JPH0656941B2 - Ladder type piezoelectric filter with improved group delay characteristics - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a ladder-type piezoelectric filter having good group delay characteristics and low loss.

(Prior Art) In order to improve group delay characteristics of piezoelectric resonant components such as a piezoelectric filter and a ceramic discriminator used in an FM / PM demodulation circuit, conventionally, a low Qm piezoelectric material is used or an elastic rubber sheet is used. It was used for mechanical damping. If such a measure is taken, the linearity of the phase is improved, but as shown in FIG. 2, this is equivalent to increasing the equivalent resistance Re in the equivalent circuit of the piezoelectric resonator. In the case of the filter,
It has a drawback that the insertion loss increases and the S / N ratio deteriorates. This state is shown in FIGS. 3 and 4. That is, FIG. 3 shows an example in which the group delay characteristics are not improved, and FIG. 4 shows an example in which the insertion loss is increased although the group delay characteristics are improved. This tendency is seen in ladder type filters, so-called three-terminal type filters that use a single mode of area vibration and length vibration, and energy trapping dual mode filters.

In recent years, car telephones have become widespread, but a filter having good group delay characteristics and small insertion loss is required because it is an FM modulation system. Further, the present invention is not limited to this, and is not limited to the AM stereo reception filter and the FM / P filter.
Similar characteristics are required for the elastic discriminator used in the M demodulation circuit.

(Object of the Invention) An object of the present invention is to provide a ladder-type piezoelectric filter having a flat group delay characteristic, in other words a linear phase characteristic, and a small insertion loss.

That is, according to the present invention, in a ladder-type piezoelectric filter in which a series resonator and a parallel resonator are alternately connected, a first resistor is provided in parallel with the series resonator and a second resistor is provided in series with the parallel resonator. This is a ladder-type piezoelectric filter in which the bodies are connected together.

(Embodiment) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of the first embodiment, and the first embodiment is an application of the present invention to an n-0.5 stage ladder type filter. Series resonator X _S1, X _S2, ..., parallel resistance R _S1 each X _Sn, R _S2, ..., connected to R _Sn, parallel resonators X _P1, X _P2, ..., the X _{P (n-1)} Series resistors R _P1 , R _P2 , ..., R _{P (n-1)} are connected to each other. In the illustrated example, the resistors are always connected in series or in parallel to each resonator, but the present invention is not limited to this, and the resistors may be connected only to necessary portions. Further, the insertion positions of the series resistors R _P1 , R _P2 , ..., R _{P (n-1)} are parallel resonators X _P1 , X _P2 , ..., X in the illustrated example.
_Although it is the ground side of _{P (n-1)} , it may be inserted in the hot side.

With such a configuration, the series resonators X _S1 , X _S2 , ...
The impedance and phase change of X _{Sn at} the anti-resonance frequency fa change from the solid line state (before resistance connection) to the broken line state (after resistance connection) as shown in FIGS. 5 and 6, respectively. Similarly, the impedance and the phase change of the parallel coaxes X _P1 , X _P2 , ..., X _{P (n-1) at} the resonance frequency fr are as shown by solid lines (before the resistance is inserted) as shown in FIGS. 7 and 8, respectively. Changes to the broken line state (after inserting the resistor). That is, the impedance and phase change of the series resonators X _S1 , X _S2 , ..., X _{Sn at} the antiresonance frequency fa, and the resonance frequency fr of the parallel resonators X _P1 , X _P2 , ..., X _{P (n-1).} The impedance and phase changes in the state change from the steep state to the slow state. This means that the phase change becomes linear in the phase characteristic of the filter shown in FIG. 9. In other words, as shown in FIG. 10, the solid line state (resistive connection, before insertion) to the broken line state ( Resistance connection, after insertion), that is, the group delay time becomes constant. It should be noted that the resistor (first resistor) was connected in parallel to the series resonator and the resistor (second resistor) was inserted in series in the parallel resonator, so that the high frequency side and the low frequency side of the filter were Resonance is suppressed only in the vicinity of each attenuation pole, and Q is reduced, and Q does not change in the vicinity of the center frequency (f ₀ ), so that the group delay time characteristic can be improved without increasing the insertion loss of the filter. is there.

If the resistance value is out of the proper range, there is not much difference compared to before resistance connection and insertion, or conversely, it will be in a state of being convex upward as shown by a dashed line in FIG. Also, resistance connection,
Depending on the resistance value, depending on the resistance value, the adverse effects such as deterioration of the shape factor and reduction of the maximum attenuation may occur, so the resistance value is set on a case-by-case basis according to the user's request.

