JPS5837725B2 - Surface acoustic wave parametric device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave parametric device that functions as a variable frequency selection device, in which a pump electrode is divided into a plurality of parts, and the parts are arranged along the propagation direction of the surface acoustic wave. , relates to a parametric device in which the frequency characteristics of the variable frequency selection device can be designed as desired by applying pump voltages of different values corresponding to desired frequency characteristics to each pump electrode. .

As previously disclosed in Japanese Patent Application No. 52-107271, the present applicant has invented a surface acoustic wave device having a variable frequency selection function as shown in FIG.

In FIG. 1, reference numeral 1 denotes a semiconductor substrate, and an insulating film 2 and a piezoelectric layer 3 are sequentially laminated on this semiconductor substrate 1.
Furthermore, a rectangular pump electrode 4 for applying a DC bias voltage and a pump voltage, and input/output transducers 5 and 6 are disposed on the piezoelectric layer 3, respectively.

7 is a DC power supply for applying DC bias, 8 is an inductor for blocking AC, 9 is a high frequency power supply for applying pump voltage,
10 is a DC blocking capacitor, and 11,
12 is a surface acoustic wave absorbing material for preventing unnecessary surface acoustic waves from being reflected at the end face.

Then, a DC bias voltage is applied to the pump electrode 4 from the DC power source 7 to generate an appropriate space charge layer capacitance on the surface of the semiconductor substrate 1 directly under the pump electrode 4, and then a high frequency power source 9 is applied to the pump electrode 4 to generate an appropriate space charge layer capacitance at the selected center frequency fo. A pump voltage with twice the frequency 2fo is similarly applied to the pump electrode 4, and the space charge layer capacitance is excited at the frequency 2fo, thereby changing the space charge layer capacitance at the frequency 2fo.

On the other hand, when an electrical signal is input to the broadband input transducer 5, this input electrical signal is converted into a surface acoustic wave signal and the surface of the piezoelectric layer 3 is directed to the left and right of the input transducer 5 in FIG. and propagate.

Then, the frequency f of the surface acoustic wave input signal 13 propagating to the right.

As the nearby signal component propagates through the operating region below the pump electrode 4, its piezoelectric potential is transferred to the semiconductor substrate 1.
The space charge layer capacitance nonlinear effect on the surface causes a parametric interaction with the pump voltage and is amplified, and the amplified surface acoustic wave signal 14 is converted back into an electrical signal by the output transducer 6 and output to the outside.

At the same time, from the pump electrode 4 towards the left side of the figure,
A frequency fi corresponding to the magnitude of the surface acoustic wave input signal 13
A surface acoustic wave signal 15 of (fi = 2fO-fs, fs: frequency of input signal) is generated.

This surface acoustic wave signal 15 can also be appropriately outputted to the outside as an output signal.

Here, FIGS. 2 and 3 show the frequency characteristics of the above-mentioned output surface acoustic wave signals 14 and 15, assuming that the magnitude of the input signal 13 is 1. In FIG. 2, the pump voltage is relatively small. In this case, FIG. 3 shows the case where the pump voltage is relatively large.

As shown in both figures above, in a surface acoustic wave device equipped with a rectangular pump electrode 4, the desired center frequency f is selected.

When the output at is determined, the response of the signal passband and the spurious response are almost determined.

For this reason, when a conventional surface acoustic wave device is used as a frequency selection device, there is extremely little freedom in designing frequency characteristics, and the width of the spurious response cannot be said to be sufficiently small for practical use.

The present invention provides a surface acoustic wave parametric device that is capable of adjusting the response of a signal passband to specifications and reducing spurious responses to a level that does not cause any practical problems when used as a frequency selection device. That's what I tried to do.

That is, the present invention was made based on the following theory.

According to current electric circuit theory, the output frequency characteristics of a linear circuit are determined by Fourier transforming the time response (time change) of the output end when an impulse is applied to the input end.

