JPH02140007A - Low pass filter - Google Patents

Abstract

PURPOSE:To reduce the loss at the cut-off frequency by increasing the anti- resonance point of a circuit including a parallel resonance circuit of the final stage higher than that of the one stage preceding. CONSTITUTION:An inductance of a coil L4' of a parallel resonance circuit A2, of the final stage among inductances of coils L1'-L4' calculated in a way that the inductance is smaller from the 1st stage toward the final stage, and the inductance of the coil L3' of the parallel resonance circuit A3, of one preceding stage is decreased from its calculated value while it is larger than the induct ance of the coil L4'. Thus, the anti-resonance point P4' of the circuit B4' of the final stage is moved toward a high frequency and the antiresonance point P3' of the circuit B2' of one preceding stage is moved toward a low frequency conversely, and the frequency characteristic of each stage is not steep, but even so the entire frequency characteristic of the filter obtained through the synthesis has a steep leading.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low-pass filter including a plurality of parallel resonant circuits connected in series in a circuit.

Conventionally, as the above low-pass filter, a printed coil (L) and a capacitor (C) are formed on a dielectric substrate.
An equivalent circuit as shown in Figure 5 (3 to 4 stages of parallel resonant circuit)
．． 82143. (including A4 size). The design of this configuration is generally a combination of Chebyshev (Tc
This is done using the design formula of hbyceff).
When forming an LC circuit using a dielectric substrate, the element value of the circuit B shown by the broken line in the first stage (left end) and the coil L of the parallel resonant circuit AI included therein should be , parallel resonant circuits AX and Ih included in circuits B2°Bs and B4 of each subsequent stage, respectively.

It is necessary to make the element values of each coil Lx, Lz, and L4 of A4 successively smaller.

Figure 6 shows the frequency characteristics SAs of the final stage circuit B4 in the filter designed as described above, the frequency characteristics SA3 of the circuit B one stage before it, and the frequency characteristics SAs of both circuits B3 and B4. FIG. 2 is a diagram showing the results obtained by simulation. As can be understood from this figure, in the designed filter, the antiresonance point (ball) B4 of the frequency characteristic SA4 in the final stage circuit B is the antiresonance point (ball) B4 of the frequency characteristic SA in the circuit B3, one stage before it. The circuit Bl+B2. is located on the lower frequency side than the resonance point P3. It is located on the lowest frequency side among B3° and B4.

In order to obtain the desired filter characteristics in this configuration,
As shown by D4 in the figure, it is necessary that the rise of the waveform of the circuit B4 at the final stage is considerably steeper than that of the circuit B3 at the preceding stage.

When the dielectric substrate is used, the Q of the printed coil is small, so the frequency characteristic S of the final stage circuit B4 is
A4 does not rise as steeply as required by design, but becomes gradual in the cutoff frequency range. Therefore, there is a problem in that the loss in the cutoff frequency range of the overall frequency characteristic obtained by superimposing the frequency characteristics of all stages becomes large.

The present invention is intended to solve this problem, and aims to provide a low-pass filter having a structure in which the overall frequency characteristics can be designed according to the design standard, and the loss in the cutoff frequency region can be reduced. .

The present invention includes a circuit in which a conductive film is formed in a predetermined pattern on the lower surface of a dielectric substrate, and a plurality of circuits connected in series in the circuit. In a low-pass filter that includes a parallel resonant circuit, among the coil element values calculated in a state that decreases sequentially from the first stage side to the final stage side of the parallel resonant circuit based on the simultaneous Chebyshev design formula, the value in the final stage parallel resonant circuit is The element value of the fill is made larger than the calculated value, and the element value of the coil in the parallel resonant circuit one stage before it is made larger than the element value of the final stage coil. It is characterized by being smaller than.

In this case, the element value of the coil of the last stage parallel resonant circuit of multiple parallel resonant circuits connected in series is made larger than the calculated value, and the element value of the coil of the last stage parallel resonant circuit is made larger than the calculated value, and Since the coil value of the parallel resonant circuit of The anti-resonance point should shift to the lower frequency side. Therefore, as shown in Fig. 3, the anti-resonance point p% of the frequency characteristic SA4' in the final stage is smaller than the anti-resonance point Pff' of the frequency characteristic SA, ' in the previous stage. If the calculated value is changed so that the frequency is also located on the high frequency side, the frequency characteristic SA before each stage loading as designed can be obtained, and as a result, the loss in the force/y-to-off frequency range is reduced.

