JPS632169B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention generally relates to mechanical filters, and more particularly to mechanical filters with good temperature characteristics. Figure 1 shows a conventional example of a mechanical filter, in which 1 and 2 are signal input terminals, 8 and 9 are signal output terminals,
3 and 3' are a pair of tuning transformers consisting of transformers 3a, 3a' and capacitors 3b, 3b' connected to the signal input terminal and signal output terminal, respectively; 5 and 5' are piezoelectric elements 5a, 5a' and constant elastic Alloy 5b,5
A pair of electrical-mechanical and mechanical-electrical converters (hereinafter referred to as input and output transducers) are connected to each tuning transformer, and 4 is a lead wire for connecting the transducer and the tuning transformer. , 6 is a resonator disposed between the transducers, and 7 is a coupler for coupling between each resonator and between the resonator and the transducer. When an electric signal is applied to the input terminals 1 and 2 with the above configuration, the signal is converted into an elastic vibration wave by the input transducer 5 via the tuning transformer 3, and propagated to the coupler 7 and the resonator 6. .
The elastic vibration waves propagated through each resonator are converted into electrical signals again by the output transducer 5', and then supplied to the output terminals 8 and 9 via the tuning transformer 3'. Here, the transducers 5, 5' and the resonator 6 perform torsional vibration, and the coupler 7 performs longitudinal vibration. The tuning transformers 3 and 3' absorb the constraint capacitance C _D of the piezoelectric elements 5a and 5a', and also change the signal source impedance R _S connected to the input terminal and the output terminal by selecting the winding ratio of the transformers 3a and 3a'. It has a function of matching the connected load impedance R _L with the impedance Z _L of the main body of the mechanical filter consisting of the transducers 5, 5', the resonator 6, and the coupler 7. Mechanical filters with the above configuration are currently in general use, but the impedance Z _L of the mechanical filter body has a large temperature dependence, so it is necessary to change the turns ratio of the tuning transformer in response to changes in impedance Z _L. However, this is not possible and has the disadvantage of ultimately deteriorating the temperature characteristics of the mechanical filter. Therefore, the present invention aims to improve the above-mentioned drawbacks of the prior art, and its object is to provide a mechanical filter with good temperature characteristics. A feature of the present invention to achieve this object is to use a gyrator composed of passive elements as a matching circuit instead of a tuning transformer, and to create a relationship between the temperature coefficient of the gyrulator's passive element and the temperature characteristic of the piezoelectric element of the transducer. This is a mechanical filter that is constructed by correlating the Examples will be described below with reference to the drawings. FIG. 2 shows a first embodiment of a mechanical filter according to the present invention, and the same reference numerals as in FIG. 1 indicate the same parts. The gyrator 10 inserted between the input terminals 1 and 2 and the input transducer 5 is a π-type connection in which an inductance L ₁ is inserted in a parallel connection of two capacitances C ₁ and C ₂ as shown in the figure.
The gyrator 11 inserted between the output terminals 8 and 9 and the output transducer 5' is similarly constructed of capacitances _C3 and _C4 and inductance _L2 . Here, assuming that the input terminals 1 and 2 are terminated with the signal source impedance R _S , the impedance Z ₁ looking into the signal source from the lead wire 4 through the gyrator 10 is, if the center frequency of the filter is ω ₀ , however

[Formula] ω ² ₀ L ₁ C ₁ = 1, C ₂ = C ₁ − C _D. Similarly, assuming that the output terminals 8 and 9 are terminated with the load impedance R _L , the impedance Z ₂ considering the load from the lead wire 4 through the gyrator 11 is: however

[Formula] ω ² ₀ L ₂ C ₃ = 1, C ₄ = C ₃ − C _D. From (1), (2) and the relationship Z ₁ = Z ₂ = Z _L , R ₁
and R ₂ , R ₁ =√ _S・_L , R ₂ =√ _L・_L (3). As is clear from equations (1) and (2), R ₁ and R ₂ are L ₁ ,
Since it is the ratio of L ₂ to C ₁ and C ₃ , if the respective temperature characteristics are selected appropriately, the temperature characteristics of Z ₁ and Z ₂ can be made to perfectly match the temperature characteristics of the impedance Z _L of the mechanical filter body. It is possible. The temperature characteristics of Z _L are determined by the temperature characteristics of the piezoelectric elements 5a and 5a', and can be expressed by the following equation. -1/Z _L・ΔZ _L /ΔT=1/C _D ΔC _D /ΔT+2/k
Δk/ΔT+2/v Δv/ΔT (4) where k is the electromechanical coupling coefficient of the piezoelectric element, v is the sound speed of the elastic vibration wave propagating in the piezoelectric element, and ΔT is the minute temperature fluctuation.

[Table] Table 1 shows the temperature characteristics of piezoelectric ceramics (materials A, B, and C) that are generally commercially available as piezoelectric elements. However, k is k ₁₅ and v is the sound speed of the thickness shear wave. As is clear from Table 1, the value of 1/C _D ΔC _D /ΔT is an order of magnitude larger than the others, so the temperature characteristics of Z _L are almost determined by the temperature characteristics of C _D. From formula (1), 1/Z ₁ ΔZ ₁ /ΔT=1/Z _L ΔZ _L /ΔT=(1/L ₁ ΔL ₁
/ΔT-1/C ₁ ΔC ₁ /ΔT) (5) Also, since ω ² ₀ L ₁ C ₁ = 1, 1/L ₁ ΔL ₁ /ΔT=-1/C ₁ ΔC
₁ /ΔT, so from equations (4) and (5), 1/C ₁ ΔC ₁ /ΔT=1/2C _D ΔC _D /ΔT (6) Similarly for the load side, 1/L ₂ ΔL ₂ / ΔT=-1/C ₃ ΔC ₃ /ΔT 1/C ₃ ΔC ₃ /ΔT=1/2C _D ΔC _D /ΔT (7) However, C ₁ , C ₃ ≫C _D. If the temperature characteristics of the capacitances C ₁ to _{C 4} are selected to be 1/2 of the temperature characteristics of C _D from equations (6) and (7), matching conditions corresponding to changes in Z _L can be obtained. At this time, the temperature characteristics of the inductances L ₁ and L ₂ must be selected to have the opposite signs of the temperature characteristics of C ₁ to _{C 4} . Figure 3a shows a second specific example of the matching circuit, and Figure 3b shows the parallel resonant circuit consisting of _L3 and _C7 in Figure 3a, which is equivalent to an inductance _L0 at the center frequency _ω0 of the filter. It has been replaced.
Therefore, L ₀ is From equation (8) However, ω _l1 ² =1/L ₃ C ₇ and ΔL ₃ /ΔT=0. FIG. 4a shows a third specific example of the matching circuit, in which the inductance L ₀ is equivalently replaced as in FIG. 4b. Similar to above, From equation (9) However, ω ² _l2 =1/L ₄ ·C ₁₀ and ΔL ₄ /ΔT=0. Among the embodiments explained above, the first embodiment is C _D
It is used when the temperature characteristics of L ₁ and L ₂ can be selected because the temperature characteristics of L 1 and L 2 are relatively small, and the second and third embodiments are equivalent when the temperature characteristics of L ₁ and L ₂ cannot be selected. It is used by controlling the temperature characteristics of L ₀ at C ₇ and C ₁₀ . Table 2 shows the constants of the passive elements constituting each of the matching circuits described in Figs. 2, 3, and 4.
R _S = R _L = 600Ω, Z _L = 15KΩ, C _D = 65pF, f ₀ =
129, 85KHz, f _l1 = 170KHz, f _l2 = 80KHz. Note that α _L = 1/L ΔL/ΔT (ppm/°C), and α _C = 1/C ΔC/ΔT (ppm/°C).

[Table] Figure 5 is an experimental example of a mechanical filter according to the present invention, using the circuit shown in Figure ₃ as a matching circuit.
C ₆ is composed of a piezoelectric element C material (Table 1), C ₇ is a magnetic capacitor, and the mechanical filter body consists of two transducers and six resonators as shown in Figures 1 and 2. Insertion loss was measured using the 16th order specification. In addition, the temperature range is 20
~60℃. In the figure, curve a is a conventional mechanical filter, and curve b is a mechanical filter according to the present invention.As is clear from the comparison between the two, it can be seen that the temperature characteristics of the filter can be significantly improved according to the present invention. In addition,
If the difference in temperature fluctuation between C ₂ , C ₄ , C ₆ , C ₉ and C _D becomes a problem, insert a magnetic capacitor with negative temperature characteristics in parallel with C ₂ , C ₄ , C ₆ , C ₉ . or use a different material. FIG. 6a shows a fourth specific example of the matching circuit, and FIGS. 6b and 6c equivalently show the temperature characteristics of the inductance L ₅ with the capacitances C ₁₃ and C ₁₈ , respectively. This is as explained in connection with FIG. As explained above, according to the present invention, since there is a correlation between the temperature characteristics of the passive elements constituting the matching circuit and the temperature characteristics of the piezoelectric element of the transducer, a mechanical filter with good temperature characteristics can be obtained. can be provided.

[Brief explanation of the drawing]

FIG. 1 shows a conventional example of a mechanical filter, and FIG. 2 shows an example of a mechanical filter according to the present invention.
3a and 3b show a second specific example of a matching circuit, FIGS. 4a and 4b show a third specific example of a matching circuit, and FIG. 5 shows an experiment of a mechanical filter according to the present invention. Examples FIG. 6a, FIG. 6b and FIG. 6c are a fourth specific example of a matching circuit. 1, 2: Signal input terminal, 3, 3': Tuning transformer, 4: Outgoing wire, 5, 5': Transducer, 6: Resonator, 7: Coupler, 10, 11: Gyretor.

Claims (1)

[Claims] 1. A pair of input/output transducers having piezoelectric elements, a plurality of resonators coupled and arranged between the transducers via connectors, and two matching circuits coupled to each of the above transducers. In the mechanical filter having the following, the matching circuit is a gyrator composed of passive elements having capacitance and inductance, and the temperature characteristics of each passive element of the gyrator are correlated with the temperature characteristics of the piezoelectric element of the transducer. Characteristic mechanical filter.