Filter unit having two attenuation poles

Details Filter unit having two attenuation poles Filter unit having two attenuation poles

2001-09-25

2003-03-11

Tokar, Michael (Department: 2819)

Wave transmission lines and networks

Plural channel systems

Having branched circuits

C333S193000

Reexamination Certificate

active

06531933

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to filter units for passing signals of a desired frequency band, and to duplexers comprising such filter units.
DESCRIPTION OF THE RELATED ART
FIG. 12
is a graph showing the common characteristics of a low-pass filter (hereinafter referred to briefly as “LPF”) for passing low frequencies.
FIG. 13
is a block diagram showing the internal construction of the low-pass filter. The filter comprises an inductance L
1
, a capacitance C
1
connected in parallel thereto, and capacitances C
2
, C
3
connected between the inductance L
1
and the ground. With reference to
FIG. 12
, fc is a cutoff frequency. The frequency characteristics gently attenuate from the cutoff frequency fc toward higher frequencies and have an attenuation pole (point where the level of passage is minimized) at a frequency of fp
1
. Thus, the pass level is minimum at fp
1
. As is known well, the cutoff frequency fc is dependent on the inductance L
1
and capacitances C
2
, C
3
, and the attenuation pole frequency fp
1
on the inductance L
1
and capacitance C
1
.
However, conventional filters have the following problem.
LPFs have frequency characteristics which attenuate gently from the cutoff frequency fc toward higher frequencies. Accordingly, if there is a narrow frequency band wherein signals are to be blocked for the prevention of noise between the cutoff frequency fc and the attenuation pole frequency fp
1
as indicated at fp
2
in
FIG. 12
, this frequency band can not be fully attenuated.
To give sharp attenuation characteristics to the LPF to block the passage of signals of the frequency band fp
2
in the vicinity of the cutoff frequency fc, it appears feasible to arrange in series many combinations of capacitance C and inductance L connected thereto as shown in FIG.
15
and to provide a plurality of attenuation poles as seen in FIG.
14
. Nevertheless, this makes the circuit construction of the LPF complex, rendering the device incorporating the LPF large-sized.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a simple construction for blocking signals of a frequency band positioned between a cutoff frequency and an attenuation pole frequency.
The present invention provides a filter unit comprising a band-pass filter
2
for passing signals of a specified frequency band, a SAW filter
3
or dielectric resonator for passing signals of a band around a frequency fp
2
positioned between a cutoff frequency fc of the band-pass filter
2
and an attenuation pole frequency fp
1
thereof, and an inversion circuit
4
for inverting the phase of the signal passing through the SAW filter
3
or dielectric resonator and superposing the phase-inverted signal on an output signal from the band-pass filter
2
.
The SAW filter
3
passes a signal of a band around a frequency fp
2
positioned between the cutoff frequency fc of the band-pass filter
2
and the attenuation pole frequency fp
1
thereof. This signal has its phase inverted by the inversion circuit
4
. Stated more specifically, the signal of the band around the frequency fp
2
to be blocked is attenuated as indicated by a chain line in FIG.
4
and superposed on the output signal form the band-pass filter
2
. Consequently, the output signal passing through the filter unit has two attenuation poles fp
1
, fp
2
, as shown in
FIG. 2
, sharpening the attenuation characteristics between the cutoff frequency fc and the frequency fp
2
to be blocked. Thus, the signal of a frequency band positioned in the vicinity of the cutoff frequency fc can be blocked by a simple arrangement. This serves to reduce the size of the device into which the filter unit is to be incorporated.


REFERENCES:
patent: 5936483 (1999-08-01), Ikada
patent: 6057744 (2000-05-01), Ikada
patent: 5-251905 (1993-09-01), None

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