JPH02182012A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To obtain a structure in which phase noise characteristic is not deteriorated due to a varied frequency by providing surface acoustic wave resonators in parallel close in the direction of cross width so that a harmonic waveguide mode and e fundamental waveguide mode in the electrode cross width direction of an interdigital transducer are made in a duplicate mode. CONSTITUTION:A crystal substrate is used for a piezoelectric substrate 12, number of pairs of interdigital transducers 17, 21 provided in the middle is selected as 250, reflectors 15, 16, 19, 20 at both sides are made of 250 metallic strip arrays to form surface acoustic wave resonators 13, 14 whose electrode cross width is nearly five wavelengths. The 1st and 2nd surface acoustic wave resonators are arranged at an interval of nearly one wavelength and coupled elastically mutually to form a lateral coupling duplicate mode surface acoustic wave filter. when the surface acoustic wave device formed to be in duplicate mode is used for a VCO, the phase noise characteristic over the entire frequency variable range is decreased to -110dBc/Hz or below and the characteristic is improved by nearly 11dB in comparison with a conventional device.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to improvement of a two-bottom surface acoustic wave device, and in particular, to improvement of phase characteristics in a surface acoustic wave device suitably used in a voltage controlled oscillator circuit. related to what was done.

[Conventional technology]

2. Description of the Related Art In recent years, voltage controlled oscillation circuits (hereinafter referred to as ``CO''), which change the oscillation frequency depending on an applied voltage, have been frequently used in wireless communication devices. As an oscillation element for this vCO, an LC tuned circuit or a strip line is generally used, but if phase noise is important, a surface acoustic wave device with a high equivalent Q and which can be made without adjustment is used. is often used.

As a surface acoustic wave device for vCO, a one-bottom surface acoustic wave resonator and a transpersion type surface acoustic wave delay line are well known.

Like a crystal resonator, an l-port surface acoustic wave resonator is mainly used in a Colpitts oscillation circuit as shown in FIG. Here, an inductance 2 is connected in series to the surface acoustic wave resonator 1 in order to expand the frequency variable range.

However, as a substrate for a surface wave device as described above,
When a crystal substrate with excellent temperature characteristics is used, the frequency variable range tends to be narrow in a one-bottom surface acoustic wave resonator. Therefore, the variable range is expanded by connecting the inductances 2 in series as described above, but as a result, the equivalent Q decreases and the phase noise characteristics tend to deteriorate.

Also, the influence of the value of inductance 2 is subtle,
It was difficult to achieve no adjustment.

On the other hand, in the case of a surface acoustic wave delay line, it is used as a so-called feedback type oscillation circuit in which a surface acoustic wave delay line 3 is inserted in the return path, as shown in FIG.

In this case, if the delay time is shortened to make the oscillation frequency variable using the frequency characteristics of the phase, the slope of the phase characteristics becomes gentler and the frequency variable range can be widened.
Since the number of electrode pairs decreases, insertion loss increases in the case of a crystal substrate, and the gain of the oscillation circuit must be increased. Therefore, power consumption and cost increase.

Therefore, it has been considered to use a two-point surface acoustic wave resonator shown in FIG. 4 in place of the surface acoustic wave delay line. In the two-point surface acoustic wave resonator 4, grating reflectors 6.7 are formed on the piezoelectric substrate 5 at a predetermined distance. Between the reflectors 6 and 7, 2t11 interdigital transducers 8 and 9 are arranged.

In the two-point surface acoustic wave resonator 4, the pitch of the interdigital transducers 8, 9 and reflectors 6,7 is approximately equal to the pitch of 1/2 of the wavelength of the surface wave propagating on the piezoelectric substrate 5. , and each interdigital transducer 89 is arranged so as to efficiently detect the standing wave generated between the reflectors 6 and 7.And when a crystal substrate is used as the piezoelectric substrate 5, Also, the insertion loss can be reduced and the frequency variable range can be made relatively wide.

[Technical problem to be solved by the invention] However,
It has been found that the general two-bottom surface acoustic wave resonator 4 as described above has a problem in that the phase characteristics are not linear in the passband, and the phase noise characteristics deteriorate.

Therefore, an object of the present invention is to provide a structure in which the phase characteristics of a two-bottom surface acoustic wave resonator can be made to have a substantially constant slope in the pass band, and the phase noise characteristics do not deteriorate even when the frequency is varied. be.

[Means for solving technical problems]

In the surface acoustic wave device of the present invention, a plurality of surface acoustic wave resonators each including a pair of reflectors formed at a predetermined distance and an interdigital transducer formed between the reflectors are mounted on a piezoelectric substrate. configured above.

The plurality of surface acoustic wave resonators are arranged close to each other in the cross-width direction so that the higher-order waveguide mode and the fundamental waveguide mode in the cross-electrode width direction of the interdigital transducer are made into a double mode. They are installed in parallel.

[Effect]

With the above configuration, the transverse mode higher-order waveguide mode in the cross width direction and the fundamental waveguide mode are made into a double mode by elastic coupling. Therefore, by adjusting the electrode crossing width and the distance between the plurality of surface acoustic wave resonators, it is possible to realize a phase characteristic that is substantially linear with respect to frequency. Therefore, for example ■
When used as a C○ surface acoustic wave device, changes in phase noise characteristics with respect to frequency changes are reduced.

[Description of Embodiment] FIG. 1 is a schematic plan view for explaining an embodiment of the present invention.

In the surface acoustic wave device 11 of this embodiment, 2&ll surface acoustic wave resonators 13 and 14 are formed on the piezoelectric substrate 12.

