JPS5851441B2 - low pass filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a coaxial line type low-pass filter.

First, the conventional low-pass filter shown in FIG. 1 will be briefly explained.

In Figure 1, 1 and 2 are the inner conductors of the filter, 3 is the outer conductor of the filter, 4 is a dielectric ring, and 5 and 6 are the inner and outer conductors of the coaxial line that is the input and output ends of the filter. be.

Note that the inner conductor 2 is fixed by sandwiching a dielectric ring of the same length between it and the outer conductor.

Further, the characteristic impedance of the coaxial line formed by the inner conductor 1 and the outer conductor 3 and the coaxial line formed by the inner conductor 2 and the outer conductor 3 are the same as the characteristic impedance of the coaxial line formed by the inner conductor 5 and the outer conductor 6. In comparison, the lines have sufficiently high impedance and low impedance, respectively.

The wave filter shown in FIG. 1 is equivalently represented by the low-pass filter circuit shown in FIG.

In FIG. 2, ~L4 is an inductance, the size of which is determined by the ratio of the length of the inner conductor 1 to the diameter of the outer conductor 3 in FIG.

Further, C1 to C3 are capacitances whose size is determined by the ratio of the length of the inner conductor 2 to the diameter of the outer conductor 3 and the dielectric constant of the dielectric ring 4 shown in FIG.

Now, the equivalent circuit as shown in FIG. 2 is established for the TEM mode wave propagating through the wave device shown in FIG.

However, since the diameter of the outer conductor 3 of the wave filter shown in Figure 1 is considerably larger than the diameter of the outer conductor 6 of the coaxial line at the input and output ends, in the high frequency band that is the stopband of the low-pass filter, As the frequency increases, higher-order modes of the coaxial waveguide can propagate.

Since the equivalent circuit shown in Figure 2 does not hold for higher-order mode waves, the higher-order mode waves generated due to the slight asymmetry in the filter propagate through the filter, and the input and output terminals The coaxial line is cut off, causing confined resonance.

In this resonant state, the fundamental mode and higher-order modes are strongly coupled, so the TEM mode wave that was converted to a higher-order mode wave near the input end is converted back to a TEM mode wave near the output end and output. It starts to come out on the edge.

Therefore, in a conventional low-pass filter configured as shown in Figure 1, at frequencies where higher-order modes cause confined resonance within the filter, the high-frequency band that is the stop band of the filter is Even if there were, there was a drawback that sufficient attenuation could not be obtained.

In this invention, multiple coupling holes long in the tube axis direction are provided on the same circumference in the outer conductor of the filter, and a cavity resonator filled with a dielectric material having an appropriate loss coefficient is connected to each coupling hole. By doing so, the above drawbacks are eliminated.

FIG. 3 shows an embodiment of the invention, and 3, 5 and 6 are the same as those in FIG.

7 is a coupling hole, 8 is a cavity resonator, and 9 is a dielectric material.

The structure of the filter is exactly the same as that shown in FIG. 1 except that a coupling hole 7 and a cavity resonator are added.

This coupling hole 7 is provided in the outer conductor 3 and has a slot shape that is elongated in the tube axis direction.

The resonant frequency of the cavity resonator 8 is made to match the frequency at which the TE11 mode of the coaxial waveguide, which can propagate at low frequencies, causes confined resonance in the filter.

Further, in this embodiment, the cavity resonator 8 is filled with a dielectric material 9 having an appropriate loss coefficient, thereby lowering the Q of the cavity resonator 8 and making it smaller.

In such a configuration, first consider the low frequency band that is the passband of the low-pass filter.

In this band, higher-order modes of the coaxial waveguide do not occur, and only the TEM mode propagates through the filter.

When the TEM mode propagates through the coaxial line, the current flowing through the inner surface of the outer conductor of the coaxial line has a distribution as shown by the solid line with arrows in FIG.

In FIG. 4, 1 is the inner conductor of the coaxial line, 3 is the outer conductor, and 10 is a line showing the current distribution.

As can be seen from FIG. 4, the current flowing through the outer conductor 3 has only a component in the tube axis direction and no circumferential component.

Therefore, the TEM mode wave propagating through the filter in FIG. 3 has no current component flowing across the coupling hole 7, which is elongated in the tube axis direction, and is therefore almost coupled to the cavity resonator 8 through the coupling hole 7. Not done.

Furthermore, the cavity resonator 8 is in a non-resonant state at this frequency.

Therefore, the wave transmitter according to the present invention has characteristics that are almost the same as those of the conventional wave transmitter in the pass frequency band.

Next, consider a case where the TEL mode, which is a higher-order mode of the coaxial waveguide that can propagate at a low frequency, causes confined resonance in the filter in the stop band of the low-pass filter.

When the TE11 mode propagates through the coaxial line, the current flowing through the inner surface of the outer conductor has a distribution as shown by the solid line with arrows in FIG.

In FIG. 5, 1, 3 and 10 are the same as in FIG.

As can be seen from FIG. 5, the current flowing through the outer conductor 3 has a component in the tube axis direction as well as a circumferential component.

Therefore, in FIG. 3, if the coupling hole 7 is provided at a position where a circumferential current exists, the TE1 confined in the filter
1 mode is coupled to at least two of the four cavity resonators 8.

Therefore, the TE,1 mode that is converted from the TEM mode at the input end and causes confined resonance is absorbed by the low Q resonator 8, so that the amount that is reconverted to the TEM mode at the output end is significantly reduced.

As described above, the wave filter according to the embodiment has the advantage that it is possible to prevent a decrease in the attenuation of the stopband of the wave filter due to the TE of the coaxial waveguide and one-mode confined resonance.

In FIG. 3, the cavity resonator 8 has four rectangular parallelepiped cavities, but the number is not limited to four.

Furthermore, the number of coupling holes shown in FIG. 3 is not limited to four.

Further, the dielectric material 9 shown in FIG. 3 may not fill the entire cavity resonator 8, but may be provided at least in the center of the resonator 8.

As described above, the present invention has the advantage that it is possible to prevent a decrease in attenuation in the stop band of the filter due to confined resonance of the TE11 mode of the coaxial waveguide in the filtrate.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a conventional coaxial line type low-pass filter, FIG. 2 is an equivalent circuit diagram of FIG. 1, and FIG. 3 is a schematic configuration diagram showing an embodiment of the present invention. Figure 4 shows the current distribution in the outer conductor of the coaxial line where the TEM mode propagates.
The figure shows the current distribution in the outer conductor of the coaxial line through which the TE11 mode propagates. 1.2...Inner conductor of filter, 3...
Outer conductor of filter, 4...Dielectric ring, 5.
...Inner conductor of input/output coaxial line, 6...
- External conductor of input/output end coaxial line, 7... Coupling hole, 8... Cavity resonator, 9... Dielectric material,
10... Line showing current distribution. Note that -1 or equivalent parts are indicated by the same reference numerals.

Claims (1)

[Claims] 1. In a low-pass filter consisting of an outer conductor and two types of inner conductors with different diameters, elongated coupling holes in the tube axis direction are formed in the outer conductor at equal intervals on the same circumference upwind on three sides. A rectangular cavity resonator having a resonant frequency equal to the frequency of the TE11 mode confined resonance in the low-pass filter is coupled to each of the coupling holes. Low pass filter.