GB2206760A

GB2206760A - Frequency modulation demodulator utilizing a surface acoustic wave delay line

Info

Publication number: GB2206760A
Application number: GB08815058A
Authority: GB
Inventors: Young Ho Choi; Hee Seong Kwon; Seong Jae Joh
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1987-06-24
Filing date: 1988-06-24
Publication date: 1989-01-11
Also published as: DE3821247A1; KR890001271A; GB8815058D0; KR890004157B1; JPS6423605A

Description

/ 2 2' 0 6 r C - 1 FREQUENCY MODULATION DEMODULATOR UTILIZING A SURFACE

ACOUSTIC WK7E DELAY LINE

DESCRIPTION

The present invention relates to a frequency modulation (FM) demodulator for demodulating F1,1 signals applied from the intermediate frequency (IF) stage of a satellite communication receiver, and more particularly to an FM demodulator which utilizes a surface acoustic wave (SAW) delay line and, further, which can also be used in the-receiving stage of spread spectrum communication systems or other wide band communication systems.

Many kinds of l,'M signal demodulators such as an F1,1 demodulator utilizing a differentiator and an envelope detector, a phaselocked loop (PLL) type FM demodulator and an FM demodulator utilizing a multiplier are well. known. However, such FM demodulators have some drawbacks in that tuning is necessary 20 in the case of the FM demodulator utilizing a differentiator and an envelope detector, and usable bandwidth is limited in the case of the PLL type 1M demodulator. In the meantime, an FM demodulator employing a time delay means for signals of wide band width, a multiplier and a low pass filter is also known. Such type of demodulator can achieve a phase difference of 90-degree by means of a inductance-capcitance (LC) time delay circuit and can thereby demodulate FM signals through the multiplier and the low pass filter. Also, other time delay means such as utilizing strip lines can be employed to a region of several GHz. However, in the low frequency range of teri M11z to several Vitindred MlIz, Llic size of Llic, strip lines should increase and thus the use of the above-described LC time delay circuit is necessary. Consequently, such demodulators have drawbacks in that tuning is necessary and thus it is difficult to make the precise 90- degree phase difference required for demodulation of FM signals.

Reffering now to FIGs. 1 and 2, the conventional demodulator employing a time delay means 11, a multiplier 12 and a low pass filter 14 fis shown. 7he T7M signal applied to an input terminal IN is demodulated and the demodulated signal is then obtained from the output terminal OUT. This type of demodulator shown as a block diagram in FIG. 1 is illustrated in FIG. 2 in more detail as the embodimdnt thereof. In this embodiment, a tuning circuit 11', which acts as the time delay means 11, is comprised of a variable tuning coil L1 and a capacitor Cl C r and provides the 90-degree phase difference to the applied FM signal. This delayed signal of 90-degree phase difference and the FM signal from the input terminal IN are supplied to the multiplying integrated cuicuit 12' as two input signals thereof and thereby the PE signal is demodulated. However, this demoldulator has problems in that tuning is required to obtain the 90-degree phase difference between the applied FM signal and the delayed signal and, further, in that obtaining in accurate 90-degree phase difference is difficult even though the manual fine tuning is made by adjusting the coil L,.

It is an object of the present invention to provide an FM demodulator utilizing the time delay characteristic of a surface acoustic wave (SAW) delay line which provides an accurate phase difference to the applied FM signal.

The FM demodulator according to the present invention comprises a SAW delay line which provides the 90-degree phase difference to the FM signal, a coil for matching the SAW delay line, a multiplication means and a low pass filter.

The SAW delay line is an element which can provide a long time delay with a.short length in the ten MHz to several hundred MHz 1 band. This element utilizes interdigital transducers usually called "IDTs") the time delay between the two interdigital transducers is preset by the installation interval of the electrodes of the interdigital transducers, and thus it is possible to increase the signal-to-noise (SIN) ratio, and to reduce size and manufacturing time because the tuning operation is unnecessary.

In the SAW delay line according to the presnet invention, the number of pairs of the electrodes of the interdigital tranducers is determined properly so as to deal with wide band signals and a piezoelectric material used for the substrate of the SASW7 delay line may be made of a simple crystal material or ceramics. The conductive electrodes are formed on the piezoelectric material by vacuum evaporation and photoetching.

