JPH0555857A - Filter circuit - Google Patents

Abstract

PURPOSE:To obtain a compact and inexpensive filter circuit having plural pass bands by connecting a BPF to a band elimination filter(BEF) in series. CONSTITUTION:The BPF 2 is serially connected to the BEF 3 having an elimination band inside the pass band of the BPF 2. Thereby the use of a power distributer is made unnecessary as compared with the parallel connection of two BPFs having different pass bands, so that a compact, inexpensive and loss- reduced filter having plural pass bands can be obtained. When a reactance circuit is connected between the BPF and the BEF, pass characteristics or the like can easily be adjusted by adjusting the circuit element of the reactance circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter circuit,
In particular, it relates to a circuit configuration of a bandpass filter having a plurality of pass bands.

[0002]

2. Description of the Related Art FIG. 7 shows a conventional bandpass filter (B
It is a block diagram which shows the circuit structure of (PF).

This bandpass filter has a center frequency f
1 and bandpass filter 11 having bandwidth ΔW1
And a bandpass filter 12 having a center frequency f2 (f2> f1) and a bandwidth ΔW2 are connected in parallel on the input side and the output side via power distributors 13 and 14, respectively. The power distributors 13 and 14 are of the Wilkinson type in which, for example, the quarter-wave lines 15 and 16 shown in FIG.

The signal input to the input terminal Pi of this bandpass filter is divided into two by the power distributor 13, and the bandpass filter 11 and the bandpass filter 12 are respectively divided.
And be transmitted to. Then, the signals that have passed through the bandpass filter 11 and the bandpass filter 12 are combined by the power distributor 14 and output from the output end Po.

Since the bandpass filter 11 and the bandpass filter 12 are bandpass filters having different passbands from each other, the power distributor 14 outputs an output signal that has passed through either the bandpass filter 11 or the bandpass filter 12. As a result, the filter circuit as a whole obtains a pass characteristic having two bandwidths ΔW1 and ΔW2 as shown in FIG.

[0006]

In the above conventional bandpass filter, the power dividers 13 and 14 are provided on the input side and the output side of the bandpass filter 11 and the bandpass filter 12, respectively.
Since they are connected to each other, the circuit configuration becomes complicated and the filter shape becomes large.

In particular, the power distributors 13 and 14 are 1 /
Since it is composed of four wavelength lines, the lower the pass band, the longer the line length and the larger the shape of the band pass filter.

There is also a disadvantage that power loss occurs due to the power distributors 13 and 14.

The present invention has been made in view of the above problems, and an object thereof is to provide a small-sized and inexpensive filter circuit having a plurality of pass bands.

[0010]

In order to solve the above-mentioned problems, the present invention provides a band pass filter and at least one band stop filter having a stop band inside the pass band of the band pass filter connected in series. It was done.

Further, according to the present invention, a reactance circuit is interposed between the band pass filter and the band stop filter.

[0012]

According to the present invention, the pass characteristics of these filters are combined by connecting the pass band filter and at least one band stop filter in series. A pass characteristic having two or more pass bands obtained by removing the stop band of the band stop filter is obtained.

Further, since the pass band filter and the band stop filter are directly connected, the structure of the filter circuit is simplified.

Further, by adjusting the reactance circuit, for example, the passage characteristics and the like are adjusted, and the operation of adjusting the characteristics of the filter circuit is facilitated.

[0015]

1 is a diagram showing a circuit configuration of an embodiment of a filter circuit according to the present invention. A filter circuit 1 shown in the figure is a bandpass filter having two passbands in which an output end of a bandpass filter (BPF) 2 and an input end of a band elimination filter (BEF) 3 are directly connected in series. Is composed of. The input end of the bandpass filter 2 and the output end of the band elimination filter 3 are the input end and the output end of the filter circuit 1, respectively.

The bandpass filter 2 uses, for example, a dielectric resonator, and the circuit shown in FIG. 2 or 3 is used.

In the bandpass filter shown in FIG. 2, the dielectric resonators .theta.1 to .theta.4 are connected in cascade via coupling capacitors C1 to C3, and the coupling point a of the first-stage dielectric resonator .theta.1 is set.
And a capacitor C4 is connected between the coupling points d of the final-stage dielectric resonator θ4. Note that Ci and Co are the input capacitance and the output capacitance, respectively.

Further, the bandpass filter shown in FIG.
In the bandpass filter shown in FIG.
, And θ3, instead of connecting the series circuits 4 and 5 of the dielectric resonator θ and the capacitor C, the capacitor C4 is deleted.

The circuit shown in FIG. 4 is used as the band elimination filter 3 using, for example, a dielectric resonator.

This band elimination filter is
The series circuits 6 to 9 of the dielectric resonator θ and the capacitor C are connected in cascade via the coils L1 to L3, and the capacitors C5 to C8 are connected between the respective connection points a to d and the ground.

In the filter circuit 1, for example, the band pass filter 2 and the band elimination filter 3 formed separately are housed in one case, and the two filters 2 and 3 are connected by a lead wire or stripped. The filters 2 and 3 are mounted on a substrate on which connection lines are formed by lines, and the strip lines connect the filters 2 and 3.

Alternatively, the bandpass filter circuit 2 and the band elimination circuit 3, which are formed independently of each other, may be coupled by a coaxial cable or the like to form the filter circuit 1.