Therefore, if we conversely find a time response that has a Fourier transform relationship with the desired frequency characteristic and construct a linear circuit so that this time response and impulse response are equal, an output signal with the desired frequency characteristic can be obtained. It will be done.

Here, the case where the above theory is applied to the conventional device shown in FIG. 1 will be described in more detail with reference to FIGS. 4A to 7B.

In the surface acoustic wave parametric device shown in FIG. 1, the parametric interaction is set not to be very strong, and an ideal impulse e1 with a duration of O as shown in FIG. 4A is sent to the input transducer 5. Time 1=
Assume that 0 is input.

At this time, among the two surface acoustic wave signal outputs output from the pump electrode 4 in the left and right directions, if we take the time response ■1 for the output signal 15 (frequency fi) that travels in the opposite direction to the input surface acoustic wave 13. What is shown in FIG. 4B is obtained.

Here, in FIG. 4B, time t1 is the time required for the surface acoustic wave signal to travel back and forth between the input transducer 5 and the front end 4' of the pump electrode 4, and time t2 is the time required for the surface acoustic wave signal to travel back and forth between the input transducer 5 and the front end 4' of the pump electrode 4. represents the time required for the input transducer 5 to travel back and forth between the input transducer 5 and the rear end 4'' of the pump electrode 4.

To explain the input impulse e1 and the output time response 1 in more detail, the ideal impulse e1 includes an infinite number of frequencies.

When this impulse e1 is input from the input transducer 5, among the above frequency components (the pump frequency is
f) frequency f.

Only a certain portion of the nearby signal undergoes mainly parametric interaction and is output as a backward surface acoustic wave signal 15 that travels in the opposite direction to the input surface acoustic wave 13.

The envelope when a certain frequency portion of this output signal is subjected to AM detection is the output time response I1 shown in FIG. 4B.

On the other hand, instead of the impulse e1, the input transducer 5 receives a frequency f as shown in FIG. 5A.

FIG. 5B shows a frequency spectrum 15c of the backward surface acoustic wave output signal 15 when a signal e2 with a constant amplitude is applied.

By the way, is the frequency spectrum 15c in FIG. 5B the same as the symbol in FIG. 2? This corresponds to the output frequency characteristic shown in 5a.

Here, the above-mentioned frequency spectrum 15C corresponds to the Fourier transform of the above-mentioned time response (1).

Therefore, let us assume that the frequency spectrum 1 shown in Fig. 5B is
Assuming that 5C is the desired frequency characteristic, the relationship between this and the Fourier transform is the time response ■1 shown in Figure 4B.
The linear circuit exhibiting this time response (1), ie, the surface acoustic wave parametric device, is the device equipped with the rectangular pump electrode 4 shown in FIG.

However, the above ideal impulse e1 is not actually realizable.

Therefore, instead of this, a carrier frequency f as shown in FIG. 6A is used.

, RF pulse e1' of duration t (1<12-11)
Use.

At this time, the time response ■1' for the backward wave output signal 15
The result shown in FIG. 6B is obtained, and the envelope obtained by AM detection is almost the same as that shown in FIG. 4B.

Incidentally, the carrier frequency of the time response output signal 1' shown in FIG. 6B is fi = 2 fo - fo = f.

Also, regarding the signal e2 of constant amplitude in FIG. 5A,
The actual signal e2' has a center frequency f as shown in FIG. 7A.

There is a tendency for the amplitude to attenuate at frequencies far away from .

However, even when such a signal e2' is input from the transducer 5, the frequency spectrum 150' of the backward wave output signal shown in FIG. 7B is obtained.

This frequency spectrum 15c' is almost the same as that shown in FIG. , actually as an impulse and a constant amplitude signal in Figures 6A and 7.
The signals shown in Figure A are used.

Next, the present invention based on the above-mentioned technical method will be specifically explained with reference to the embodiments shown in the drawings.

In the following drawings, the same reference numerals as above are given to the same members as in FIG. 1.

