JPH0654847B2 - Frequency modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a frequency modulator, and more particularly to a frequency modulator using a Lamb wave delay line oscillator having a interdigital transducer for modulation and a variable resistance means.

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

(Object of the Invention) The present invention is a frequency modulator using such a interdigital transducer and a variable resistance means, which has good frequency characteristics,
It is an object of the present invention to provide a frequency modulator which has high modulation sensitivity, can use harmonics, and has a simple structure.

(Structure and Action of the Invention) FIG. 1 is a structure diagram showing an embodiment of the present invention. In the figure, 10 is a Lamb wave device, which is composed of a piezoelectric substrate 11, modulation electrodes 12 and 13, an input electrode 14, and an output electrode 15. Reference numeral 11 denotes a piezoelectric substrate, which is a thin plate having a thickness formed of a piezoelectric material and being equal to or less than the wavelength λ of a propagating wave. Reference numeral 12 is a modulation electrode, which is a transducer having a comb-shaped electrode in which comb-teeth-shaped electrode fingers are alternately arranged in an interdigital manner, and a modulation signal is transmitted through a modulation signal input terminal 30 and two FETs 16 and 17. Supplied. Reference numeral 13 is a modulation electrode, which is a flat plate electrode formed by vacuum-depositing a metal such as aluminum and is grounded. The modulation electrode 12 is the piezoelectric substrate 11
, And the modulation electrode 13 is located behind the modulation electrode 12. Reference numeral 14 is an input electrode, which is a transducer having interdigital electrodes in which comb-teeth-shaped electrode fingers are alternately arranged in an interdigital manner. Reference numeral 15 is an output electrode, which has the same structure as the input electrode 14. One electrode of each of the input electrode 14 and the output electrode 15 is connected to the amplifier circuit 20 as shown, and the other electrode is grounded. Reference numerals 16 and 17 are FETs, which are used as variable resistance elements for modulation. R _DS can be changed by changing V _GS.
Can be changed. Reference numeral 20 denotes an amplifier circuit, whose input is connected to the output electrode 15 and whose output is connected to the input electrode 14 to form a feedback circuit. The modulated signal is available via output terminal 40.

FIG. 2 is a configuration diagram specifically showing the Lamb wave device of FIG. The number of electrode pairs of the input electrode 14 and the output electrode 15 is 1
4. The electrode cycle length is 333 μm, the distance between the centers is 39 cycles, or 12.987 mm, and the electrode weight width is 10 cycles, or 3.33 mm. The number of electrode pairs of the modulation electrode 12 is 7,
In addition, the direction of the source of the Lamb wave (from the modulation electrode 12 to the output electrode
In order to cancel the reflection at the electrode part, the electrode fingers facing each other are displaced by ¼ cycle length. However, although the above is described in this embodiment, the present invention is not limited to this.

FIG. 3 is a circuit diagram of the amplifier circuit 20 of FIG. The circuit shown in the figure is a wide band amplifier which can obtain a maximum amplification factor of about 200 times by using one operational amplifier 733 having a wide band characteristic.

Next, the operation will be described.

When the electric signal from the amplifier circuit 20 is applied to the input electrode 14, the electric signal is converted into a Lamb wave, which is the piezoelectric substrate 11.
It propagates through the inside, reaches the output electrode 15, is converted into an electric signal in the output electrode 15, and is input to the amplifier circuit 20. The amplifier circuit 20 compensates for the insertion loss due to the Lamb wave device 20 and outputs a signal. A Lamb wave device (10) and an amplifier circuit (20) constitute a feedback loop oscillator. In this state, two F electrodes are provided between the modulation electrode 13 and the modulation electrode 12 directly behind this.
When ET is provided and the resistance value between the drain and the source of this FET is changed, the termination condition changes accordingly and the oscillation frequency changes. This frequency shift is obtained via the output terminal 40. This is the modulation electrodes 12, 13 and the two F
The ETs 16 and 17 can be treated equivalently to arranging the phase shifter depending on the termination condition on the Lamb wave device 10.
Electrically changing this termination condition changes the oscillation frequency.

Next, an experimental example of the present invention will be described. In the Lamb wave device 10 used in this experiment, the piezoelectric substrate material is a piezoelectric ceramic 101A material manufactured by TDK Corporation, and its dimensions are 20.0 mm x 8.5 m.
It is m × 0.18 mm, and is composed of the modulation electrodes 12 and 13, the input electrode 14 and the output electrode 15. The amplifier circuit shown in FIG. 3 was used.

FIG. 4 shows the frequency-to-impedance absolute value characteristic and the frequency-to-phase characteristic of the Lamb wave device 10 with the above specifications. As can be seen from the attachment, there are resonance points around 5, 8, 11, and 13 MHz.

FIG. 5 shows insertion loss characteristics between the two input electrodes 14 and the output electrode 15. As you can see from the figure, 13, 8,
There is a pass band with less loss in the order of 5 and 11 MHz. Here, A ₀ , S ₀ , A ₁ , and S ₁ in the figure are 5, 8, 11, and ₁₃ M, respectively.
Shows a plot of Hz.

FIG. 6 is a diagram in which the FD value and the phase velocity near the resonance point of the Lamb wave device 10 are plotted on the data of PZT-5H. The oscillation frequency is determined by the amplification degree and phase relationship of the amplifier. Therefore, the oscillation mode used in the experimental example of the present invention was the first-order symmetric mode Lamb wave (S ₁ mode in FIG. 6) which is the easiest to oscillate.

FIG. 7 shows V _GS vs. R _{DS of} the FET used in the experimental example of the present invention.
Show the characteristics. The FET used in the experimental example of the present invention can change R _DS as shown in the figure by changing V _GS .

