JP3112996B2 - Surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter.

As devices utilizing surface acoustic waves, surface acoustic wave resonators, surface acoustic wave filters, surface acoustic wave delay devices, and the like are known. Among them, the surface acoustic wave filter is a chip made of a piezoelectric crystal in which a surface acoustic wave propagates, an input transducer for converting energy of an electro-surface acoustic wave formed on the chip, and an excitation by the input transducer. And an output-side transducer for converting the generated surface acoustic wave into an electric signal. This type of surface acoustic wave filter is often used as a component of a high-precision voltage-controlled oscillator because of its good stability of the center frequency of its pass band.

[0003]

2. Description of the Related Art FIG. 3 is a block diagram of a conventional voltage controlled oscillator (VCO) provided with a surface acoustic wave filter. The VCO is configured to form an oscillation loop by connecting the surface acoustic wave filter 2, the phase adjuster 4, and the amplifier 6 in series, and to take out an oscillation signal from the output terminal of the amplifier 6. By adjusting the signal phase with the phase adjuster 4, a desired oscillation frequency can be obtained.

[0004]

In the conventional VCO, a phase adjuster is required to make the oscillation frequency variable, and this phase adjuster becomes a large-scale circuit depending on the oscillation frequency and its variable range. That is the current situation. Therefore, there is a demand for a VCO that does not require a phase adjuster. If it is possible to realize a surface acoustic wave filter in which the center frequency of the pass band is variable, it is considered that the above-described problem is achieved.

The present invention has been made in view of such circumstances, and has as its object to provide a surface acoustic wave filter having a variable center frequency in a pass band. It is another object of the present invention to provide a VCO that does not require a phase adjuster using the surface acoustic wave filter.

[0006]

A surface acoustic wave filter according to the present invention comprises a chip made of a piezoelectric crystal, an input transducer and an output transducer formed on the surface of the chip, and a signal supplied to the input transducer. First and second common electrodes to be applied and third and fourth common electrodes for extracting signals from the output-side transducer;
A DC bias electrode formed on the back side of the chip so as to face the first to fourth common electrodes;
A variable DC power supply connected between the bias electrode and the first and third common electrodes or the second and fourth common electrodes, and an input connected between the first and second common electrodes A side inductor and an output inductor connected between the third and fourth common electrodes; an input capacitor connected between the first or second common electrode and a signal input terminal; and the third or fourth common electrode. An output-side capacitor connected between the fourth common electrode and the signal output terminal is provided.

A voltage controlled oscillator according to the present invention is configured by connecting the signal input terminal and the signal output terminal of the surface acoustic wave filter according to the present invention to an output terminal and an input terminal of an amplifier, respectively.

[0008]

The physical constant such as the piezoelectric constant of the chip can be changed by applying a DC bias to the chip made of the piezoelectric crystal. When the physical constant of the chip changes, the propagation speed of the surface acoustic wave changes, and accordingly, the center frequency of the pass band also changes.

In the structure of the present invention, the input inductor is connected between the first and second common electrodes, and the output inductor is connected between the third and fourth common electrodes.
This is because, when a bias voltage is applied between the C bias electrode and the first to fourth common electrodes, the first to fourth common electrodes are required to have substantially the same potential. That is, if the above requirement is satisfied without providing the inductor, the first common electrode and the third common electrode are short-circuited, or the second common electrode and the fourth common electrode are short-circuited, and the signal Since input or signal output becomes difficult, an inductor is connected between the common electrodes as described above.

In the configuration of the present invention, an input-side capacitor is connected between the first or second common electrode and the signal input terminal, and an input capacitor is connected between the third or fourth common electrode and the signal output terminal. The reason why the output side capacitor is connected is to prevent the DC bias from affecting the external circuit.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. FIG. 1 is a configuration diagram of a surface acoustic wave filter showing an embodiment of the present invention. Two sets of IDTs (interdigital transducers) each formed by combining a plurality of interdigital electrodes 12 in an interdigital manner are formed on the surface side of a chip 10 made of a piezoelectric crystal such as quartz, one of which is an input transducer 14. , The other is the output transducer 1
6.

Each of the comb electrodes 12 of the input transducer 14 is alternately provided with a first common electrode 18 and a second common electrode 18.
Are connected to the common electrode 20. Further, every other comb-shaped electrode 12 of the output-side transducer 16 is connected to the third common electrode 22 and the fourth common electrode 24 every other one.

The comb electrodes 12 and the common electrodes 18, 20,
22 and 24 are formed by depositing or sputtering aluminum on the chip 10 through a suitable mask. There is a relationship of p = λ / 2 between the pitch p of the comb-shaped electrode 12 and the wavelength λ of the surface acoustic wave excited.