In FIG. 8, reference numeral 1 denotes a semiconductor substrate made of silicon (Si) material, as an example.
An insulating film 2 and a piezoelectric layer 3 are sequentially laminated thereon to form a laminate.

The insulating film 2 acts as a stabilizing film on the surface of the semiconductor substrate 1, and an example is a silicon dioxide film (S i02
).

Further, the piezoelectric layer 3 is formed of a piezoelectric material such as zinc oxide (ZnO') or aluminum nitride (AI, N).

Next, an input transducer 5 and an output transducer 6 are respectively disposed near both left and right ends of the laminate in the figure.

The human output transducers 5, 6 are formed to have sufficiently broadband characteristics.

Then, on the propagation path of the surface acoustic wave between the input transducer 5 and the output transducer 6, a plurality of divided pump electrodes M1, M2, etc. are arranged along the propagation direction of the surface acoustic wave. .

As an example, each pump electrode M1, M2, etc. has a rectangular shape as shown in the figure, and is arranged with the long sides of this rectangle parallel to each other. The gap length between the electrodes, etc. are selected in conjunction with each pump voltage value applied to each, and are made to correspond to the desired output frequency characteristics.

Next, capacitors C1, C2, etc. having different capacitance values are connected to each of the pump electrodes M1, M2, etc., and a pump voltage is applied to the other end of each capacitor C1, C2,... High frequency power supply (pump power supply) 9
Commonly connected to.

Thus, the high frequency voltage drop in each capacitor C1, C2, . . . is made different, so that pump voltages of different values can be applied to each pump electrode M1, M2, .

Incidentally, each of the capacitors C1, C2, . . . also functions as a DC blocking capacitor.

FIG. 9 shows an example of a specific arrangement of the above-mentioned capacitors C1, C2, etc., and in this case, each capacitor C
1, C2, . . . are formed by thin film capacitors, and these thin film capacitors are integrated on the piezoelectric layer 3, that is, on the same plane as the surface on which each pump electrode is disposed.

In the figure, reference numeral 16 is a thin dielectric layer, 17 is an upper electrode, and lower electrodes 1 8 . By changing the area of 1 8'..., each capacitor C1, C2
...to have different capacitance values.

In FIG. 8, symbols L1, L2, . . . are inductors for blocking AC, and 7 is a DC power source 11.1 for applying DC bias.
2 is a surface acoustic wave absorbing material for preventing unnecessary surface acoustic waves from being reflected at the end face.

Note that the element for adjusting the pump voltage value applied to each pump electrode M1, M2, etc. is not limited to a capacitor, but may be another impedance element such as an inductor L1 or a resistor R. Capacitor C1 in Figure 8
, C2, . . . function as DC blocking, and an inductor L1 or a resistor R having a different impedance value is interposed in series with each of these capacitors C1, C2, .

Next, a procedure for designing a pump voltage value to be applied to the parametric interaction region, that is, each pump electrode M1, M2, . . . based on the above-mentioned technical method will be described.

Now assume that the desired frequency characteristic is an ideal bandpass filter with a bandwidth B as shown in FIG.

At this time, if we calculate the time response ■2 which has a Fourier transform relationship with this frequency characteristic E2, we get si as shown in Figure 11.
In an nx/x-shaped diagram, it extends infinitely to the left and right.

Here, the magnitude of the surface acoustic wave output signal SAW3 generated by the impulse e1 is related to the strength of the parametric interaction region, and one of the main factors determining the strength of this parametric interaction is the magnitude of the pump voltage. There is.

Therefore, when other conditions are held constant, if the pump voltage is different, the strength of the parametric interaction will be different, and the surface acoustic wave output signal SA generated by the impulse e1 will be
The size of W3 also differs.

