KR100382382B1 - Surface Acoustic Wave Transducer Drives in Differential Mode - Google Patents

Abstract

The differentially driven transducers according to the invention are at least a first transducer 5 and a second transducer 6 which are connected in parallel in order to obtain the symmetry of the differential inputs, each forming a first channel and a second channel. These two transducers 5, 6 represent a particular longitudinal offset with respect to each other.

Description

Surface acoustic wave transducer driven in differential mode

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to surface acoustic wave transducers, and more particularly, to surface acoustic wave distributed acoustic reflection transducers known as DART and driven in differential mode and having low impedance input / output. There is an increasing demand for improvements of this type of drive, especially integrated circuits.

Since the basic structure of the DART is presumed to be well known, it will not be described in detail in the description of the present invention.

To drive the transducer in differential mode, it is necessary to use a structure with a symmetrical differential input. The basic structure of this DART cannot be used directly in differential mode. This is because the reflector of the DART is asymmetrical between the connection pads individually linked to the differential inputs.

Transducers and filters used in these transducers can be applied to many applications, especially intermediate frequencies, such as radar or mobile wireless communications systems.

DART with a well-known differential input is disclosed in patent 92 10966, entitled “Transducteur d'ondes unidirectionnel [Unidirectional wave transducer]”, filed Sep. 15, 1992 and published as No. 2 695 771. It is disclosed in detail. This structure enables differential feed between positive and negative sources relative to ground, includes two emitter cells linked to the positive source, and a third Emmy linked to the negative source. The cells are flanked by the sides and these three cells are spaced apart by the first and second reflecting cells and grounded to the ground, so naturally these cells are taken to ground potential by symmetry. This configuration uses a maze structure in which the grounded electrodes are interposed between those linked to the positive source and those linked to the negative source. This maze structure has problems of sampling and high impedance, as can be seen by the input / output of the structure.

It is an object of the present invention to overcome the above disadvantages.

For this purpose, the object of the present invention consists of at least a first transducer and a second transducer connected in parallel to form a symmetry of a differential input and forming a first acoustic channel and a second acoustic channel, respectively. Surface acoustic wave transducer driven in a differential mode, characterized in that the two transducers exhibit a predetermined longitudinal offset with respect to each other.

An advantage of the structure of the transducer according to the invention is to give a low impedance to the input / output of such a structure for a structure with two parallel acoustic channels driven in differential mode.

Other features and advantages of the invention will be apparent from the following description with reference to the accompanying drawings.

1A and 1B are cross-sectional views from the top, respectively, of a known example of a DART type transducer having a non-differential input.

2A, 2B and 2C are cross-sectional views, respectively, viewed from above, showing an embodiment of a transducer composed of two transducers connected in parallel to form a transducer having a differential input.

Figure 3 is a first structure of a transducer according to the invention with two transducers connected in parallel driven in differential mode and longitudinal offset.

4A, 4B and 4C are cross-sectional views of a second structure of the transducer according to the invention with two transducers connected in parallel driven in differential mode and offset of [lambda] / 2, respectively.

5a, 5b and 5c are cross-sectional views of a third structure of a transducer according to the invention with two transducers connected in parallel, each driven in a differential mode and an offset of [lambda] / 3.

6A, 6B and 6C are cross-sectional views of a fourth structure of a transducer according to the present invention having two transducers connected in parallel, each driven with a differential mode and an offset of [lambda] / 2.

7A, 7B and 7C are cross-sectional views of a fifth structure of a transducer according to the present invention having two transducers connected in parallel, each driven at an offset of λ / 2 and in differential mode.

8 shows a shape of a transducer according to the present invention offset by an even number of half wavelengths and a symmetrical acoustic wave generated by the transducer.

Fig. 9 shows a transducer according to the present invention offset by an odd multiple of half wavelength, and the shape of an asymmetric acoustic wave generated by the transducer.

In these figures, corresponding components are denoted by the same reference numerals.

1A and 1B show an example of a structure of a known DART having a non-differential input. It consists of a first electrode 1 and a second electrode 2 each comprising a predetermined number of fingers. The first electrode 1 is linked to the negative potential V ⁺ , and the second electrode is linked to the reference potential; Each electrode 1, 2 comprises a connection pad capable of applying a predetermined potential to each finger of the electrodes. Thus, each of the electrodes forms a comb, and some fingers of these two electrodes are interleaved to form emitter cells and acoustic reflector cells. An acoustic signal is generated when a potential difference exists.

FIG. 1B shows a cross section along xx 'of the structure shown in FIG. 1A. In this cross-section you can see various fingers of two electrodes deposited on the surface of a piezoelectric substrate (not shown). Also shown are the potentials of these fingers.

