JPH0654846B2 - FM modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to an FM modulator, and more particularly to an FM modulator using a Lamb wave device.

(Background Art) A comb-shaped transducer, that is, a transducer having a comb-shaped electrode in which comb-teeth-shaped electrode fingers are alternately arranged in an interdigital manner on one surface of a piezoelectric substrate, is widely used as the most important surface acoustic wave technology. It is used in filters and delay lines.

(Object of the Invention) The present invention is an FM modulator using such a interdigital transducer, and in particular, by using a Lamb wave as a sound wave propagating through a piezoelectric substrate, high efficiency, high performance, small size, light weight, and general purpose are achieved. It is an object of the present invention to provide a highly efficient FM modulator.

Hereinafter, the present invention will be described with reference to the drawings.

(Structure and Action of the Invention) FIG. 1 is a diagram showing an embodiment of the present invention. In the figure, 10 is a Lamb wave device, which includes a piezoelectric substrate 12 and a modulation electrode
And 14 ', an input electrode 16 and an output electrode 18. The piezoelectric substrate 12 is a thin flat plate formed of a piezoelectric material, and its thickness is set to be equal to or less than the wavelength λ of the propagating sound wave so that the sound wave propagating through the piezoelectric substrate is a Lamb wave. The modulation electrodes 14 and 14 'are flat plate electrodes formed by vacuum-depositing a metal such as aluminum, and are provided substantially at the center of the piezoelectric substrate 12 and facing each other so as to sandwich the substrates. A modulation signal is supplied to the modulation substrate 14 via a modulation signal input terminal 30. On the other hand, the modulation electrode 14 'is grounded. The input electrode 16 and the output electrode 18 are spaced apart from each other on the piezoelectric substrate 12 with the modulation electrode 14 interposed therebetween, and comb-teeth-shaped electrode fingers are alternately arranged in an interdigital manner. One electrode of each of the input electrode 16 and the output electrode 18 is connected to the amplifier circuit 20 as shown, and the other electrode is grounded. Although not shown, flat electrodes may be provided on the surface of the piezoelectric substrate 12 facing the input electrode 16 and the output electrode 18, respectively, for grounding. As a result, only the Lamb wave of the desired mode can be propagated.

Reference numeral 20 denotes an amplifier circuit, whose input is connected to the output electrode 18 and whose output is connected to the input electrode 16 to form a feedback circuit. The feedback circuit and the Lamb wave device 10 constitute an oscillation circuit (Lamb wave oscillator). The modulated wave is obtained via the FM signal output terminal 40. Here, a configuration example of the amplifier circuit 20 is shown in FIG. The circuit shown in the figure is a wide band unregulated amplifier having a two-stage configuration of an operational amplifier 733C having wide band characteristics. As apparent from the figure, does not use a resonant circuit in the circuit, the unnecessary adjustments in response to changes in frequency, 100kH _z ~
Until 70MH _z it is possible to obtain a flat characteristic. The above-mentioned Lamb wave oscillator using this amplifier oscillates only by adjusting the gain (feedback rate) and adjusting the circuit constants slightly (phase correction), and it is extremely reproducible under the same conditions. Can be done. Further, if the condition optimally, it is possible to suppress the harmonics without going through the filter circuit, as shown in the spectral distribution diagram generated by the oscillator in 7.180MH _z of FIG. 3, the second harmonic - It can be suppressed to 50 dB. That is, this modulator also works well as an oscillator. Also, as will be described later, by increasing the feedback rate, the fundamental oscillation frequency is almost unchanged, and the oscillation waveform can be distorted to generate many harmonics. The input impedance is high (860kΩ100kΩ / 100H _z ~10k
H ₂ ) is another feature.

Next, the operation will be described.

When an electric signal is applied to the input electrode 16 from the amplifier circuit 20, the electric signal is converted into a Lamb wave, which is the piezoelectric substrate 12.
After propagating through it, it reaches the output electrode 18, is converted into an electric signal, and is applied to the input of the amplifier circuit 20. The amplifier circuit 20 compensates for the insertion loss due to the Lamb wave device 10 and outputs a signal. When a modulation signal is applied to the modulation electrode 14 in such a state, distortion occurs in the sound wave propagation direction, and the propagation distance changes. That is, the delay time changes, the phase condition changes, and the frequency of the Lamb wave oscillator shifts. This frequency shift is obtained via the FM signal output terminal.

Here, the sound wave propagating through the piezoelectric substrate is a Lamb wave. Unlike the Rayleigh wave, the zero-order symmetric Lamb wave has a displacement on both front and back surfaces of the medium in which the sound wave propagates, and since it is a symmetric mode, the displacement characteristics are the same. Since the thickness of the piezoelectric substrate 12 is extremely thin, which is equal to or less than the wavelength λ of the sound wave, the efficiency is good. In other words, it is not necessary to increase the modulation voltage.

Next, an experimental example of the present invention will be described. In the Lamb wave device 10 used in this experiment, the piezoelectric substrate is a piezoelectric ceramic 101A material manufactured by TDK Corporation, and its dimensions are 40 mm × 20 mm × 180 μ.
m, the interdigital electrode having an electrode period of 400 μm, the number of electrode pairs is 10, and the modulation electrode having an effective area of 30 mm ² (5 mm × 6 mm) are vacuum-deposited with aluminum. Also,
The amplifier circuit 20 used is that shown in FIG.

In the above specification, showing the carrier and sidebands when modulated with a modulation frequency 3KH _z in Figure 4. As is clear from the figure,
It can be seen that the same modulation pattern is obtained by the conventional modulation circuit.