That is, in such a ladder type filter, generally, a combination of the resonator and the resistor is selected so as to satisfy the filter characteristics required by the user. This combination is determined by simulating various combinations of resonators and resistors, and selecting the one that most closely matches the required characteristics. For reference, FIGS. 12 to 14 show examples in which the present invention is applied to a conventional structure having the characteristics shown in FIG. In addition, this conventional structure has a practical construction of 56-31.
718 has a structure as shown in FIG. FIG. 12 shows parallel resistors (first resistors) R _S1 , R _S2 , ..., R _Sn = 150 kΩ,
Series resistance (second resistance) R _P1 , R _P2 , ..., R _{P (n-1)} = 60
Ω example, FIG. 13 shows parallel resistance (first resistance) R _S1 ,
R _S2 , ..., R _Sn = 50 kΩ, series resistance (second resistance)
An example of R _P1 , R _P2 , ..., R _{P (n-1)} = 200Ω, and FIG. 14 shows parallel resistances (first resistances) R _S1 , R _S2 , ..., R _Sn = 15 kΩ,
Series resistance (second resistance) R _P1 , R _P2 , ..., R _{P (n-1)} = 6
An example of 00Ω is shown.

11 to 14, a solid line indicates an amplitude characteristic, a broken line indicates an enlarged amplitude characteristic of a part thereof, and a dashed-dotted line indicates a group delay time. The amplitude characteristic indicated by the solid line shows the attenuation amount corresponding to the wide frequency scale in the figure, and the large attenuation amount scale in the figure shows the amplitude characteristic, and the amplitude characteristic indicated by the broken line shows a part of the amplitude characteristic shown by the solid line in enlarged form. The attenuation amount is shown by a narrow frequency scale and a small attenuation scale in the figure. The group delay time indicated by the one-dot chain line indicates the group delay time corresponding to the narrow frequency range in the figure.

In each of these embodiments, various structures are conceivable for embodying the resistance in the product, but some examples will be given below.

In the case of the energy trapping type, it is preferable to form a resistance film in the open space on the piezoelectric plate. Alternatively, the resistance film may be formed on the substrate to which the piezoelectric plate is attached. In the case of the non-energy confinement type, by connecting the opposing electrodes with a resistance film at the vibration node portion of the piezoelectric plate, for example, in the center of the side in the spreading vibration mode of the rectangular plate, the first resonator is connected in parallel with the series resonator. It can realize resistance. It is conceivable that the second resistor inserted in series with the parallel resonator is formed of a resistive vibrating electrode film of the piezoelectric plate. Alternatively, the terminal plate itself, which is pressed against the vibrating electrode film provided on the piezoelectric plate to connect the vibrating electrode film to the outside and holds the piezoelectric resonator, may be made of a resistive material. In the case of a ladder type filter, there are some which realize a ladder structure by forming a connection pattern on the inner surface or the outer surface of the base portion of the case. (Preferably) may be mounted on a pattern, or a resistive film may be configured together with the connection pattern. Also, as one structure of the ladder type filter, the chip resistor or the resistor is used for the holding plate that covers the case opening so that the piezoelectric resonator, terminal plate, insulating spacer, spring plate, etc. housed in the case do not protrude. You may comprise a pattern. A specific example of this will be described below. Figure 15 shows a 3.5-stage ladder type filter. The inside of the case of this filter is shown in FIG. 16 as a schematic plan view. It should be noted that in the illustrated example, internal parts are illustrated with a space therebetween for the sake of clarity, and some parts are not shown. In the figure, 10 is a case, 11 is various terminal plates, and 12 is an insulating plate. Series resonator X
_The terminal plates connecting _S1 and X _S2 , X _S2 and X _S3 , and X _S3 and X _S4 are integrally formed with the connecting portion 11a. The input pin P ₁₁ and the adjacent terminal board 11 are connected, and the output pin P 11
₂₁ and the adjacent terminal board 11 are connected. The holding plate 13 is provided with a resistance film or resistance components R _{S1 to} R _S4 , R _{P1 to} R _P3 and a conductive film or a terminal electrode film connecting these. Cover the opening of the case 10 with such a holding plate 13,
The terminal plates 11, 11 existing on both sides of the series resonator X _S1 are connected to both electrodes of the resistor (first resistor) R _S1 . Similarly, the terminal plates 11 and 11 existing at both ends of the series resonator X _S2 are connected to both electrodes of the resistor (first resistor) R _S2 , and the series resonator X _S3 is connected.
Terminal plates 11 and 11 present at both ends of the and resistance (first resistance) R
The electrodes of both ends of _S3 are connected, and the terminal plates 11, 11 present at both ends of the series resonator X _S4 are connected to the electrodes of both ends of the resistor R (first resistor) _S4 . Both ends of the conductive pattern connecting the respective one ends of the resistors R (second resistors) _{P1 to} R _P3 are connected to the input side ground pin P ₁₂ and the output side ground pin P ₂₂ , respectively. Further, the non-earth side electrode of the resistor R _P1 and the earth side terminal plate 11 of the parallel resonator X _P1 are connected. Similarly, the non-ground side electrode of the resistor R _P2 and the ground side terminal plate 11 of the parallel resonator X _P2 are connected, and the non-ground side electrode of the resistor R _P3 and the ground side terminal plate 11 of the parallel resonator X _P3 are connected. When connected, the wiring will be as shown in Fig. 15. Next, it is easy to connect the input pin P ₁₁ and the output pin P ₂₁ to the related terminal board, and to easily connect the input side ground pin P ₁₂ and the output side ground pin P ₂₂ to the conductive pattern of the holding plate. FIG. 18 shows a holding plate 14 that does not require a terminal plate having a connecting portion 11a.