FIG. 8 shows the DC characteristics of V _GS vs. oscillation frequency of the FET. As can be seen from the figure, there is no linear relationship over the entire range. However, this is due to the non-linear characteristic of the FET as shown in FIG. However, V _GS is −0.
It is relatively straight between 45 and -0.60V. FIG. 9 shows a detailed measurement of this straight line portion. As can be seen from the figure, it is understood that V _GS is a straight line between −0.45 and −0.61V. Here, an AC signal is applied around -0.53V as the center point of the operation. By the way, −0.
53V is called offset voltage.

FIG. 10 shows the modulation signal voltage versus frequency deviation characteristic. From the figure, the modulation sensitivity is 2 △ / V _PP = 26KHz / V, △ / V
_RMS = 36.8 KHz / V.

FIG. 11 shows the relative frequency deviation amount (output level ratio) when the frequency of the modulation signal is changed while the modulation signal voltage is kept constant at 0.1 V _PP . As you can see from the figure, D
Since it is constant from C to 100 KHz or more, a considerably high speed can be achieved for serial data transmission.

FIGS. 12 and 13 show the frequency response with time of a sine wave and a triangular wave having a modulation frequency of 0.1 Hz. As you can see from both figures, it has a very good fidelity. It can be seen that this can be applied to the sensor as a voltage-versus-frequency transducer including the slow changing region.

FIG. 14 shows the frequency deviation amount and the relative amplitude of each harmonic component in a state where the amplification degree of the amplifier circuit 20 is unnecessarily increased and distorted. As can be seen from the figure, the relationship between the order and the frequency shift amount is a straight line, which is effective when the harmonics are used.

(Effects of the Invention) As described in detail above, in the present invention, the two pairs of interdigital electrodes that generate and detect the first-order symmetric mode Lamb wave by setting the thickness of the piezoelectric substrate to the wavelength λ of the propagating sound wave or less. The resistance value between the drain and source of the two FETs is changed by the modulation electrode composed of a pair of interdigital electrodes and plate electrodes, which are provided so as to face each other so as to sandwich the piezoelectric substrate at a position sandwiched between them. By doing so, the variable resistance means for changing the phase termination condition of the feedback loop oscillation configuration including the Lamb wave device and the amplifier circuit to change the oscillation frequency of the feedback loop oscillation configuration is connected.

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.
Thus, by constructing the piezoelectric substrate thin so that the sound wave propagating through the piezoelectric substrate becomes a Lamb wave, it is possible to provide a frequency modulator with excellent sensitivity, and the high efficiency and high performance required for this type of converter are provided. The characteristics can be sufficiently satisfied. Moreover, since the Lamb wave delay line oscillator and the two FETs are used as the variable resistance means, the frequency characteristic of modulation is good and the modulation sensitivity is higher. Further, since it is possible to directly oscillate up to several tens of MHz, harmonics can be used. In particular, it is possible to provide a frequency modulator that has a very simple structure and can be miniaturized. It goes without saying that the present invention can be used for a voltage sensor.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a configuration diagram specifically showing the Lamb wave device shown in FIG. 3, FIG. 3 is a circuit diagram of the amplifier circuit shown in FIG. 1, and FIG. 4 is a diagram showing absolute value characteristics of frequency versus impedance and frequency versus phase characteristics of the Lamb wave device. FIG. 5 is a diagram showing insertion loss characteristics between two input electrodes and output electrodes, and FIG. 6 is a plot of FD values and phase velocities near the resonance point of the Lamb wave device on the data of PZT-5H. FIG. 7 is a diagram showing the V _GS vs. R _DS characteristics of the FET used in the experimental example of the present invention, and FIG. 8 is the V of the FET.
Fig. 9 is a diagram showing DC characteristics of _GS vs. oscillation frequency, Fig. 9 is a measurement diagram in which V _{GS of} Fig. 8 is -0.45 to -0.61 V in detail, and Fig. 10 is a modulation signal voltage vs. frequency deviation characteristic. Fig. 11 shows the relative frequency deviation when the frequency of the modulation signal is changed to 0.1 V _PP and the modulation signal voltage.
12 view and FIG. 13 is denoted a sine wave and a triangular wave of modulation frequency 0.1H _z with respect to the frequency response time diagram, the harmonics in the state Fig. 14 which is distorted to increase unnecessarily the amplification degree of the amplifier circuit It is the figure which described both the frequency shift amount and relative amplitude of a wave component. 10 …… Lamb wave device, 11 …… Piezoelectric substrate, 12,13 …… Modulation electrode, 14 …… Input electrode, 15 …… Output electrode, 16,17 …… FET, 20 …… Amplification circuit, 30 …… Modulation signal input terminal, 40 …… Output terminal.

Claims (2)

[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, a comb-shaped electrode provided on one surface of the piezoelectric substrate, and the piezoelectric substrate facing the comb-shaped electrode. A Lamb wave device having a modulation electrode composed of a flat plate-shaped electrode provided on the back surface, and an input electrode and an output electrode composed of interdigital electrodes spaced apart on both sides of the interdigital electrode. An amplifier circuit that is connected to the input electrode and the output electrode, amplifies an electrical signal from the output electrode, outputs the amplified signal to the input electrode, and outputs a modulated signal; By changing the resistance value between the drain and the source of the two FETs connected between the interdigital electrode and the flat electrode of the modulation electrode, a feedback loop oscillation configuration including the Lamb wave device and the amplifier circuit is formed. And a variable resistance means for changing the oscillation frequency of the feedback loop oscillation configuration by changing the phase termination condition. 2. The frequency modulator according to claim 1, wherein the interdigital electrode fingers of the modulation electrode are displacement electrodes having a quarter cycle length.