Double Sideband Suppressed Carrier Modulator Using Surface Wave Device

This invention pertains to a modulator and a method of the type for generating a double sideband suppressed carrier (DSBSC) signal and more particularly to such a modulator which utilizes a surface wave device.

The generation of DSBSC signals by means of a balanced modulator has required relatively expensive elements such as transformers and modulating amplifiers. In addition the balanced modulator required exact tuning in order to cancel out the carrier signal. If a single sideband suppressed carrier signal was desired, a filter for blocking transmission of one of the sidebands was used.

Surface acoustic wave devices, known as SAW's, have been used as frequency discriminators because: they are compatible with integrated circuit techniques, they require very little tuning, and they are relatively inexpensive when compared against more conventional devices.

The particular advantages associated with utilizing SAW devices as frequency discriminators are retained when the SAW devices are used as a modulator.

In the preferred embodiment of the present invention three equally spaced surface wave transducing means are positioned on a surface wave propagating medium along a common acoustic channel. An RF signal generating means is connected to one of the outermost positioned transducing means. A means for amplitude modulating the RF signal is also connected to the RF signal generator to provide an amplitude modulated RF signal to the other outermost transducer means. An output terminal is connected to the centrally positioned transducer means for detecting the double sideband suppressed carrier signal received by the centrally positioned transducer.

From the foregoing it can be seen that it is a primary object of the present invention to provide a novel double sideband suppressed carrier signal generator.

It is a further object of the present invention to provide a unique surface wave modulator for generating a double sideband suppressed carrier signal generator.

A further object of the present invention is to provide a modulator which requires no transformers and a minimum amount of tuning.

These and other objects of the present invention will become more apparent and better understood when taken in conjunction with the following description and drawings, wherein like characters indicate like parts and which drawings form a part of the present invention.

FIG. 1 illustrates schematically and pictorially a preferred embodiment of the surface wave modulator constructed in accordance with the present invention.

FIG. 2 illustrates the spectrum content of an amplitude modulated carrier signal.

FIG. 3 illustrates the spectrum content of an amplitude modulated carrier signal with a suppressed carrier.

FIG. 1 depicts a surface wave modulator 10 comprised of two input transducers 12 and 16 and an output transducer 14, all of which are formed on a surface of a piezoelectric substrate 15. The substrate 15 may be constructed of a suitable piezoelectric material such as PZT, quartz, lithium niobate, lithium tantalate, ZnO, ZnS, CdS, or other suitable material for propagating acoustic surface waves in the frequency range of interest.

The transducers 12, 14 and 16, in the simplest arrangement, are identical and are constructed of two comb-type electrode arrays. Input transducer 12 has a first comb of electrodes 11 connected to circuit ground and a second comb of electrodes 13 connected to the output of a modulator 22. Electrodes 11 and 13 have interleaved fingers which are spaced apart from each other a distance which corresponds to one-half of the acoustic wavelength of the signal wave for which it is desired to achieve maximum response. The electrodes are formed from a material such as gold or aluminum which may be vacuum deposited or photoetched onto the surface of the piezoelectric substrate 15.

The output transducer 14 is positioned equidistant between the input transducers 12 and 16. One comb 18 of output transducer 14 is connected to circuit ground. The other comb 17 is connected to an output terminal 30.

The input transducer 16 has one comb 20 connected to circuit ground and the other comb 19 connected to the output of an RF carrier signal generator 21. The output of generator 21 is also connected to the input of modulator 22. An amplitude modulating signal a(t) is applied to an input terminal 25, which terminal is connected to the modulating signal input of modulator 22. The output signal from modulator 22 is an amplitude modulated RF carrier signal of the well-known type.

In the novel method of operation the input transducers 12 and 16 operate as transmitters. An electrical signal applied to the transducers, through piezoelectric coupling, is transformed into a traveling acoustic surface wave on the substrate 15. This occurs when the strain components produced by the electric fields in the substrate are substantially matched to the strain components associated with the surface wave mode. Transducer 12 will produce the RF carrier wave and its associated sidebands. Transducer 16 will produce the RF carrier wave only. Each of these produced surface waves propagate along the substrate common acoustic channel to the output transducer 14. Transducer 14 will convert the received acoustic waves into an electrical signal. Due to the fact that the RF carrier wave components transmitted by the input transducers will be 180.degree. out of phase when received by the output transducer 14 they will cancel and not be converted into an electrical signal. The signal from output transducer 14 will therefore be a double sideband suppressed carrier (DSBSC) signal. To achieve perfect cancellation of the RF carrier it may be necessary to adjust the amplitude of the RF carrier signal fed to transducer 16 so as to compensate for any losses incurred in signal strength in modulator 22.

FIG. 2 illustrates the spectrum content of the amplitude modulated signal applied to the input transducer 12. The RF carrier at frequency f.sub.c which is also applied to the input transducer 16 is shown with an amplitude many times that of the sideband signals f.sub.c -f.sub.a and f.sub.c +f.sub.a, where f.sub.a is the frequency of the modulating signal a(t). A denotes the amplitude of the unmodulated RF carrier signal, and the quantity m.sub.a represents the amplitude modulation factor. The sidebands therefore have an amplitude of maA/2.

FIG. 3 illustrates the spectrum content of the output signal present at the output transducer 14. The unmodulated RF carrier signal component, f.sub.c, is eliminated and only the two sideband signals remain.

While there has been shown what is considered to be the preferred embodiment of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as fall within the true scope of the invention.