Transducers with differential inputs, starting from this basic structure with non-differential drives, can easily be composed of two transducers with non-differential inputs, filed on October 18, 1991 by the applicant and 2 682 833, "Filtre a ondes de surface et a trajet acoustique replie [surface wave filter with folded acoustic path]". For this purpose, two transducers which are identical in structure to FIGS. 1A and 1B are connected in series. The grounded contact pads of each of the two transducers are identical. These two transducers each form two acoustic channels. These two acoustic channels may appear as a single channel by offsetting the two channels in the horizontal direction by λ / 2, where λ is the wavelength corresponding to the center frequency of the transducer.

Such a structure is shown in FIG. 2A. The electrical connection is symmetrical with respect to ground, so when referenced to ground, it will give the same impedance, as can be seen between each differential input (V +, V-).

2b and 2c show sectional views along the axes xx 'and yy' of the first transducer 3 and the second transducer 4, respectively. As shown in Fig. 1B, various potentials are indicated on top of each of the fingers corresponding to the electrodes linked to the differential inputs through their respective connection pads.

The main disadvantage of this structure is that the impedance as seen between the differential inputs is equal to four times the impedance of the structure with the non-differential input having the same aperture as using two channels.

The first structure of the transducer according to the invention, which can reduce this impedance, is shown in FIG.

In this structure, the two transducers already described in the structure shown in FIG. 2A are connected in parallel rather than in series. Each transducer (5,6) is a differential input (V ^+, V ^-) and the wiring directly between the reference potential (ground) is no longer present. The symmetry of the two paths is restored by inverting the polarity on the two DARTs. This polarity inversion can be compensated by offsetting the two transducers 5 and 6 by an odd multiple of half wavelength in a single path, thereby making it possible to obtain a symmetric wavefront. However, this is not essential and it is acceptable to generate an asymmetric wavefront, and the device, for example the receiving transducer, must be designed accordingly. This results in an impedance seen between each differential input that is approximately equal to the differential input seen between the non-differential inputs of the transducer having the same aperture as the sum of the apertures of the two acoustic channels. Compared to the initial serially connected structure, four factors were obtained.

In the second structure of the transducer according to the present invention, shown in Fig. 4A, the wiring to the same differential input as the previous structure uses only two connection pads of the two electrodes 7 and 8, and each connection pad has its own. two differential inputs (V ^+, V ^-) to becomes each wired to one of the fingers linked together in a way that by linking to (9i, 10i), of the same Shi sequence polarity on the number of electrodes responding to each channel It is done.

The first acoustic channel is formed from a first electrode connected to the first differential input V ⁻ through its connecting pad. This electrode 7 comprises a predetermined number of fingers of a specific width at a particular position along the electrode 7. The second channel is formed from a second electrode 8 in the form of a comb and comprises the same number of fingers as the first electrode 7 and is connected to another differential input (V ⁺ ) via its connecting pad. do.

The same predetermined number of fingers 9i and 10i respectively belonging to the two electrodes 7 and 8 are interleaved with other fingers of the electrodes 7 and 8. The characteristic of this structure is that the predetermined number of fingers 9i of the first electrode 7 belong to the second acoustic channel, and the same number of fingers 10i of the second electrode 8 belong to the first acoustic channel. . 4B and 4C illustrate how the various potentials are distributed on each finger, with respect to the first transducer along the cross section axis xx 'and the second transducer 8 along the cross section axis yy'.

The transducer according to the invention takes advantage of the fact that acoustic waves are generated by the potential difference applied to two adjacent fingers rather than by absolute potential.

Compared with the cross sections of FIGS. 2B and 2C, respectively, the cross sections of FIGS. 4B and 4C show that the potential difference between two adjacent fingers of FIGS. 4A, 4B and 4C is measured between two adjacent fingers of FIGS. Appears twice the potential potential. This is due to the fact that there is no connection to the reference potential (ground). Thus, as shown in Fig. 4A, the same polarity is connected in two channels. The fingers 9i, 10i of the electrodes 7, 8 interleaved by the other fingers are made by extending the other fingers as in the case with the first electrode, or the potential (V ⁺ ) of the second electrode 8. By extending the first finger 11 hatched in the figure, which is added so as to enable connection to it. Similarly, a hatched second finger 12 of the first electrode 7 is added to the first acoustic channel to retain the same number of fingers in each channel. This differential inputs (V ^+, V ^-) by ensuring that the same impedance seen between, thereby ensuring the impedance symmetry of the transducer.

As shown in Fig. 5A, it is possible to add two fingers 13i and 14i to two opposite ends of two channels, respectively. This makes it possible to align the stages of the two channels while maintaining the same number of identical fingers for the two channels. 5b and 5c show how different potentials are distributed on each finger, for the first transducer 7 along the cross section axis (xx ') and the second transducer along the cross section axis (yy'), respectively. Explain.