FIG. 5 shows the modulation signal voltage (peak-peak) vs. frequency shift characteristic. As can be seen from the figure, the characteristics are linear. This shows that it is not necessary to use a separate equalizer circuit. Further, it is shown that when the present invention is used for frequency shift keying (FSK), a sufficient frequency change can be obtained with a small control voltage.

Next, regarding the frequency characteristics, FIG. 6 shows fluctuations in the detection output of the demodulator when the modulation signal voltage is set to a constant value. As can be seen from the figure, this characteristic indicates that it can be used not only for FSK data communication and mobile radio communication but also for audio signal transmission.

The embodiments and experimental examples of the present invention have been described above.

Further, according to the present invention, by increasing the feedback rate, it is possible to distort the oscillation waveform and generate many harmonics while the fundamental oscillation frequency remains almost unchanged. That is, it is possible to easily generate harmonics without specially modifying the circuit. At this time, in the case of the above-described embodiment, that is, when the same modulation as the fundamental wave is applied, the frequency deviation of the second and higher harmonics increases in proportion to the order (FIG. 9). The frequency relative output level characteristics obtained in the above-mentioned specification example are shown in FIG. Thus, if a bandpass filter that generates a high frequency and passes only a desired frequency is used, the Lamb wave device can be used as a high frequency oscillator and a modulator without designing it for high frequency with high manufacturing accuracy. It has the advantage that it can. FIG. 8 is a spectrum distribution diagram of the 14th harmonic, and FIG. 9 is a modulation signal voltage (peak −
(Peak) vs. frequency shift characteristic diagram, and FIG. 10 is a frequency characteristic diagram. As is clear from these figures, good characteristics can be obtained even when harmonics are used.

Further, according to the present invention, by providing a plurality of modulation electrodes, it is possible to expand the function of the signal processing device. As an application example in this case, there is a method of transmitting the change of the oscillation frequency in a time-division method in response to ON / OFF of a 2-input pulse signal.

(Effects of the Invention) As described in detail above, in the present invention, the thickness of the piezoelectric substrate is set to be equal to or less than the wavelength λ of the propagating sound wave, and the piezoelectric substrate is propagated on both front and back surfaces so that both front and back surfaces are symmetrical. At least one pair of flat plates provided facing each other so as to sandwich this piezoelectric substrate at a central position sandwiched by two pairs of interdigital electrodes for generating and detecting Lamb waves of zero-order symmetric mode The FM modulation signal is applied to the rectangular modulation electrode.

The Lamb wave can be used by setting the thickness of the piezoelectric substrate to be equal to or less than the wavelength of the Lamb wave. As a result, even if the applied voltage is the same, the electric field increases and the dimensional change of the substrate due to the piezoelectric effect increases. As a result, the change in the propagation path length between the input and output becomes large, and the change in the oscillation frequency becomes large, resulting in high sensitivity. That is, a particular effect that satisfactory modulation can be performed even if the input voltage is suppressed low can be obtained.
As described above, by making the piezoelectric substrate thin so that the sound wave propagating through the piezoelectric substrate becomes a Lamb wave, it is possible to provide an FM modulator having good sensitivity and excellent frequency characteristics as an FH modulator. It is possible to sufficiently satisfy the high efficiency and high performance characteristics required as. Moreover, it is possible to provide an FM modulator that is compact and lightweight and has high versatility.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG.
Circuit diagram of the amplifier circuit of FIG., FIG. 3 is the spectral distribution diagram of a Lamb wave oscillator oscillates at 7.180MH _z, Figure 4 is a view showing a carrier and sideband in the case of modulation at the modulation frequency 3KH _z,
FIG. 5 is a diagram showing the frequency deviation of the modulation signal voltage, FIG. 6 is a diagram showing the frequency characteristic of the detection output of the demodulator when the modulation signal voltage is a constant value, and FIG. 7 is obtained by the present invention. Fig. 8 is a diagram showing harmonics, Fig. 8 is a spectrum distribution diagram of the 14th harmonic, Fig. 9 is a diagram showing modulation signal voltage vs. frequency deviation when the harmonics are used, and Fig. 10 is a diagram showing harmonics. It is a frequency characteristic diagram when using. 10 …… Lamb wave device, 12 …… Piezoelectric substrate, 14,14 ′ …… Modulation electrode, 16 …… Input electrode, 18 …… Output electrode, 20 …… Amplifier circuit, 30 …… Modulation signal input terminal , 40 …… FM signal output terminal.

Claims (1)

[Claims] 1. A piezoelectric substrate having a thickness equal to or less than a wavelength λ of a sound wave propagating through the piezoelectric substrate, and a piezoelectric substrate which is provided substantially at the center of the piezoelectric substrate so as to face both sides of the piezoelectric substrate so as to sandwich the piezoelectric substrate. , At least one pair of flat plate-shaped modulation electrodes to which the FM modulation signal is applied, and each pair are spaced apart on the same surface with one of the modulation electrodes interposed therebetween, and propagate on both front and back surfaces of the piezoelectric substrate. A Lamb wave device having two pairs of interdigital electrodes for generating and detecting a Lamb wave of zero-order symmetric mode which gives a displacement of the symmetric mode on both front and back surfaces of the piezoelectric substrate, and two pairs of the interdigital electrodes. And an amplification circuit that amplifies an electric signal from the comb-shaped electrodes of one pair, returns the amplified signal to the comb-shaped electrodes of the other pair, and outputs the modulated signal as a modulated signal. FM modulator.