On the back side of the chip 10, a DC is provided so as to face the first to fourth common electrodes 18, 20, 22 and 24.
The bias electrode 26 is also formed of aluminum.

Reference numeral 28 denotes an input-side capacitor provided between the first common electrode 18 and the signal input terminal 30, and 32 denotes an input-side inductor provided between the first common electrode 18 and the second common electrode 20. , 34 are output-side capacitors provided between the third common electrode 22 and the signal output terminal 36, and 38 is an output-side inductor provided between the third common electrode 22 and the fourth common electrode 24. .

A variable DC power supply 40 is connected between the DC bias electrode 26 and the second and fourth common electrodes 20 and 24. With this configuration, the DC bias electrode 2
A desired DC bias can be applied between the first common electrode 6 and the first to fourth common electrodes 18, 20, 22, 24. The variable DC power supply 40 may be provided between the DC bias electrode 26 and the first and third common electrodes 18 and 22.

When the DC bias voltage is changed by the variable DC power supply 40, the piezoelectric constant of the chip 10 changes accordingly. This changes the propagation speed of the surface acoustic wave,
By changing the resonance frequency given by f = v / 2p, the center frequency of the pass band of the surface acoustic wave filter can be changed. Here, v is the propagation speed of the surface acoustic wave, p is the pitch of the comb-shaped electrodes, and f is the resonance frequency.

On the input side and the output side, inductors 32 and 38 are provided.
Is provided, the first to fourth common electrodes 18, 20, 22, and 24 are set to substantially the same potential while inputting / outputting a high-frequency signal, and these common electrodes and the DC bias electrode 26 are turned on.
Thus, a DC bias can be effectively applied. Further, since the capacitors 28 and 34 are provided, there is no possibility that the DC bias affects an external circuit (not shown).

Here, since the capacitors and inductors on the input side and the output side are of a high-pass filter type, they can be used also as the impedance matching circuit of the input / output section. That is, the inductors 32 and 38
By setting the inductance of the surface acoustic wave filter and the capacitance of the capacitors 28 and 34 according to the circuit constant of the impedance matching circuit, the loss characteristics of the surface acoustic wave filter can be improved.

FIG. 2 is a block diagram of a VCO (voltage controlled oscillator) constructed using the surface acoustic wave filter shown in FIG. This VCO is connected to the signal input terminal 30 and the signal output terminal 36 of the surface acoustic wave filter 42 shown in FIG.
Are connected to the output terminal and the input terminal of the amplifier 44, respectively, and an oscillation signal is obtained from the output terminal of the amplifier 44.

According to this configuration, a phase adjuster is not provided unlike the conventional configuration, but a required oscillation frequency can be obtained by changing the DC bias by the variable DC power supply 40 (FIG. 1). Further, frequency stability according to the stability of the DC bias can be obtained.

As described above, according to the present embodiment, it is possible to provide a highly stable voltage controlled oscillator which is simple in structure, suitable for miniaturization.

[0023]

As described above, according to the present invention,
There is an effect that it is possible to provide a surface acoustic wave filter in which the center frequency of the pass band is variable. Further, by using this surface acoustic wave filter, it is possible to realize a highly stable voltage controlled oscillator which is simple in configuration, suitable for miniaturization.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a surface acoustic wave filter according to an embodiment of the present invention.

FIG. 2 is a block diagram of a voltage controlled oscillator configured using the surface acoustic wave filter shown in FIG.

FIG. 3 is a block diagram of a conventional voltage controlled oscillator.

[Explanation of symbols]

 10 chip 14 input side transducer 16 output side transducer 18, 20, 22, 24 common electrode 26 electrode for DC bias 40 variable DC power supply

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kageharu Yoshizawa 1015 Ueodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-3-201611 (JP, A) JP-A-47- 31557 (JP, A) US Patent 4,636,678 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/25 H03B 5/24 H03B 5/32 H03H 9/64

Claims (1)

(57) [Claims] 1. A chip made of a piezoelectric crystal, an input-side transducer and an output-side transducer formed on a surface side of the chip, and a first and a second part for giving a signal to the input-side transducer.
And a third and a fourth common electrode for extracting a signal from the output side transducer and a DC bias electrode formed on the back surface of the chip so as to face the first to fourth common electrodes. A variable DC power supply connected between the DC bias electrode and the first and third common electrodes or the second and fourth common electrodes; and a variable DC power supply connected between the first and second common electrodes. The input-side inductor and the output-side inductor connected between the third and fourth common electrodes; the input-side capacitor connected between the first or second common electrode and the signal input terminal; third or a connected output side capacitor between the fourth common electrode and the signal output terminal, the inductance arrangement of the input side and output side inductor
The capacitance of the input and output capacitors is
Satisfies the circuit constants of the impedance matching circuit
A surface acoustic wave filter characterized by being set as follows .