JPS6232642B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a surface acoustic wave device. [Technical background of the invention and its problems] The video intermediate frequency filter of color television receivers currently in practical use is composed of an LC filter. Therefore, when manufacturing color television receivers, highly skilled engineers adjust the filter characteristics of the LC filter circuit for each receiver. This adjustment by technicians takes a considerable amount of time,
This was a problematic process in the mass production process. In consideration of this point, research has been conducted on configuring an image intermediate frequency filter (abbreviated as a PIF filter) using a surface acoustic wave device. When a PIF filter is constructed using a surface acoustic wave device, there is no need for adjustment, which not only eliminates the need for an engineer for adjustment, but is also extremely convenient for the mass production process. As is well known, this surface acoustic wave device uses a piezoelectric substrate as a surface wave propagation medium.
Interdigital input and output transducers are provided on this board. In this case, a substrate with good efficiency in converting an electric signal into a surface wave and a high efficiency in converting a surface wave into an electric signal is put into practical use. When connecting such a surface acoustic wave filter to an external circuit, due to the nature of the transducer (surface waves are generated on both sides of the transducer), if you try to reduce the conversion loss, the reflected waves at the transducer will increase ( For example, April 1975 IEICE lecture number 79). This reflected wave travels back and forth between the input and output transducers and becomes a spurious signal. This spurious signal is typically a Triple Transit Echo
(TTE), which causes ripples in the passband. This TTE level is approximately given by the following formula. TTE=-10log(1+R _I /R _g ) ² (1+R _O /R _l
) ² ... (1) In this case, R _I : Reciprocal of the input conductance of the input transducer R ₀ : Reciprocal of the input conductance of the output transducer R _g : Reciprocal of the conductance of the circuit coupled to the input transducer R _I : Reciprocal of the conductance of the circuit coupled to the output transducer Therefore, in order to lower this TTE level and reduce ripple, R _I > R _g or R _O > R _l
Therefore, it becomes necessary to increase the conversion loss. However, there is a problem in that increasing conversion loss increases voltage loss and deteriorates the signal-to-noise ratio. [Object of the invention] The present invention has been made in view of the above points, and
It is an object of the present invention to provide a surface acoustic wave device that can suppress the TTE level to reduce ripples in the passband and reduce deterioration of the signal-to-noise ratio. [Summary of the Invention] The present invention provides a surface acoustic wave device in which interdigital input and output transducers are provided on a piezoelectric substrate, in which the conversion loss in the input transducer is greater than the conversion loss in the output transducer. The impedances of the input and output transducers are set in relation to the circuit loads connected to these input and output transducers. [Effects of the Invention] According to the present invention, the TTE level can be suppressed and the voltage transmission ratio can be increased, so that deterioration of the signal-to-noise ratio can also be reduced. [Examples of the Invention] Hereinafter, the present invention will be explained in detail with reference to Examples.
First, the input and output transducers will be explained. Let R _I and R _O be the reciprocals of the input and output conductances of the filter, and let L be the power transfer ratio of the filter, then when matching the output side (R _O = R _l ), the following relationship exists between the input voltage Vin and the output voltage Vout. A relationship is established. This relational expression can be expressed as a general expression that does not consider output side matching: becomes. Furthermore, this equation (3) is It can be expressed as This equation (4) can be obtained as follows. That is, considering the conversion loss of the input and output transducers, first, the maximum power (available power) P _g that can be supplied from the power source E _g in the input transducer is P _g =E _g ² /4R _g ...(5 ). Also, the surface wave power P _a radiated to one side of the input transducer is P _a =Vin ² /R _I ×1/2=1/2R _I {R _I
/R _g +R _I E _g } ² (6). Therefore, the conversion loss η _io = P _a /P _g of the input transducer is obtained from equations (5) and (6). becomes. Output transducer conversion loss Vout
Similarly becomes. Here, for L and _ηput , let Pin be the power supplied to the input transducer, and Pout be the power that can be taken out to the output load resistor. L=Pout/Pin=Pout/2P _a = 1/2η
_put ……(9) is the relation. Therefore, by substituting this equation (9) into equation (3), we get Therefore, equation (4) can be obtained. Now, in such a surface acoustic wave device, in order to increase the voltage transmission ratio Vout/Vin, it can be seen from equation (4) that R _O / _{R I} should be increased such that R _O >>R _I. However, the determination of R _I and R _O cannot be done arbitrarily, and depends largely on the required frequency characteristics of the filter. For example, in a video intermediate frequency filter for television, it is necessary to provide a deep trap on the high frequency side of the frequency amplitude characteristic in order to prevent interference from adjacent audio channels. For this purpose, it is necessary to provide not only the input side transducer but also the output side transducer with frequency characteristics to ensure the amount of trapping. That is, it is necessary to increase the logarithm of the output transducer. Generally, as the logarithm increases, the reciprocal of the transducer's radiation conductance, ie, R _O, tends to decrease, which means that there is an upper limit to R _O . On the other hand, the value of R _I is determined almost uniquely once the frequency characteristics of the output side transducer described above are determined, and it is difficult to set it arbitrarily. That is, R _I ,
The R _O ratio is almost determined by the desired frequency characteristics and cannot be set arbitrarily by the designer in order to improve the voltage transmission ratio. Therefore, R _I /R _O cannot be arbitrarily increased. On the other hand, it is clear from equation (4) that it is better to increase the voltage transmission ratio Vout/Vin by decreasing R _O /R _l . However, if R _O /R _I is made small, the TTE level will deteriorate from the above-mentioned equation (1). To compensate for this, from equation (1), R
This means _{that g} can be made smaller to increase the conversion loss in the input transducer.
However, R _g cannot be made infinitely small due to restrictions on the external circuit, and a value of about R _g R _I /50 is practical. Furthermore, since the practically permissible value of the TTE level is -40 dB or less, applying these conditions to equation (1) means that it is desirable to approximately set R _o =R _l . Further, when R _o =R _l , the output side of the surface acoustic wave device is in a matching state, the power conversion loss in the output transducer is minimized, and the signal-to-noise ratio is improved. For these reasons, it is desirable that R _o =R _l . From the above, in order to reduce the TTE level and increase the voltage transmission ratio, it is necessary to increase the conversion loss at the input transducer and set the conversion loss at the output transducer to be small. Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which an oxide piezoelectric material, that is, a ceramic material, for example, 0.07In (Li ₃ /
Interdigital _input and output transistors _{2 and} 3 are provided on one side of a 300 μ thick ceramic substrate (1) made of O ₃ -0.49PbTiO ₃ -0.44PbZrO ₃ (5 W 2 / 5 ). In this transducer, for example, the input transducer 2 has 13 logarithmic pairs, and the output transducer 3 has 3 logarithmic pairs. For example, the input transducer 2 is weighted to obtain PIF filter characteristics. This surface wave device has R _O /R _I =19 and power transmission loss of 15 dB. In this elastic surface PIF filter, the voltage transmission ratio when suppressing the TTE level to −40 db or less will be explained. When the conversion losses at the input and output transducers are each equal to 7.5 dB, the voltage transmission ratio is