That is, the conductive film 15 that replaces the connecting portion of the terminal plate 11 is provided and the conductive pattern that extends to the pins P ₁₁ , P ₁₂ , P ₂₁ , and P ₂₂ is a feature. It should be noted that although holes for inserting the pins P ₁₁ , P ₁₂ , P ₂₁ , P ₂₂ and the connecting portions of the terminal plates 11 are provided, the holes are not limited to the holes and may be slits or notches recessed from the sides. It may be. Further, the resistance may be formed not only on one surface of the plate but also on two surfaces. Further, it may be provided on a plurality of plates.

(Effects of the Invention) As is apparent from the above embodiments, according to the present invention, the group delay time characteristics can be improved without increasing the insertion loss by devising the connection of the electric resistance and the insertion position. It was Therefore, the S / N ratio is not deteriorated, and a low distortion receiver can be manufactured without increasing the gain or the number of stages of the amplifier. This kind of merits is particularly effective for objects that are subject to severe conditions, such as the receiver of a communication device.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of an embodiment of the present invention, FIG. 2 is an equivalent circuit diagram of a piezoelectric resonator, FIGS. 3 and 4 are amplitude characteristics and group delay characteristics of filters, and FIG. 5 is series resonance. Fig. 6 shows the same phase characteristic diagram as Fig. 6, Fig. 7 shows the impedance curve of a parallel resonator, Fig. 8 shows the same phase characteristic diagram, Fig. 9 shows the phase characteristic diagram of a filter, and Fig. 10 shows the same group. Delay characteristic diagram, FIG. 11 is a measured characteristic diagram of a conventional example, FIGS. 12 to 14 are measured characteristic diagrams of one embodiment of the present invention, FIG. 15 is an equivalent circuit diagram of another embodiment of the present invention, FIG. 16 is a plan view of the internal structure of the ladder type filter, and FIGS. 17 and 18 are plan views of the holding plate. X _S1 , X _S2 , ..., X _Sn are series resonators, X _P1 , X _P2 ,.
X _{P (n-1)} is parallel _{resonator, R S1, R S2, ...} , R Sn is first
, R _P1 , R _P2 , ..., R _{P (n-1)} are the second resistors.

Claims (5)

[Claims] 1. A ladder type piezoelectric filter in which a series resonator and a parallel resonator are alternately connected, a first resistor is connected in parallel with the series resonator, and a first resistor is connected in series with the parallel resonator. A ladder-type piezoelectric filter with improved group delay characteristics, characterized in that two resistors are connected. 2. The first resistor and the second resistor are
The ladder-type piezoelectric filter with improved group delay characteristics according to claim 1, wherein the ladder-type piezoelectric filter is provided on the case member. 3. The first resistor and the second resistor are
A ladder-type piezoelectric filter with improved group delay characteristics according to claim 1, wherein the ladder-type piezoelectric filter is provided on the internal component pressing member. 4. The first resistor and the second resistor are
The ladder-type piezoelectric filter with improved group delay characteristics according to claim 1, wherein the ladder-type piezoelectric filter is provided on the piezoelectric plate. 5. The first resistor and the second resistor are
The ladder-type piezoelectric filter with improved group delay characteristics according to claim 1, wherein the ladder-type piezoelectric filter is provided on a substrate on which the piezoelectric plate is mounted.