The electrodes are arranged so as to provide the time dealy corresponding to the width of the electrodes. In other words, as the time delay between two input interdigital transducers corresponds to the distance of>,14, wherein --,\ is the wavelength of the F.M signal, an accurate 90-degree phase difference can be obtained. In addition, a matching circuit utilizing a coil or a substituteis included to minimize the insertion loss of the input and output interdigital transducers.

- 5 The FM signal and the time delayed signal through the input interdigital transducers are applied to the two input trminals of the multiplication means after being detected by the output interdigital transducer, and then the output of the multiplication means, passes through the low pass filter resulting in that the FM demodulated signal is obtained.

The present invention will now be described by way of illustrative example with referece to the accompanying drawings, in which:

FIG. 1 is a general block circuit diagram of the conventional FM demodulator utilizing a multiplier; FIG. 2 is a detailed circuit diagram of the FM demodulator of FIG. 1 FIG. 3 shows a preferred embodiment of an FM demodulator according to the present invention; FIG. 4 shows the characteristic of interdigital transducers of the present invention; and FIGs. 5 to 7 show other embodiment of the FM demodulator according to the present invention.

Referring now to FIGs. 3 to 7, an FM demodulator according to the present invention comprises a SAW delay line 31, a multiplier 12 and a low pass filter 13. Interdigital transducers such as shown in FIG. 4 are designed and arranged on a piezoelectric material 32 to constitute the SAW delay line 31 as shown in FIG. 3.

The SAW delay line 31 shown in FIG. 3 is comprised of three interdigital transducers IDT31, IDT32, IDT33 wherein aluminium electrodes or a substitute are evaporated on the piezoelectric material 32. In the SAW delay line, an electric signal applied to the interdigital transducer IDT3, is converted into a mechanical deformation due to the inverse piezoelectric effect of the piezoelectric material 32, the mechanical deformation travels to the other interdigital transducers IDT32 and IDT33 and thereby the electric signal can be detected in the other interdigital transducers IDT32 and IDT33 respectively due to the piezoelectric effect of the piezoelectric material 32. The mechanical deformation, which is, in fact, called the surface acoustic wave.

is propagated along the upper surface of the piezoelectric material and its propagating speed is reduced by about 1/100,000 comprared with the speed of a common electromagnetic wave. Therefore, 1 1 utilizing the characteristic of this surface acoustic wave, it is possible to obtain a time delay corresponding to the 9b-degree phase difference by adjusting the diatance between the interdigital transducers IDT32 and IDT33 as shown in FIG. 3. In the preferred embodiments of the present invention, the distance "to" between the two interdigital tranducers IDT32 and IDT33 is determined to be the width "a" of the electrodes of the interdigital transducers shown in FIG. 4.

Referring to FIG. 3, as the FM signals from the input terminal IN are applied to the interdigital transduder IDT311 the surface acouti acoutic waves are generated and propagated along the upper surface of the piezoelectric material 32, and in turn detected as the electric signals in the interdigital transducers IDT32 and IDT 33 At this moment, as the interdigital transducers IDT32 and IDT33 are installed with the location difference "to", the output signals of the interdigital transducer IDT33 experience a delay time compared with that of the interdigital transducer IDT32. If the location difference "to" equals the width 'a" of the electrodes, the two signals obtained form the interdigital transducer s IDT32 and IDT33 respectively will have the accurate 90-degree phase difference with each other. These two signals are then applied to the two input terminals of the multiplier 12 with 90-degree phase difference, the output signals of the multiplier 12 pass through the low pass filter 13, and the FM demodulated signals are thereby obtained.

Referring now to FIG. 5, there is shown another preferred embodiment of the present invention. In the embodiment, the output interdigital transducers IDT 32 and IDT33 are positioned at each side of the input interditital transducer IDT3, respectively. The output from the interdigital transducer IDT33 will have a time delay corresponding to the distance "to" compared with that of the interdigital transducer IDT32 by setting the difference of the installation intervals with respect to the interdigital transducer IDT31 to be the distance "to".