Actually, Jt, -狽1 Fig. 1 is a circuit diagram showing a low-pass filter according to the present invention, and Fig. 2 is an equivalent circuit diagram of this low-pass filter. The above low-pass filter has a dielectric base +Fj, 10
Conductive films 11 (solid line) and 12 (dashed line) are formed in a predetermined pattern on the upper and lower surfaces], Oa and 10b, respectively, and the printed coils L, ', L2 are made up of portions where both films 11 and 12 do not face each other. ', t, il, 4' and capacitors 02C4', C4', C5', consisting of opposing parts
C6', C? ', Ca', C9' and printed coil 14. A capacitor C, l consisting of a stray capacitance of
has. In the figure, S denotes an electrode that connects the upper conductive film 11 and the lower conductive film 12 through the resonator 10. This circuit consists of a plurality of parallel resonant circuits, for example, four parallel resonant circuits AA2', A3', A4', as shown in FIG.
are connected in series, and each parallel resonant circuit A, ', A2
'+ A, (,, G capacitor C5' on both sides of r,
Cb', C7', Ca', and C=r' are provided for t7, and the display of each element is shown in correspondence with FIG.

In this circuit, the anti-resonance points are exchanged in the above-mentioned simultaneous Chiebishev design formula. Specifically, each coil 1. is calculated based on the formula G in a state in which the size becomes smaller from the first stage side to the final stage side. l', L2L, l, L,
Among the element values of l, the element value of coil 1.4' of the final stage parallel resonant circuit A4' is made larger than the calculated value, and the element value of coil 3' of the parallel resonant circuit Aff' one stage before it is made larger than the calculated value. The element value is designed to be smaller than the calculated value while being larger than the element value of the enlarged coil L4'. For example, in the conventional case shown in FIG. 5, each stage of coils L, L2
．． L,, I7. The element value of each is 27.9.2669
．． 21.3.11. , I tried to make it OnH, but
In this example, the coils L+', Lz', Lt
', the element values of La are 27.9, 26.9 and ■7, respectively.
．． 8.14.5 nH, and the element values for the final two stages of coils Ll' + L4' are set so that the difference is small.

Regarding the capacitance of the capacitors, conventionally each capacitor C+, Cm, C3, C4, Cs, Cb, C
qlCe, C9 respectively 0.39.1.2.2゜3.5
．． 1.3.3.6.0.5゜2.4.3.0.9 pF
In this example, each capacitor C+, C
2, C3, Ca, Cs, CbC7' = CB
' + C9' respectively 0.39.1.2.3.1.3
．． 3.3.3.6.0.4.8.4.1.1.5 pF
It is said that

FIG. 3, which was explained in the operation section, shows the frequency characteristics SA of the final stage circuits B and I when designed in this way.

This is a diagram showing the results obtained by simulating the frequency characteristics SA3 of the circuits B and l in the previous stage, and the frequency characteristics SA of the superposition of both stages. p, l move to the high frequency side, and conversely, the anti-resonance point p, / of circuit B, /, one stage before, moves to the low frequency side, and Pa'
>Px', and even though the frequency characteristics near the cant-off frequency in each of the frequency characteristics SA4' and SA3' became gentle due to the use of the dielectric substrate, the synthesized frequency characteristics SA could be matched as designed.

Therefore, the output characteristic E of the filter according to the present invention is the fourth
As shown in the figure, the loss in the cutoff frequency region can be reduced compared to the output characteristic F of the conventional low-pass filter.

As detailed above, in the case of the present invention, by making the anti-resonance point of the circuit including the final stage parallel resonant circuit higher than that of the previous stage, each Even if the frequency range of the stages becomes gentle, the frequency characteristics of the entire filter obtained by synthesis can be made to have a steep rise, and a good low-pass filter with low loss in the cutoff frequency range can be obtained. It will be done.

[Brief Description of the Drawings] Figure 1 is a circuit diagram of a low-pass filter to which the present invention is applied;
Figure 2 is an equivalent circuit diagram of this circuit, and Figure 3 is the frequency characteristics SA of the final stage circuit B4 and the circuit B3 one stage before this.
4 is a graph showing the output characteristics of the filter of the present invention in comparison with the conventional output characteristics. FIG. 5 is an equivalent circuit diagram showing the configuration of the conventional low-pass filter. FIG. 6 is a graph showing the designed frequency characteristics of the circuit. 10... Dielectric substrate, 11.12... Conductive film. Lx', L3', L4...Coil,
C+, Cz, C+C4', C5', Cb', C?
', CB, C9... Capacitor. Figure 5 Patent applicant Murata Manufacturing Co., Ltd.

Claims (1)

[Claims] (1) A low-pass filter that includes a circuit in which conductive films are formed in a predetermined pattern on the upper and lower surfaces of a dielectric substrate, and that includes multiple parallel resonant circuits connected in series, based on the simultaneous Chebyshev design formula. Parallel resonant circuit 1
Among the element values of the coil calculated in a state that decreases sequentially from the stage side to the final stage side, the element value of the coil in the final stage parallel resonant circuit is made larger than the calculated value, and the parallel resonance of the stage one stage before it is made larger than the calculated value. A low-pass filter characterized in that the element value of a coil in a circuit is made smaller than a calculated value while being larger than the element value of a final stage coil.