The first surface acoustic wave resonator 13 is inserted between a pair of reflectors 15 and 16 arranged at a predetermined distance (
It has a configuration in which a 1 m long interdigital transducer 17 made of toothed electric paddles is arranged.

Similarly, the second surface acoustic wave resonator 14 also has a tI formed between the pair of reflectors 19 and 20 and the reflectors 19 and 20.
It has a JI interdigital transducer 21.

The feature of this embodiment is that the first and second surface acoustic wave resonators 1
3.14 for each interdigital transducer 17.
21 electrodes are arranged adjacently in the cross width direction. This first. The distance W2 between the second surface acoustic wave resonators 13 and 14 is such that the higher-order waveguide mode and the fundamental waveguide mode in the electrode crossing width direction of both surface acoustic wave resonators 13 and 14 are made into a double mode. In other words, both surface acoustic wave resonators 13
14 are chosen so that they are laterally elastically connected.

In the transversely coupled dual mode surface acoustic wave device of this embodiment, as shown in FIG.
Interdigital transducer in 17.21
By Iflff the electrode crossing width W1 and the distance Mwt between the resonators 13 and 14, the fundamental waveguide mode f1 and the higher-order waveguide mode f, whose intensity distributions are schematically shown on the right side of FIG.
It is possible to change the coupling state with the frequency, thereby making the phase characteristic almost linear with respect to frequency.

This improvement in phase characteristics will be explained in more detail.

A crystal substrate is used as the piezoelectric substrate 12, the logarithm of the interdigital transducer 17°21 provided in the center is set to 250, and the reflectors 15, 16, 19, and 20 on both sides are each constituted by an array of 250 metal strips. ,
Surface acoustic wave resonators 13 and 14 having an electrode crossing width of about 5 wavelengths were formed. The first and second surface acoustic wave resonators were arranged approximately one wavelength apart, that is, Wt "1 wavelength", and were elastically coupled to each other, thereby constructing a laterally coupled dual mode surface acoustic wave filter.

The frequency characteristics of this surface acoustic wave device were as shown in FIG. As is clear from FIG. 6, it can be seen that the phase characteristics change almost linearly and the insertion loss is very small at about 2 dB in the pass band width.

For comparison, FIG. 7 shows the attenuation and phase frequency characteristics of the general two-point surface acoustic wave device shown in FIG. The body substrate 5 is composed of a crystal substrate, the interdigital transducers 8.9 each have 50 pairs of cos-' weighting, and the reflectors 6.7 each consist of 350 metal strip arrays. A surface acoustic wave resonator is constructed. As is clear from Figure 7,
In this two-bottom surface acoustic wave resonator, the phase characteristics at points A and C are gentler than at point B, and this can be expressed as ■C
When used at point A, its phase noise characteristics are -9
9dBc/H2, -120dBc/H2 at point B
Hz and at point C, it was -102 dBc/Hz. That is, it can be seen that the phase noise characteristics are good at points where the phase characteristics with respect to frequency become steep, but conversely, the phase noise characteristics deteriorate at points A and C where the phase characteristics are gradual.

On the other hand, when the dual-mode surface acoustic wave device of the above-mentioned embodiment was used for CO, the phase noise characteristic was -110d B c / over the entire frequency variable range.
Hz or less, and therefore approximately 11 dB compared to the conventional example above.
I found that it only improved.

〔Effect of the invention〕

As described above, in the present invention, the higher-order waveguide mode and the fundamental waveguide mode are combined to form a double mode, so -
It is possible to effectively improve the phase noise characteristics compared to a two-bottom surface acoustic wave resonator used on a boat. Furthermore, compared to conventional surface acoustic wave delay lines, the loss is much lower, there is no need to increase the potential of the oscillation circuit, and therefore there is no increase in power consumption, and the frequency variable range can be expanded. It is also easy to make no adjustments.

[Brief explanation of the drawing]

FIG. 1 is a plan view schematically showing an embodiment of the present invention, and FIG.
The figure shows a Colpitts type using a one-bottom surface acoustic wave resonator■
Figure 3 is a circuit diagram of a substrate-type CO using a surface wave delay line, Figure 4 is a schematic plan view of a two-point surface acoustic wave resonator, and Figure 5 is a diagram showing an example of a CO circuit. FIG. 6 is a diagram showing the relationship between the frequency difference between the fundamental waveguide mode and the secondary waveguide mode, the crossing width of the combined surface acoustic wave resonators, and the distance between the elastic resonators in an embodiment of the invention. FIG. 7 is a diagram showing attenuation and phase-frequency characteristics of an embodiment of the present invention, and FIG. 7 is a diagram showing attenuation-frequency characteristics and phase-frequency characteristics of a two-bottom surface acoustic wave resonator. In the figure, 11 is a surface acoustic wave device, 12 is a piezoelectric substrate, 13 is a first surface acoustic wave resonator, 14 is a second surface acoustic wave resonator, 15, 16, 19°, 20 is a reflector, 17 ．．
21 indicates an interdigital transducer. Figure 4 Figure 5 Electric power 9 + 14W + Yazuhai provisional) 1'Jt's distance noodles Vb Ototsu right A' Momema color Figure 6 n days tan number (MHX)

Claims (1)

[Claims] A plurality of surface acoustic wave resonators are configured on the piezoelectric substrate, and include a pair of reflectors formed at a predetermined distance apart, and an interdigital transducer formed between the reflectors. The plurality of surface acoustic wave resonators are arranged closely in parallel in the cross-width direction so that a higher-order waveguide mode and a fundamental waveguide mode in the cross-electrode width direction of the transducer are combined to form a double mode. A surface acoustic wave device for a voltage controlled oscillator circuit, characterized in that: