JPH0351329B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a filter bank consisting of a plurality of surface acoustic wave filters 1 having different center frequencies.

[Prior art]

For example, as shown in FIG. 1, a conventional filter bank of this kind utilizes the capacitance of the interdigital electrode 2, which is a microwave/surface acoustic wave converter, to
It was constructed by connecting a plurality of surface acoustic wave filters 1 in a constant K shape, with the input admittance of the input-side interdigital electrode 2 of the surface acoustic wave filter 1 as a parallel element and the coil 3 as a series element. . 4 is a terminating resistor.

By the way, when the filter characteristics of each surface acoustic wave filter 1 are defined, the value of the capacitance of the input side interdigital electrode 2 of each surface acoustic wave filter 1 is determined. Also, in a constant K type circuit as shown in Figure 1,
As is well known, in order to obtain the required cutoff frequency and nominal impedance, the value of the inductance of the coil 3 is determined from the value of the capacitance. Therefore, in a conventional filter bank of this type, when the filter characteristics are specified, the inductance of the coil 3 becomes extremely small.
There have been cases where it has gotten bigger. In such a case, there are disadvantages such as it is difficult to manufacture the coil 3 having the required inductance value with good workability and accuracy, and the space occupied by the coil 3 becomes too large. Ta. That is, in the conventional filter bank of this type, each parallel element was composed only of the input admittance of the input-side interdigital electrode 2 of the surface acoustic wave filter 1, which made the coil 3 constituting the series element difficult to manufacture. However, there were some drawbacks such as difficulty in downsizing.

[Summary of the invention]

In this invention, the input admittance of the input-side interdigital electrode 2 of the surface acoustic wave filter 1 is made into a parallel element, the coil 3 is made into a series element, and a plurality of surface acoustic wave filters 1 are connected in a constant K shape. In this filter bank, the above-mentioned parallel element is constructed by combining the input admittance of the surface acoustic wave filter 1 and the capacitor 5, thereby eliminating the conventional drawbacks, and will be explained in detail below using examples. do.

[Embodiments of the invention]

FIG. 2 shows a first embodiment of a filter bank according to the present invention.

In the embodiment shown in FIG. 2, the parallel elements of the constant K type circuit are constructed by connecting the input admittance of the input side interdigital electrode 2 of the surface acoustic wave filter 1 in parallel with the capacitor 5, and The element is constituted by a coil 3.

Now, even in the filter bank shown in FIG. 2, when the filter characteristics of each surface acoustic wave filter 1 are defined, the capacitance of the input side interdigital electrode 2 of each surface acoustic wave filter 1 is determined. It is similar to

However, unlike the conventional filter bank shown in FIG. 2, each parallel element is configured by connecting the input admittance of the input-side interdigital electrode 2 of the surface acoustic wave filter 1 and the capacitor 5 in parallel. , the capacitance value of each parallel element can be controlled by changing the capacitance value of the capacitor 5. Therefore, the value of the inductance of the coil 3 required to obtain the required cutoff frequency and nominal impedance can be changed by changing the value of the capacitance of the capacitor 5.

Regarding this, Figures 5, 6, 7, and 8
This will be explained in detail with reference to FIG.

FIG. 5 shows the configuration of one stage of constant K type circuit. In Figure 5, let √1( _{0 0} ) = f ₀ ... (1) √ (2 ₀ ) ₀ = R ... (2), and furthermore, let f be the frequency, f/f ₀ = x ... (3) Then, the impedance Z looking from terminal AA to the load side is Z=R[1-x ² +j√2x(1-x ² /2)]/[1-x ² +j√2x]... …(4) becomes.

Here, j=√−1, f ₀ is the cutoff frequency R, and the nominal impedance. The constant K type circuit in Figure 5 is
Operates as a low-pass filter.

Note that if x≪1, equation (4) becomes Z=R (5). This indicates that in a frequency band sufficiently lower than the cutoff frequency f ₀ , the constant K-shaped circuit operates as a mere transmission line.

