Highly selective narrow-band lc-filter

FIELD: radio engineering, communication.

SUBSTANCE: invention pertains to radio electronics and can be used for frequency selection of signals. The highly selective narrow-band LC-filter comprises two structurally identical band-pass filter links, each consisting of three capacitors and an inductance coil, eleven capacitors, two inductance coils, an input and an output potential terminal and a common bus.

EFFECT: high selectivity of the filter.

6 dwg

The present invention relates to electronics and can be used for frequency selection signals.

In professional communications equipment for frequency selection signals have found an extensive use of different types of bandpass LC filters. One of them is the filter containing the inductor, the first output of which is connected through the first capacitor with a common bus, a second output connected to the second capacitor, the second terminal through which a third capacitor connected to the common bus. This scheme implements a bandpass circuit first class attenuation contains the minimum number of inductances that you can get the maximum transfer ratio compared to many other bandpass circuits, especially when the implementation of a narrow bandwidth. In addition, it is easy to manufacture. This filter is the closest to the technical nature of the present device and is selected as a prototype. The disadvantage of the filter prototype is a small value Squareness of its amplitude-frequency characteristics, because it is not possible to implement the attenuation pole at finite frequencies.

The objective of the invention is the improvement of the selectivity filter while simultaneously providing the most technologically advanced ratings of its elements.

The task of meeting the tsya fact, in the filter, consisting of two identical in structure units of bandpass filters, each of which contains the first inductor, the first output of which is through the first capacitor is connected to a common bus, the second terminal of the first inductor is connected with the second capacitor, the second terminal of which is connected to the third capacitor, the second terminal of which is connected to a common bus, in addition to the first output of the first inductor in each of the links connected to the fourth and fifth capacitors, while the second terminal of the fourth capacitor is connected to the second output of the second capacitor, the second terminal of the fifth capacitor is connected to the first output of the second inductor and with the first output of the sixth capacitor, the second terminal of the sixth capacitor is connected with the second output of the second capacitor and the second output of the second inductor is connected with a common bus, besides, the first output of the first inductor of the first link is connected to the seventh capacitor, the second terminal of which is connected to the input potential terminal of the filter and the eighth capacitor, the second terminal of which is connected to the shared bus, the second terminal of the second capacitor of the first link through the ninth capacitor is connected to the first output of the first inductor of the second ringing, the second conclusion in the showing of the second capacitor element connected to the first output of the tenth capacitor, the second output of which is connected to the output potential terminal of the filter and through the eleventh capacitor connected to the common bus.

Comparative analysis shows that the claimed technical solution differs from the prototype in that the first output of the first inductor in each of the links connected to the fourth and fifth capacitors, while the second terminal of the fourth capacitor is connected to the second output of the second capacitor, the second terminal of the fifth capacitor is connected to the first output of the second inductor and the first output of the sixth capacitor, the second terminal of the sixth capacitor is connected with the second output of the second capacitor and the second output of the second inductor is connected with a common bus, besides, the first output of the first inductor of the first link is connected to the seventh capacitor, the second the output of which is connected to the input potential terminal of the filter and the eighth capacitor, the second terminal of which is connected to the shared bus, the second terminal of the second capacitor of the first link through the ninth capacitor is connected to the first output of the first inductor of the second link, the second terminal of the second capacitor of the second link connected to the first output of the tenth capacitor, the second terminal of which is connected to the output potential terminal of the filter and across the eleventh capacitor connected to the common bus.

When comparing the claimed solution is not only the prototype, but also with other known in science and technology technical solutions are not found solutions with similar characteristics.

Figure 1 shows the electric diagram of the device.

The device consists of a first 1 and second 2 parts bandpass circuits are identical in structure. Each link contains the first inductor 3, the first output of which is through the first condenser 4 is connected with a common bus, a second output connected to the second capacitor 5, to the second output of which is connected to the third capacitor 6. To the first output of this coil is connected 7 fourth and fifth 8 capacitors, the second terminal of the fourth capacitor 7 is connected with the second output of the second condenser - 5, and to the second output of the fifth capacitor 8 connected to the second inductor 9 and the sixth capacitor 10, the second terminal of which is connected to the second output of the second capacitor 5. Input terminal device through the seventh capacitor 11 is connected to the first output of the first inductor of the first link and through the eighth capacitor 12 to the shared bus. The ninth capacitor 13 connects the second terminal of the second capacitor of the first link and the first output of the first inductor of the second link, the Tenth capacitor 14 connects the second output of the second to the of henstra second level and potential output terminal of the filter, which through the eleventh capacitor 15 is connected with a common bus.

The device operates as follows.