[Formula] becomes. Also, when the conversion loss at the input trussducer is 3 dB and the conversion loss at the output terminal is 14 dB, the voltage transmission ratio is

[Formula] becomes. Furthermore, when the conversion loss at the output trussducer is 3dB and the conversion loss at the input transducer is 14dB, the voltage transmission ratio is

[Formula] becomes. Furthermore, assuming that the conversion loss at the input and output transducers is 3 dB,
The voltage transmission ratio when the propagation loss is 14dB is

[Formula] becomes. As is clear from the above, when the conversion loss at the input transducer is increased and the conversion loss at the output transducer is increased,
The voltage transmission can be maximized. FIG. 2 shows another embodiment of the present invention, in which the configuration of the transducer is such that an input transducer 5 is provided in the middle as shown in the figure, and outputs are provided on both sides of the surface propagation path of the input transducer 5. Transducers 6 and 7 are provided, and both transducers 6,
7 are connected in parallel. In the case of such a three-electrode configuration, a similar effect can be obtained by making the conversion loss in the input transducer larger than the conversion loss in the output transducer.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram for explaining one embodiment of the apparatus of the present invention, and FIG. 2 is an explanatory diagram of another embodiment of the present invention. 1... Board, 2, 5... Input transducer, 3, 6, 7... Output transducer.

Claims (1)

[Claims] 1. In a surface acoustic wave device in which interdigital input and output transducers are provided spaced apart from each other on a piezoelectric substrate, the conversion loss in the input transducer is greater than the conversion loss in the output transducer. A surface acoustic wave device characterized in that impedances of the input and output transducers are set in relation to circuit loads connected to the input and output transducers.