On the other hand, the input admittance Yi of the input-side meandering electrode 2 of the surface acoustic wave filter 1 can generally be expressed by a radiation conductance Gi and a capacitance Ci connected in parallel, as shown in FIG.

Note that the radiation conductance Gi has a frequency characteristic, has a finite value other than zero only within the passband of the surface acoustic wave filter 1, and becomes approximately zero outside the passband.

Next, the differences between the conventional filter bank shown in FIG. 1 and the first embodiment of the filter bank according to the present invention shown in FIG. 2 will be explained.

The conventional filter bank shown in FIG.
The shaped circuit has a multi-stage configuration, and in each stage, the input admittance Yi of the interdigital electrode 2 on the input side of the elastic surface liquid filter 1 is used instead of the parallel element C ₀ .
was inserted. Here, Gi≪2πfCi. Assume that a frequency in the passband of the n-th stage surface acoustic wave filter 1 is supplied from the power supply. In the parallel elements other than the n-th stage, Gi=0, so the constant K-type circuits other than the n-th stage function simply as transmission lines. Therefore, the equivalent circuit can be expressed as shown in FIG. In Fig. 7, if the power supply voltage is expressed by V _s and Gi≪2πfCi, then the surface acoustic wave filter 1
The radiation conductance of the interdigital electrode 2 on the input side of
The voltage applied to Gi is approximately V _s /2.

On the other hand, each surface acoustic wave filter 1 is connected to a voltage
The equivalent circuit when connected to a _Vs power supply and operated is shown in Figure 8. In Figure 8,
If Gi≪2πfCi, the radiation conductance Gi of the interdigital electrode 2 on the input side of the surface acoustic wave filter 1
The voltage applied to is approximately _Vs.

Therefore, the insertion loss when each surface acoustic wave filter 1 is operated in FIG. 7 is about 6 dB compared to the insertion loss when it is operated in FIG. 8.
This will result in an increase in losses. This amount of loss increase is the same in all stages of the constant K type circuit. In other words, the number of channels in the filter bank (the number of stages in the constant K type circuit)
Even if the amount increases, the amount of increase in loss remains constant at approximately 6 dB.

By the way, when the characteristics of the surface acoustic wave filter 1 are defined, the capacitance Ci of the interdigital electrode 2 on the input side is determined as described above. Therefore, in order to set the nominal impedance R to 50Ω in the conventional filter bank, the value of L, which is the series element of the constant K-type circuit, is uniquely determined from equation (2). Furthermore, once the values of L and C are determined, the cutoff frequency f ₀ is uniquely determined from equation (1).

Let us consider the case where the value of the capacitance Ci of the interdigital interdigital electrode 2 on the input side is small. In order to set the nominal impedance R to 50Ω, the value of L also becomes smaller from equation (2). Since both the value of Ci and the value of L are small,
From equation (1), the cutoff frequency f ₀ becomes extremely high.

By the way, in the surface acoustic wave filter 1, unnecessary passbands due to bulk waves and harmonics generally occur in a frequency band higher than the passband, although there are differences in level. In such a case, in a conventional filter bank, the cutoff frequency f ₀ of the constant K-shaped circuit
Since the constant K-type circuit also passes frequencies in unnecessary passbands due to bulk waves and harmonics.
Therefore, the frequencies in this unnecessary passband are output as they are to the output terminal of the surface acoustic wave filter 1.

On the other hand, in the first embodiment of the filter bank according to the present application shown in FIG. 2, a capacitor 5 is added in parallel to the interdigital electrode 2 on the input side, and the parallel elements of the constant K-shaped circuit are It consists of 9th
The figure shows an equivalent circuit when a frequency in the passband of the n-th stage surface acoustic wave filter 1 is supplied from the power supply. In Figure 9, the capacitance of capacitor 5 is
Expressed in Cgp, the capacitance of the entire parallel element is Ci +
Becomes Cgp. Therefore, even if Ci is small, the overall capacitance can be increased. Therefore, the expression
It is possible to increase the value of L determined from (2), and also
The cutoff frequency f ₀ determined from (1) can be lowered. By appropriately selecting the value of Cgp, the value of L can be increased, resulting in the effect that the coil can be manufactured with high precision and good workability. Furthermore, capacitor 5
By setting the capacitance Cgp, the cutoff frequency of the constant K type circuit can be set higher than the passband of the elastic surface filter 1 in the subsequent stage, and lower than the passband to eliminate unnecessary waves due to bulk waves and harmonics. It becomes possible. If the cutoff frequency is set in this way, the constant K type circuit itself has low-pass characteristics, so the constant K type circuit itself has low-pass characteristics.
The shape circuit has the effect of cutting out unnecessary passbands.

In addition, in FIG. 9 as well, if the power supply voltage is expressed by V _s and Gi≪2πf (Ci + Cgp), then the voltage applied to the radiation conductance Gi of the interdigital interdigital electrode 2 on the input side of the surface acoustic wave filter 1 is approximately , _Vs /2
becomes. Therefore, in the first embodiment of the present invention as well, the loss increases by about 6 dB compared to the loss when the surface acoustic wave filter 1 is operated alone.
In other words, even if the number of channels in the filter bank (the number of stages in the constant-K circuit) increases, the loss will increase by approximately 6 dB.
is constant. Therefore, the first embodiment of the present invention is also suitable for constructing a filter bank with a large number of channels.

FIG. 3 shows a second embodiment of the filter bank according to the invention. In the example in FIG.
Unlike the embodiment shown in FIG. 2, the parallel elements of the constant K-type circuit are constructed by connecting the input admittance of the input side interdigital electrode 2 of the surface acoustic wave filter 1 and the capacitor 5 in series. There is. In the embodiment shown in FIG. 3, the same advantages as shown in FIG. 2 can be obtained by changing the value of the capacitance of the capacitor 5 used in each parallel element.

This will be explained using FIG. 10. FIG. 10 shows an equivalent circuit when a frequency in the passband of the n-th stage surface acoustic wave filter 1 is supplied from the power supply. Capacitance Ci of interdigital electrode 2 on input side
Consider the case where the value of is large. In order to set the nominal impedance R to 50Ω, the value of L also increases from equation (2). Therefore, from equation (1), the cutoff frequency
f ₀ becomes extremely low.

In such cases, traditional filter banks
Since the constant K-type circuit has a low cutoff frequency f ₀ , in a filter bank with a large number of channels, there is a problem that the cutoff frequency f ₀ becomes lower than the frequency of the highest pass band in the surface acoustic wave filter 1 . Since the constant K type circuit has a low-pass type filter characteristic, the input signal cannot be transmitted to the surface acoustic wave filter 1 in the high frequency band of the passband.

On the other hand, in the second embodiment of the filter bank according to the present invention shown in FIG. 3, an external capacitor Cgs is added in series to the input side interdigital electrode 2.
It constitutes a parallel element of a constant K type circuit. Therefore, the capacitance of the entire parallel element is CiCgs/(Ci +
Cgs). Therefore, the input side interdigital electrode 2
Even if Ci is large, the overall capacitance can be made small. Therefore, the value of L determined from equation (2) can be reduced, and the cutoff frequency f ₀ determined from equation (1) can be increased. Since the value of L can be made small, the effect and operation that the coil 3 can be manufactured with small dimensions can be obtained. Furthermore, since the cut-off frequency f ₀ can be made high, an effect and action can be obtained in which the input signal can be transmitted to the surface acoustic wave filter 1 in the high frequency band of the passband without large loss.

In the second embodiment of the present invention as well, the voltage applied to the radiation conductance Gi of the interdigital interdigital electrode 2 on the input side of the surface acoustic wave filter 1 is approximately:
It becomes V _s /2. Therefore, in the second embodiment of the present invention as well, the loss increases by about 6 dB compared to the loss when the surface acoustic wave filter 1 is operated alone. Therefore, the second embodiment of the present invention is also suitable for constructing a filter bank with a large number of channels.

