided into two signals. Thereafter, the two signals are respectively 
input to the second excitation electrodes 113 and 123 of the first and 
second elastic surface wave elements 51 and 52. Since the acousto-electric 
converters 114 and 124 of the first and second elastic surface wave 
elements 51 and 52 are formed to be shifted from each other by v/8f in the 
propagation direction of an elastic surface wave, signals each having a 
carrier frequency=0 and a wave number=4.pi.f/v generated on the substrates 
are extracted after they are being shifted by one wavelength, i.e., SIN 
and COS components are extracted. 
Therefore, data can be easily demodulated after the outputs from the 
acousto-electric converters 114 and 124 of the first and second elastic 
surface wave elements 51 and 52 are filtered through the filters 61 and 
62. 
In this embodiment, as shown in FIG. 9, the first and second elastic 
surface wave elements 51 and 52 may be formed on a single substrate 131, 
and the two first excitation electrodes of these elements may be 
integrated as an electrode 132 and/or the two second excitation electrodes 
of these elements may be integrated as an electrode 133. 
[Sixth Embodiment] 
FIG. 10 is a diagram showing the sixth embodiment of the present invention. 
The same reference numerals in FIG. 10 denote the same parts as in FIGS. 3 
to 9. In this embodiment, the first and second elastic surface wave 
elements 51 and 52 are formed on the single substrate 131, the two first 
excitation electrodes of the first and second elastic surface wave 
elements 51 and 52 are integrated as the electrode 132, and the two second 
excitation electrodes of the first and second elastic surface wave 
elements 51 and 52 are integrated as an electrode 143. Furthermore, the 
arrangement of the second excitation electrode of the second elastic 
surface wave element 52 is shifted by v/4f from that of the first 
excitation electrode of the first elastic surface wave element 51 in the 
propagation direction of an elastic surface wave. 
In this embodiment, a received signal 1 is frequency-converted by the first 
frequency converter 2, and is then input to the first excitation electrode 
132 of the first and second elastic surface wave elements 51 and 52. The 
output from the reference signal generator 3 is frequency-converted by the 
second frequency converter 4, and is then input to the second excitation 
electrode 143 of the first and second elastic surface wave elements 51 and 
52. Since the second excitation electrode portion of the first elastic 
surface wave element 51 is formed to be shifted from the second excitation 
electrode portion of the second elastic surface wave element 52 by v/4f in 
the propagation direction of an elastic surface wave, an elastic surface 
wave generated by the excitation electrode of the element 51 has a 
90.degree. phase difference from an elastic surface wave generated by the 
excitation electrode of the element 52. Therefore, a signal having a 
carrier frequency=0 and a wave number =4.pi.f/v generated at the first 
elastic surface wave element 51 side, and a signal having a carrier 
frequency=0 and a wave number=4.pi.f/v generated at the second elastic 
surface wave element 52 side have a 90.degree. phase difference 
therebetween, and when these signals are extracted by the acousto-electric 
converters 114 and 124, SIN and COS components are extracted. 
Therefore, data can be easily demodulated after the outputs from the 
acousto-electric converters 114 and 124 of the first and second elastic 
surface wave elements 51 and 52 are filtered through the filters 61 and 
62. 
In this embodiment, the first and second elastic surface wave elements 51 
and 52 are formed on a single substrate, but may be formed on separate 
substrates. In this embodiment, the second excitation electrode portions 
of the first and second elastic surface wave elements are arranged to be 
shifted from each other by v/4f. Alternatively, the first excitation 
electrode portions may be arranged to be shifted from each other by v/4f. 
Furthermore, the first and second excitation electrode portions may be 
arranged to be properly shifted from each other, so that signals each 
having a carrier frequency=0 and a wave number=4.pi.f/v generated on the 
substrate on the sides of the first and second elastic surface wave 
elements 51 and 52 have a 90.degree. phase difference therebetween. 
In the first to sixth embodiments described above, the substrates 101, 111, 
121, and 131 may consist of a piezoelectric single crystal such as lithium 
niobate. However, the present invention is not limited to this. For 
example, a material and structure which can exhibit the parametric mixing 
effect, such as a structure obtained by adding a piezoelectric film on the 
surface of a semiconductor or glass substrate, or the like, may be used. 
Also, an elastic surface wave waveguide may be formed on the substrate to 
facilitate propagation of an elastic surface wave. 
In the fourth to sixth embodiments, the excitation electrodes 112, 122, 
113, 123, 132, 133, and 143 may comprise double electrodes or 
unidirectional electrodes. 
FIG. 11 is a diagram showing an embodiment of a spread spectrum 
communication system adopting a demodulation apparatus of the present 
invention. Referring to FIG. 11, an information signal output from a 
terminal 301 is spreading-modulated by spread codes generated by a spread 
code generator 303 in a spreading modulator 302 in a transmitter 304. The 
spreading-modulated signal is transmitted from the transmitter 304 via an 
antenna 305. 
The signal transmitted from the transmitter 304 is received by a receiver 
308 via an antenna 306. The received signal is demodulated by a 
demodulator 307 in the receiver 308, and the demodulated information 
signal is input to a terminal 309. The demodulator 307 has one of the 
above-mentioned arrangements shown in FIG. 3 and FIGS. 5 to 10. 
FIG. 12 is a diagram showing another embodiment of a spread spectrum 
communication system using a demodulation apparatus of the present 
invention. The same reference numerals in FIG. 12 denote the same parts as 
in FIG. 11, and a detailed description thereof will be omitted. The 
communication system shown in FIG. 12 is substantially the same as that 
shown in FIG. 11, except that a signal is transmitted from the transmitter 
304 to the receiver 308 via a cable 310 in place of the antennas in FIG. 
11. In this system, modulation and demodulation of a signal are performed 
in the same manner as in FIG. 11. 
The present invention allows various other applications in addition to the 
above-mentioned embodiments. The present invention incorporates all these 
applications without departing from the scope of claims.