In changing the magnitude of the impulse response in accordance with the magnitude of the pump voltage applied to each pump electrode M1, M2, etc., the voltage of the pump power source 9 is kept constant as in the example shown in FIG. When this is done by changing the capacitance value (impedance value) of the capacitor, if the pump power supply 9 and the pump electrode are electrically loosely coupled and the parametric interaction is relatively weak, the capacitance value of the capacitor will change. The square and the impulse response are almost proportional.

Therefore, in order to equalize the time response (2) and the impulse response, the capacitance values of the capacitors C1, C2, etc. connected to each pump electrode M1, M2, etc. should be adjusted to the waveform of the time response (2). All you have to do is make them correspond to each other and make them different.

However, in order to correspond to the ideal bandpass filter (Fig. 10), groups of pump electrodes M1, M2, etc. are arranged infinitely in the left and right direction, similar to waveform 2 in Fig. 11.
A parametric interaction region must be generated up to this infinite position.

Therefore, the shape and dimensions of the laminate must be increased accordingly, which poses a practical problem.

For this reason, the filter characteristics (frequency selection characteristics) are optimized when applied to the device by making some sacrifices to the extent that there is no practical problem, and by adjusting the number of pump electrodes and the capacitance value of each capacitor connected to these. must be designed so that

Therefore, a design example of the parametric interaction region, in other words, the number of pump electrodes and the capacitance value of each capacitor connected thereto, will be described below, taking practicality into account.

Figure 12 shows impulse response ■3 corresponding to the waveform portion of the sin ■Each pump electrode M, , corresponding to 3.
Each capacitor C1, C2... connected to M2...
FIG. 13 shows the capacitance value.

In the example shown in FIG. 13, the number of pump electrodes is six.

The number of pump electrodes to be arranged (the number of divisions) should be as large as possible to correspond to the impulse response more accurately if it is practically possible.

When the number of pump electrodes and the capacitance settings of the capacitors C1, C2, etc. connected thereto as shown in FIG. 13 above are applied to the device shown in FIG. 8, the surface acoustic wave output signal SAW3 is A desired frequency characteristic as shown in FIG. 14 can be obtained.

When this frequency characteristic E3 is compared with the ideal bandpass filter E2 shown in FIG. 10, there is a deterioration in the shoulder characteristics of the filter, but when compared with that of the conventional example shown in FIG. 2, there is a spurious response. has decreased significantly to the extent that there is no problem in practical use.

Therefore, each pump electrode M1, M2 shown in FIG.
The capacitance value setting manner of each of the capacitors C1, C2, . . . connected to the capacitors C1, C2, .

Next, Figures 15 and 16 show each capacitor C1, C2...
This shows other design examples such as capacitance value setting, etc. In this example, the desired frequency characteristics are
The figure shows the capacitance settings of the capacitors C1, C2, etc., so that the time response (4) and the impulse response, which are in a Fourier transform relationship, are equal to each other.

In this case as well, when compared with the characteristic E2 of an ideal bandpass filter, there is a slight deterioration in the shoulder characteristic of the filter, but the spurious response has been eliminated, and the number of pump electrodes M1, M2, etc. The two designs described in Figs. 12 to 17 above can be set within a practical range even in the case of other devices, and can be applied as pump electrodes of the surface acoustic wave parametric device shown in Fig. 8. The example is about a case where the parametric interaction is relatively weak.

When the parametric interaction is large, the impulse response and the capacitance value settings of the capacitors C1, C2, . . . become slightly different from each other, compared to when the parametric interaction is weak.

However, if the effect of this deviation is taken into consideration, even when parametric interactions are large, it is possible to set the capacitance value of each capacitor to obtain the desired frequency characteristics in a manner similar to the design example described above. .

Furthermore, in each of the above design examples, the backward wave output shown by the symbol SAW3 in FIG. A similar design can be made for extracting surface acoustic waves.

However, for the surface acoustic wave with the symbol SAW2, the signal component of the frequency that does not cause parametric interaction is output as is without being changed, so the improvement effect on the spurious response is slightly reduced.