FIG. 4 shows a third embodiment of the filter bank according to the present invention. In the embodiment shown in FIG. 4, unlike the embodiments shown in FIGS. 2 and 3, the parallel elements of the constant K-type circuit are connected to the input admittance of the interdigital electrode 2 on the input side of the surface acoustic wave filter 1. , and a plurality of capacitors 5 are formed by a combination of series connection and parallel connection.

A third embodiment of the filter bank according to the application of this description, shown in FIG. 4, is a combination of the first and second embodiments described above. By using the third embodiment, the degree of freedom in setting the value of L and the cutoff frequency can be made even greater than in the first and second embodiments described above. Therefore, there are effects and operations suitable for configuring a filter bank consisting of surface acoustic wave filters 1 of more diverse frequency bands.

Although the embodiments shown in FIGS. 2, 3, and 4 have been described above, the present invention is not limited thereto. By combining the input admittance and further combining the capacitor 5,
It may be applied when configuring parallel elements.

〔Effect of the invention〕

As shown above, in the filter bank according to the present invention, the input admittance of the input side interdigital electrode 2 of the surface acoustic wave filter 1 and the capacitor 5 are combined to form a parallel element, and the coil 3 is formed as a series element. , a plurality of surface acoustic wave filters 1 are set at a constant K
By configuring the filter bank by connecting wires in a shape, there are advantages such as improved workability and accuracy compared to the conventional method.

Furthermore, by setting the capacitance of the capacitor, you can increase the degree of freedom in setting the cutoff frequency, and the cutoff frequency can be higher than the required passband of the subsequent surface acoustic wave filter and lower than the unnecessary passband. Effects and effects that can cut the passband, effects and effects that can transmit the input signal to the surface acoustic wave filter 1 in the high frequency band of the passband without large loss, and various frequency bands. Effects and effects suitable for configuring a filter bank consisting of surface acoustic wave filters 1
It has an effect.

Further, the cutoff frequency can be changed for each stage, or can be the same for all stages. Changing the cutoff frequency for each stage has the advantage of being able to deal with cases where the unnecessary passbands are different for each stage. Also,
If the cutoff frequency is made the same for all stages, the same coil 3 can be used for all the stages, which is a great advantage in manufacturing the filter bank.

[Brief explanation of drawings]

Figure 1 is a conventional filter bank configuration diagram;
3 and 4 show examples of the filter bank according to the present invention, FIG. 5 shows a constant K-type circuit, and FIG. 6 shows the input side electrode of a surface acoustic wave filter. A diagram showing an equivalent circuit of input admittance, FIG. 7 is a diagram showing an equivalent circuit of a conventional example, and FIG. 8 is a diagram showing an equivalent circuit of the conventional example.
A diagram showing an equivalent circuit when a surface acoustic wave filter is operated alone, FIG. 9 is a diagram showing an equivalent circuit of the first embodiment of the present invention, and FIG. 10 is a diagram of the equivalent circuit of the second embodiment of the present invention. FIG. 3 is a diagram showing an equivalent circuit. In the figure, 1 is a surface acoustic wave filter, 2 is an interdigital electrode, 3 is a coil, 4 is a terminating resistor, and 5 is a capacitor. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

Claims (1)

[Claims] 1. In a filter bank formed by connecting a plurality of surface acoustic wave filters in a constant K shape, with the input admittance of the input-side interdigital electrode of the surface acoustic wave filter as a parallel element and the coil as a series element,
The parallel element is configured by combining the input admittance of the surface acoustic wave filter and a capacitor,
The above constant K determined by the above parallel element and the above series circuit
The capacitance of the capacitor is set so that the cutoff frequency of the shaped circuit is higher than the required passband of the surface acoustic wave filter in the subsequent stage and lower than the unnecessary passband of the surface acoustic wave filter. filter bank.