The distance "to" is set to be the same as the width "a" of the electrode in order to obtain the accurate 90-degree phase difference, and thereby the FM signal.is demodulated through the multiplier 12 and the low pass filter 13.

In still another embodiment of the present invention shown in FIG. 6, a 3 dB multistrip coupler MSC is disposed between the interdigital 0 -1 transducers IDT31, IDT32Y IDT33. In this embodiment, the surface acoustic waves propagated from the input interdigital transducer IDT31 are supplied to the interdigital transducer IDT32 with their loci unconverted and also supplied to the interdigital transducer IDT33 with their loci converted. Thus, the 90-degree phase difference is obtained between the two electric signals output from the interdigital transducers IDT32 and IDT33.

0 Referring now to FIG. 7, thereis shown still another embodiment of the present invention. In this embodiment, the input and output of the SAW delay line 31 of FIG. 3 are interchanged. The FM signals from the input terminal IN are transmitted to the interdigital transducer IDT3, after having the 90-degree phase difference through the interdigital transducers IDT32 and IDT33. Ihus, the added signal of the two input signals can be obtained from the interdigital transducer IDT31 as its output and FM demodulation is performed as the output signal passes through an envelope detector 71.

As described above, it will be apparent that the present invention provides advantages such as small size, an increased SIN ratio and a simplification of the manufacturing process of FM demodulators since the accurate phase difference can be obtained in bands of ten M11z to several hundred M11z by utilizing the SAW delay line.

1 Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be.understood that other and further modifications, apart from those shown or suggested herein may be made within the spirit and scope of this invention.

11 1 I> 1

Claims

CLAIMS il 1 1. An FM demodulator for use in communication systems

compring:

(a) a surface acoustic wave delay line providing a time delay of an FM signal, said delay line comptising:

an input interdigital transducer converting said FM signal into a surface acoustic wave; a piezoelectric material propagating said surface acousti wave from said input interdigital transducer; and two output interdigital transducers converting said surface acoustic wave into an electric signal; wherein said input and output interdigital transducers are evaporated on said piezoelectric material and one of said output interdigital transducers is constructed and arranged to provide the time delay of said converted signal compared with the other of said output interdigital transducers; (b) a means for multiplying two output signals of said output interdigital transducers; and (c) a low pass filter connected to said multiplying means and through which an FM demodulated signal is obtained.
2. An FIA demodulator as claimed in claim 1 wherein said output interdigital transducers are constructed and arranged at one side or both sides of said input interdigital transducer with a distance for the time delay with each other.
3. An nI demodulator as claimed in claim 1 or claim 2 wherein said distance for the time delay is set to be the width of the electrodes of said input and output interdigital transducers so that said output signals of said output interdigital transducers have an accurate 90-degree phase difference with each other.
4. An FM demodulator as claimed in claim 1 wherein a multistrip coupler is installed between said input and output interdigital transducers for partly converting the loci of said surface acoustic wave propagated from said input interdigital transducer.
5. An FM demodulator as claimed in claim 1 wherein said 1,11 signal is applied to said output interdigital transducers, the output signals of said output interdigital transducers are added by said input interdigital transducer, and the output signal of said input interdigital transducer passes through an envelope detector to perform the FM demodulation.
6. An M demodulator as claimed in claim 1 wherein said piezoelect.ric material is made of a simple crystal material or ceramics.

1^ t
7. An PM demodulator for use in communication systems comprising a SAW including a substrate and input and output electrode means for respectively propogating and receiving a surface acoustic wave in the substrate in response to an input PM signal, said input and output electrode means being so arranged as to produce first and second signals to be received by said output electrode means having a substantially orthogonal phase difference therebetween for at least a predetermined range of carrier frequency of said PM input signal; and means for combining said first and second signals as received by said output electrode means for demoulating said PM signal.
8. An PM demodulator substantially as hereinbefore described with reference to the accompanying drawings.

-Published 1985 at 'The Patent Office. State I-louse. 6671 High Rolborn, London WC1R 4TP. Further copies may be obtained from The Patent office, Stdes Branch, St Mary Gray. Orpington, Kent BM 3RD. Printed by Multiplex schniques ltd, St Mary Gray. Kent. Con. 1187.