JPS6141451B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dispersive delay line essential to chirp radar systems, and particularly to a reflective grating type surface acoustic wave dispersive delay line that utilizes surface acoustic waves.

In general, to extend the detection distance of a radar, it is necessary to increase the energy of the transmitted pulse, that is, the product of peak voltage and pulse width, but the former is limited by the withstand voltage of the transmitting tube, while increasing the latter increases the distance resolution. descend. There is a chirp radar system as a method to solve this dilemma, and its principle is that when a frequency-modulated transmission pulse (pulse width △T, frequency band △) is reflected from a target object and guided to a receiving circuit, The pulse is compressed into a short pulse using a distributed delay line with delay characteristics opposite to that of the pulse.

As a distributed delay line that has been used in practice, there is a reflection grating type that uses surface acoustic waves. It consists of input and output transducers consisting of interdigital electrodes and a pair of grating-type reflectors, which in turn are composed of a plurality of reflection gratings. A reflector is provided adjacent to the input transducer in the direction of propagation of the surface waves, and the output transducer and the second reflector are arranged adjacent to the input transducer and the first reflector in the direction of propagation of the surface waves.
It is axially symmetrical with the reflector.

The surface wave emitted from the input transducer propagates in the first reflector, but is strongly reflected in the direction perpendicular to the propagation direction at the point where the wavelength of the surface wave matches the spacing of the reflection grating. It is again reflected perpendicularly by the second reflection grating and reaches the output transducer. In this case, the spacing between each reflection grating is based on the assumption that each grating reflects only surface waves with a wavelength approximately equal to the spacing between the front and rear gratings.When used in a linear radar radar, the delay time changes with respect to frequency changes. is a straight line, and when used in a nonlinear radar radar, the frequency characteristics of each delay time, which becomes the amplitude, are determined to satisfy certain conditions, usually conditions for a matched filter.
However, in reality, since the change between each grating is extremely gradual, the number of reflection gratings that contribute to the reflection of a surface wave having a certain wavelength is not just one but a plurality. In this case, the number of gratings that contribute to the reflection of surface waves near the upper or lower frequency limits within the band is higher than the number of gratings that contribute to the reflection of surface waves with other frequencies within the band. decreases by about half as it approaches the upper or lower limit of the range. This is because in the former case, there are approximately the same number of gratings contributing to reflection on both sides of the grating where the wavelength and grating spacing are almost the same, but in the latter case, at the end of the grating, the gratings contributing to reflection are only on one side. be.

Therefore, even if the reflection coefficients of the reflection gratings near both ends are made larger than the reflection coefficients of the reflection gratings near the center, it is extremely difficult to provide a certain target amplitude characteristic, for example, a humming characteristic. Moreover, for surface waves having frequencies at both ends of the band, gratings that contribute to reflection are always present at both ends of the reflection grating, so the center of reflection will be slightly inward from both ends of the reflection grating. Therefore, the difference in delay time between the surface wave at the upper limit frequency and the surface wave at the lower limit frequency within the imperial region becomes shorter than the target characteristic, and the frequency characteristic of the delay time also deteriorates.

Therefore, the position of each reflection grating can be determined by assuming that only surface waves with a certain frequency are reflected by one grating, and the frequency characteristics of the amplitude and delay time can be adjusted to the target characteristics. On the other hand, it would be extremely useful if a reflection grating could be obtained that does not deteriorate.

An object of the present invention is to provide a surface acoustic wave dispersion type delay line with excellent delay time frequency characteristics.

The delay line of the present invention is composed of an input and output transducer and a plurality of reflectors, and the reflector is provided adjacent to the input transducer in the propagation direction of the surface wave. The output transducer and the second reflector are axially symmetrical with respect to the input transducer and the first reflector with respect to the direction of propagation of the surface waves. The reflector consists of a plurality of reflection gratings, and dummy reflection gratings are provided at both ends of these reflection gratings, and the intervals between the dummy reflection gratings are constant.

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a plan view showing an embodiment of a surface acoustic wave dispersive delay line according to the present invention. 1 is a piezoelectric substrate made of lithium niobate (LiNbO ₃ ) and crystal (SiO ₂ )
or BGO (Bi ₁₂ GeO ₂₀ ). Each element such as an input converter is formed on this substrate. Reference numeral 2 denotes an input transducer for converting an electric signal into a surface acoustic wave signal, and numeral 3 an output converter for converting the surface acoustic wave signal into an electric signal, both of which are composed of interdigital electrodes.