The bandpass filter 2 is, for example, as shown in FIG.
Each intended with types of the dielectric resonator θ1~θ4 and capacitors C1~C4 to a pass band between the lower limit frequency f _L and an uppermost frequency f _H of the two passbands of the filter circuit 1 The element constants are set.

The band elimination filter 3 has an upper limit frequency f1 (> f _L ) of the lower pass band and a lower limit frequency f2 of the upper pass band of the filter circuit 1.
The series circuit 6 to so that the stop band is between (<f _H ).
9, the dielectric resonator θ, the capacitor C, and the coils L1 to
The constants of each element of L3 and capacitors C5 to C8 are set.

Therefore, the input terminal Pi of the filter circuit 1 is
From the signal inputted from the above, the band pass filter 2 removes a signal smaller than the lower limit frequency f _{L and a} signal larger than the upper limit frequency f _H , and further, the band elimination filter 3 has a frequency larger than the frequency f1 and a frequency f2. The smaller signal is removed, and f is output from the output Po.
Only the signals in the two frequency bands _L <f <f1 and f2 <f <f _H are extracted, and the filter circuit 1
It has the pass characteristic shown in FIG.

That is, the filter circuit 1 has a pass band Δ including two desired pass bands ΔW1 and ΔW2.
A bandpass filter 2 having W3 and the passband Δ
A desired bandpass characteristic is obtained by connecting in series a band elimination filter 3 having an attenuation band ΔW4 between W1 and ΔW2 as a stop band and combining the bandpass characteristics of both filters 2 and 3.

With such a configuration, a power divider inserted at the input and output ends is not required as compared with the conventional configuration in which two band pass filters having different pass bands are connected in parallel. Cost reduction can be achieved. There is also an advantage that power loss in the power distributor is eliminated.

In the above embodiment, the bandpass filter 2 and the band elimination filter 3 are directly connected, but if necessary, the bandpass filter 2 and the band elimination filter are connected as shown in FIG. The reactance circuit 10 may be interposed between the filters 3.

The reactance circuit 10 is an inductance (L) circuit, a capacitance (C) circuit, or a combination circuit of an inductance and a capacitance (LC circuit).
Etc., and can be configured using a coil or a capacitor as a single component, or a coil or a capacitor formed by a strip line or the like.

In the case where the reactance circuit 10 is provided,
The mutual effect of the band pass filter 2 and the band elimination filter 3 is mitigated, and the dielectric resonator θ, the capacitor C and the coil L in both filters 2 and 3 are reduced.
By adjusting the circuit constants in the reactance circuit 10 in addition to the adjustment of each element such as, the adjustment range of the pass characteristic and the input / output impedance of the filter circuit 1 can be widened and the adjustment work can be facilitated. There are advantages.

In the above embodiment, the case where two filter circuits are connected in series to form a filter circuit having two pass bands has been described, but three or more filter circuits are connected in series and three filter circuits are connected in series. It is also possible to configure a filter circuit having the above pass band.

Although a dielectric resonator is used as a specific example of the filter circuit, the present invention is not limited to this, and is formed by combining an inductance L, a capacitor C, a resistor R and other circuit elements. It can also be applied to a filter circuit.

[0033]

As described above, according to the present invention, a plurality of pass bands are formed by connecting in series a band pass filter and at least one band stop filter having a stop band inside the pass band of the band pass filter. Since the bandpass filter having the band is configured, the conventional power divider for coupling a plurality of filter circuits is not required, and the filter circuit can be downsized and the cost can be reduced accordingly. Moreover, the power loss in the power distributor can be reduced.

Further, since the reactance circuit is interposed between the band pass filter and the band stop filter, the circuit element of the reactance circuit can be adjusted to easily adjust the characteristics such as the pass characteristic of the filter circuit. You can

[Brief description of drawings]

FIG. 1 is a diagram showing a circuit configuration of an embodiment of a filter circuit according to the present invention.

FIG. 2 is a diagram showing a circuit configuration of an embodiment of a bandpass filter using a dielectric resonator.

FIG. 3 is a diagram showing a circuit configuration of another embodiment of a bandpass filter using a dielectric resonator.

FIG. 4 is a diagram showing a circuit configuration of an embodiment of a band elimination filter using a dielectric resonator.

FIG. 5 is a diagram showing a pass characteristic of the filter circuit according to the present invention.

FIG. 6 is a diagram showing a circuit configuration of another embodiment of the filter circuit according to the present invention.

FIG. 7 is a diagram showing a circuit configuration of a conventional bandpass filter.

FIG. 8 is a diagram showing a configuration of a power distributor.

FIG. 9 is a diagram showing pass characteristics of a conventional bandpass filter.

[Explanation of symbols]

 1 Filter Circuit 2 Band Pass Filter (BPF) 3 Band Elimination Filter (BEF) 4-9 Series Circuit of Dielectric Resonator and Capacitor 10 Reactance Circuit C, Ci, Co, C1-C8 Capacitor L1-L3 Coil θ, θ1 to θ5 Dielectric resonator Pi input end Po output end

Claims (2)

[Claims] 1. A filter circuit comprising a band pass filter and at least one band stop filter having a stop band inside a pass band of the band pass filter, which are connected in series. 2. A filter circuit according to claim 1, wherein a reactance circuit is interposed between the band pass filter and the band stop filter.