This structure works even if more fingers are added to one channel besides the other channel, but the difference in the number of fingers of the two channels should be kept small enough so as not to change the capacitance of the differential input to the ground.

For these equal impedances, a comparison of the structures of FIGS. 2A and 4A shows that the structure of FIG. 4A is approximately four times smaller than the structure of FIG. 2A. This is because the structure of Fig. 2A is made by the series connection of two transducers 3, 4 on the basis of the reference potential, that is, the ground. The potential difference between the various polarities of the structure of FIG. 4A is twice the potential difference of the structure of FIG. 2.

The λ / 2 offset of the first transducer 5 relative to the second transducer 6 makes it possible to obtain a phase shift of zero between the two channels at the center frequency f0. Moreover, this phase shift makes it possible to obtain zero at the frequency 2f0, which makes it possible to eliminate the second harmonic.

The offset of the two transducers 5, 6 is not essential, but it is nevertheless important as it is possible to solve the connection problem.

6A, 6B and 6C show the application of the transducer according to the present invention to a DART differential structure using a finger having a width of λ / 6 and a source having a period of λ / 3. In this structure, the interleaved fingers 15i, 16i are not directly along the extension of the already existing fingers of the electrodes 7, 8. In particular, for fingers 16i belonging to electrode 8 linked to positive potential input V ⁺ , in view of the problem of electrical connection, these fingers 16i are interleaved fingers with potential V ⁺ . It is added to the ends of two adjacent fingers of the same electrode 8 which are slightly elongated to allow electrical connection of 16i.

As shown in FIG. 4A, the two transducers 7, 8 are offset by λ / 2, and the interleaved fingers 15i of the first electrode 7 are interleaved fingers (i) of the second electrode 8. Undergo an offset of λ / 2 for 16i).

The interleaved fingers 15i of the first electrode 7 are similarly offset with respect to the other fingers of the electrode 7 supporting them, which are elongated to provide for electrical connection with the potential V-.

In Figures 7a, 7b and 7c another structure of the transducer according to the invention is shown.

In this structure, there is an offset of λ between each transducer 5, 6. This structure is partially similar to that of FIGS. 6A, 6B and 6C and will not be described again.

In order to correspond to the same number of fingers for each channel, three fingers 17i and 18i, which are hatched in the figure, are respectively added to two opposite ends of the two channels.

8 and 9 respectively show two types of offsets possible by the transducer according to the invention.

In FIG. 8, the offset between two acoustic channels is an odd multiple of λ / 2, for example 3λ / 2. This can be used to find the remaining zero in the transducer's transfer function. The generated acoustic waves A (y) are symmetrical because the acoustic waves of the two channels are in phase.

In FIG. 9, the offset between two acoustic channels is an even multiple of λ / 2, for example 0 or λ. Thus, the generated acoustic wave A (y) is asymmetrical.

It is also possible to use two different transducers on two channels, as long as the impedance and capacitance in the band used remain the same order of magnitude.

The invention is not limited to the specific structure just described. In order to be able to operate in differential mode, any other structure may implement the main feature of the invention, namely the parallel arrangement of two transducers exhibiting asymmetry in their two connection pads.

Claims (4)

2 channel structure, Each channel comprises first and second channel transducers 5, 6 comprising a plurality of fingers 12, 10i, 11, 9i linked to one of the differential inputs V ⁺ , V ⁻ . Consisting of, In each channel, the plurality of fingers 12, 10i, 11, 9i are contiguous with the width of the same finger in two channels offset by multiples of λ / 2 and are offset by multiples of λ / 2 Single connection pads 7, 8 that are continuous with the same or opposite electrical polarity in the channel and each of said first and second channel transducers 5, 6 are linked to one of the differential inputs V ⁺ , V ⁻ . And the fingers 10i, 9i of the channel having an electrical polarity opposite to the connection pads 7, 8 of the channel in a given channel include the fingers 11, 12 attached to the latter channel. Surface acoustic wave transducer driven in differential mode characterized in that it is linked to the connection pads (7, 8) of the other channel. The method of claim 1, The offset between the first transducer 5 and the second transducer 6 is an odd multiple of half wavelength; A surface acoustic wave transducer, characterized in that the wavelength corresponds to the center frequency of the transducer operation. The method of claim 1, The offset between the first transducer 5 and the second transducer 6 is an even multiple of half wavelength; And its wavelength corresponds to the center frequency of the transducer operation. The method according to any one of claims 1, 2 and 3, Surface acoustic wave transducer, characterized in that the first transducer (5) and the second transducer (6) is a surface acoustic wave distributed acoustic reflection transducer (DART).