A surface acoustic wave parametric device according to an embodiment of the present invention is constructed as described above and operates as follows.

First, a DC bias voltage of an appropriate value is applied from the DC power supply 7 to each pump electrode M1, M2..., and each pump electrode M1, M2...
M2... Produces an appropriate space charge layer capacitance on the surface of the semiconductor substrate 1 immediately below, and further generates a frequency 2 twice the center frequency fo in a desired frequency selection band from the high frequency power source 9.
The pump voltage of fo is the same for each pump electrode M1, M2...
* is applied to excite the space charge layer capacitance at a frequency of 2fo, and this space charge layer capacitance is changed at a frequency of 2fo.

On the other hand, the input electrical signal applied to the broadband input transducer 5 is converted into a surface acoustic wave and propagates on the surface of the piezoelectric layer 3 toward the left and right of the input transducer 5 in FIG.

Then, the frequency f of the surface acoustic wave input signal SAW1 propagating to the right.

The nearby signal strength is the pump electrode group M1, M2, M3...
In the process of propagating through the lower parametric interaction region, the piezoelectric potential is amplified by parametric interaction with the pump voltage due to the nonlinear effect of the space charge layer capacitance on the surface of the semiconductor substrate 1, and the output surface acoustic wave is In the figure, it occurs from the pump electrode groups M1, M2, . . . toward the left and right.

The output surface acoustic wave SAW2 traveling in the same direction as the input surface acoustic wave SAW1 is converted back into an electrical signal by the output transducer 6 and output to the outside.

On the other hand, the output surface acoustic wave SAW3 traveling in the opposite direction to the input surface acoustic wave SAW1 can be generated by using the input transducer 5 or by using other appropriate means (for example, as disclosed in Japanese Patent Application No. 54-64923 by the present applicant). Similarly, it can be outputted as an electrical signal to the outside by an output means (equipped with a multi-strip coupler as disclosed).

If the surface acoustic wave SAW 3 traveling in the opposite direction is to be taken out as an output, at least only the input transducer 5 of the input transducer 5 and the output transducer 6 should be provided to serve as an input/output transducer. You can also do it.

And as mentioned above, surface acoustic waves SAW2 and SAW3
In the process of outputting, the pump electrode of the parametric interaction region forming section is divided, and each capacitor C1, M2, etc. connected to each of the divided pump electrodes M1, M2...
C2..., the capacitance value of each pump electrode M1, M2,...
Since it is set so that the pump voltage corresponding to the desired frequency characteristic is applied to each of the input surface acoustic waves SAW1, only a certain portion of the signal of the frequency corresponding to this desired frequency characteristic is applied to the corresponding elastic Surface wave SAW2 or SA
It is selectively output as W3.

That is, as an example, the output surface acoustic wave SAW3 in the opposite direction to the input surface acoustic wave SAW1 is output as a signal having the desired frequency characteristics as shown in FIG. 14 or FIG. Parameter l-IJ
The locking device functions as the desired bandpass filter.

Furthermore, although the output surface acoustic wave SAW2 in the same direction as the input surface acoustic wave SAW1 is inferior to the above output surface acoustic wave SAW3 in terms of spurious response, it can be output as a signal having the desired frequency characteristics in the same manner as above. Ru.

As described in detail above, according to the present invention, the pump electrode is divided into a plurality of parts along the propagation direction of the surface acoustic wave, and the value of the pump voltage applied to each pump electrode is adjusted to the desired frequency of the output surface acoustic wave. Because they are made to differ according to their characteristics, when using a surface acoustic wave parametric device as a variable frequency selection device (bandpass filter), it has an extremely excellent effect in that the output frequency characteristics can be designed as desired according to specifications. do.

In addition to the above effects, the surface acoustic wave parametric device inherently has the following effects: it has variable tuning characteristics over a wide frequency range, and the stability of the output center frequency depends on the stability of the external pump power source. and S
It is possible to combine various effects such as good /N, and to provide an extremely excellent surface acoustic wave parametric device for use as a variable frequency selection device.