4 to 10 are first reflection gratings (including dummy reflection gratings), and the surface waves radiated from the input converter 2 are as follows:
These reflection gratings reflect the surface waves in a direction perpendicular to the direction of propagation. 11 to 17 are also reflection gratings similar to 4 to 10, but are axially symmetrical with respect to the virtual axes O ₁ to O ₂ . These gratings consist of groove-shaped reflectors with grooves cut into a piezoelectric substrate, or metal reflectors made of metal strips such as Al, and in order to control the reflection coefficient and make the amplitude frequency characteristics meet a certain target characteristic. This is achieved by changing the depth of the groove in the former case and the width of the metal strip in the latter case.

Among reflection gratings 4 to 10, those from 4 to 6 are 1
Under the assumption that the book grating reflects only surface waves having a wavelength equal to the interval between the gratings before and after it, the position of each grating is determined so that the frequency characteristic of the delay time becomes the target characteristic. However, with this reflection grating alone, the frequency characteristics of amplitude and delay time deteriorate near both ends of the frequency band, so a dummy reflection grating consisting of 7, 8 and 9, 10 was provided to improve this. It is. The lattice spacing of this dummy electrode is constant.

The spacing d ₁ between the gratings 7 and 8, and the spacing d ₂ between the gratings 9 and 10 are the center frequency o, the frequency band Δ, the propagation speed of the surface wave V, and assuming a down-chirp, each is given by In the case of an up-down, the right-hand side of equation (1) is d ₂ and the right-hand side of equation (2) is d ₁ .

In general, the number of reflection gratings Neff that contributes to reflection near the center frequency is given by the following equation, where the delay time is △T: is given by For example, o is 30MH, △
By substituting 2MHz for ΔT and 24 μs for ΔT into equation (3), Neff becomes approximately 73 lines. At this time, approximately 10 to 20 dummy reflection gratings are appropriate.

In this way, when a surface wave propagates through a reflector with a dummy reflection grating, the number of gratings contributing to the reflection of the surface wave at the upper and lower frequencies o+Δ/2 and o−Δ/2 within the band is equal to the number of gratings of the dummy reflection grating. The frequency characteristics of the amplitude and delay time are therefore significantly improved compared to the case without the delay time.

FIG. 2 is a diagram showing amplitude frequency characteristics, and 11 is a target characteristic. Usually this is designed for linear radar radar with a humming characteristic. 12 is the characteristic of a conventional delay line, where the number of cycles is o+
There is a large deviation from the target characteristics near Δ/2 and o−Δ/2. This tendency is o+△/2, o-△/2
No matter how much you increase the reflection coefficient of the grating, which mainly contributes to the reflection of surface waves with a frequency of , it hardly changes. However, as shown in Figure 1, 7, 8, 9, 10, 1
In the delay line according to the present invention provided with dummy reflection gratings such as 4, 15, 16, and 17, the o
The magnitude of the amplitude at +Δ/2 and o−Δ/2 approaches the target characteristic.

Figure 3 is a diagram showing the frequency characteristics of delay time.
4 is the target characteristic. This figure shows one for a linear chirp radar, and the function of delay time versus frequency is a straight line.

15 is the characteristic of the conventional delay line, and 16 is the characteristic of the delay line according to the present invention. As in the case of amplitude, it can be seen that the characteristics are improved by the dummy reflection grating.

In the embodiment shown in FIG. 1, the number of gratings constituting the dummy reflection grating is two, but it goes without saying that two or more gratings may be provided to better match the target characteristics.

Furthermore, in Figures 2 and 3, we talked about the distributed delay line for linear/cheap radar, but this dummy reflection grating is also used for the distributed delay line for nonlinear/cheap radar that satisfies the conditions for mated filters. Needless to say, it has a similar effect.

As explained above, according to the present invention, good frequency characteristics, frequency-delay time relationships, etc. can be obtained.

Although the embodiments of the present invention have been described above, the patent rights of the present invention extend to all surface acoustic wave dispersion type wires described in the claims.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the surface acoustic wave dispersion type delay line according to the present invention. 1... piezoelectric substrate, 2... input transducer, 3
...Output converter, 4 to 17...Reflection grating (7 to 1 of them
0, 14 to 17 indicate dummy reflection gratings). FIG. 2 is a diagram showing the frequency characteristics of the amplitude of the surface acoustic wave dispersion type delay line. 11...Target characteristics, 12...Characteristics of the conventional delay line, 13...Characteristics of the delay line of the present invention provided with a dummy reflection grating. shows. FIG. 3 is a diagram showing the frequency characteristics of the delay time of the surface acoustic wave dispersion delay line, 14...target characteristics, 15...
16 shows the characteristics of a conventional delay line before providing a dummy reflection grating, and 16 shows the characteristics of the delay line of the present invention provided with a dummy reflection grating.

Claims (1)

[Claims] 1. In a surface acoustic wave dispersion type delay line equipped with an input transducer, an output transducer, and a reflector consisting of a plurality of reflection gratings, dummy reflection gratings with equal grating intervals are provided on both sides of the reflection grating. A surface acoustic wave dispersive delay line featuring