[Brief explanation of drawings]

Figure 1 is a perspective view showing the conventional example, Figures 2 and 3 are characteristic diagrams showing the output frequency characteristics of the same as above, and Figure 2 is the parameter l-
Figure 3 shows the case where the IJtsuk interaction is relatively weak and the parametric interaction is relatively large, respectively.
FIG. 4A is a waveform diagram showing an ideal impulse used for designing the parametric interaction area, FIG. 4B is a characteristic diagram showing an example of the time response when the same impulse is applied to the device shown in FIG. 1, Figure 5A is a frequency characteristic diagram of an ideal input surface acoustic wave, and Figure 5B is a characteristic diagram showing the frequency spectrum of backward wave surface acoustic wave output when the same input surface acoustic wave as above is applied to the apparatus of Figure 1. , FIG. 6A is a waveform diagram showing an example of an RF pulse used in place of FIG. 4A, and FIG. 6B is a characteristic showing an example of the time response when the same RF pulse is applied to the device shown in FIG. Figure 7A is a frequency characteristic diagram of an input surface acoustic wave used in place of Figure 5A, and Figure 7B is a backward wave elastic surface when the same input surface acoustic wave is applied to the device in Figure 1. A characteristic diagram showing the frequency spectrum of the wave output. Fig. 8 is a diagram showing a surface acoustic wave parametric device as an embodiment of the present invention, and the laminate part is shown in a perspective view. Fig. 9 is a capacitor group in the same device. Fig. 10 is an enlarged perspective view showing an example of integrally integrated on a laminate, with some parts omitted. FIG. 12 is a characteristic diagram showing the time response related to Fourier transform. FIG. 12 is a characteristic diagram showing the impulse response obtained by removing the waveform portion other than -π<x<π from the characteristics in FIG. 11. FIG. Fig. 14 is a setting curve diagram showing an example of setting the capacitance value of each capacitor corresponding to the above impulse response; Fig. 14 is a characteristic diagram showing an example of the output frequency characteristic when the setting example same as above is applied to Fig. 8; Fig. 15 is a characteristic diagram showing another example of the impulse response applied to the present invention, the first
Figure 6 is a setting curve diagram showing an example of setting the capacitance value of each capacitor corresponding to the above impulse response, and Figure 17 is a characteristic diagram showing the output frequency characteristics when the above setting example is applied to Figure 8. . 1: Semiconductor substrate, 2: Insulating film, 3: Piezoelectric layer, 5: Human power transducer, 6: Output transducer, 9: High frequency power supply, 11, 12: Surface acoustic wave absorber, 16: Dielectric thin film, 17: Upper electrode, 18.18': lower electrode, C
1, C2, C3...: Capacitor, L1, L2, L3
...: Inductance M1, M2, M3...: Pump electrode.

Claims (1)

[Claims] 1. At least an input transducer of an input transducer and an output transducer, and a plurality of pump electrodes are disposed on the piezoelectric body in a laminate including a semiconductor and a piezoelectric body; A high frequency power source for applying a pump voltage is connected to each of the pump electrodes through a separate impedance element having a different value corresponding to the desired frequency characteristics of the output surface acoustic wave, and further, a high frequency power source for applying a pump voltage is connected to each of the pump electrodes. A surface acoustic wave parametric element characterized in that a DC power supply for applying a DC bias voltage is separately connected. 2. The surface acoustic wave parametric device according to claim 1, wherein each pump electrode has a rectangular shape, and the long sides of the rectangular pump electrodes are arranged in parallel. 3. The surface acoustic wave parametric device according to claim 1 or 2, wherein the impedance element is a capacitor. 4. The surface acoustic wave parametric device according to claim 3, wherein the plurality of capacitors are formed of thin film capacitors, and the thin film capacitors are integrated on the same plane as the surface on which each pump electrode is disposed.