This filter can be obtained near equivalent transformations, the original circuit is a bridge circuit containing each pair of branches reactance Z_andand Z_b. Electrical circuits of these two-terminal device shown in figure 2. If the frequency dependence of the impedance Z_aand Z_bselected in accordance with figure 3, will receive bandpass filter of the second class by attenuation, which can realize two poles of attenuation at finite frequencies. The bandwidth of the filter is located between the frequencies ω₁and ω₂. The method of calculation of the elements of such a filter is given in [2]. The circuit elements are determined from the relations:

$$\begin{array}{l}{L}_1=\frac{M{Z}_m}{\Delta \omega }{C}_{01}=\frac{1}{{\omega }_0{Z}_{m}M}\text{<msub> <mi> C </mi> <mn> 1 </mn> </msub> <mo> = </mo> <msub> <mi> C </mi> <mrow> <mn> 01 </mn> </mrow> </msub> <mfrac> <mrow> <mi> Δ </mi> <mi> ω </mi> </mrow> <mrow> <msub> <mi> ω </mi> <mn> 0 </mn> </msub> </mrow> </mfrac>}\\ L_2=\frac{Z_m}{\Delta \omega M}{C}_{02}=\frac{M}{{\omega }_0{Z}_m}{C}_2=C_{02}\frac{\Delta \omega }{{\omega }_0}\\ \Delta \omega ={\omega }_2-{\omega }_{1}M=\frac{1+m_1{m}_2}{m_1+m_2}\text{the}\left(1\right)\end{array}$$

$$$$$$m_1=\sqrt{\frac{{\omega }_2^2-{\omega }_{\infty 1}^2}{{\omega }_1^2-{\omega }_{\infty 1}^2}}$$$$m_2=\sqrt{\frac{{\omega }_2^2-{\omega }_{\infty 2}^2}{{\omega }_1^2-{\omega }_{\infty 2}^2}}$$$$m_0=\frac{{\omega }_1+{\omega }_2}{2}$$

When symmetric (with respect to the frequency ω₀) whic is ogenyi poles of attenuation, when m₁=1/m_2,size M<1. In this case, the container₀₂less capacity₀₁. We bring the container₀₂beyond the bridge circuit and let her through With₃and then select from the remaining part of the resistance Z_aparallel capacity With₂and at the same time sequential container₂of the remaining part of the resistance Z_bobtained on the basis of known equivalent conversion of the bridge circuit is in blocking T-shaped [2] the diagram in figure 4, for which values of its elements can be found from the formula:

$$\begin{array}{l}{C}_3=C_0\\ C_4=C_{01}-C_{02}-C_2\left(2\right)\end{array}$$

Next, by introducing additional capacity With₀between the container₃and the remaining fragment of a scheme of figure 4, we get the following modification of the circuit shown in figure 5, which is equivalent to the circuit shown in figure 4 when the following conditions are met:

$$\begin{array}{l}{C}_{10}=\frac{C_0^2{C}_1}{(C_0-C_2-C_4)(C_0-C_2-C_4-C_1)}\\ C_{20}=\frac{C_0{C}_2}{C_0-C_2}\left(3\right)\\ L_{10}=L_1{\left(\frac{C_0-C_2-C_4}{C_0}\right)}^2\\ C_{40}=C_4\frac{C_0^2}{(C_0-C_2)(C_0-C_2-C_4)}\end{array}$$

Note that the introduction of a capacity₀allows you to decrease the value of L₁and increase value With₁With₂With₄to more convenient in practice. A pair of capacitors₀and C₃input and output of the filter allows using known conversion Norton [1], transformed with the transformation ratio n<1 all values of the elements of the T-shaped part of the schema.

After the cascade connection of two filters, made by the scheme of figure 5, and the corresponding equivalent transformation (star-Delta) group of capacitors connecting both filters, get the scheme Phi is tra, shown in figure 1. Thus, the proposed filter circuit will correspond to the bandpass filter of the fourth class in attenuation, as shown by the analysis of the scheme, the most technologically advanced element values are obtained by implementing the relative bandwidth of no more than 5-8%. Figure 6 shows the amplitude-frequency characteristic of the bandpass filter at the frequency of 80 MHz with a relative bandwidth of 2.5%. The schema contains the minimum possible number of coils inductance whose value is equal to 20 NH, and which can be implemented in a miniature version with good quality more than 500-600 units Insertion losses of the filter is not more than 3 dB, when the Squareness ratio levels 60/3 dB less than 3% and the attenuation in the band of retaining more than 70 dB.

Sources of information

1. Černá HI inductive coupling and transformation in electric filters. - M.: Svyazist, 1962, 318 pages

2. Velikin YA, Gelmont SA, Zeljah EV Piezoelectric filters. M. Bond, 1966, 396 pages

Highly selective narrow-band LC filter consisting of two identical in structure units of bandpass filters, each of which contains the first inductor, the first output of which is through the first capacitor is connected to a common bus, the second terminal of the first inductor is connected to the second conden what ATOR, the second output of which is connected to the third capacitor, the second terminal of which is connected to a shared bus, characterized in that the first output of the first inductor in each of the links connected to the fourth and fifth capacitors, while the second terminal of the fourth capacitor is connected to the second output of the second capacitor, the second terminal of the fifth capacitor is connected to the first output of the second inductor and the first output of the sixth capacitor, the second terminal of the sixth capacitor is connected with the second output of the second capacitor and the second output of the second inductor is connected with a common bus, besides, the first output of the first inductor of the first link connected to the seventh capacitor, the second terminal of which is connected to the input potential terminal of the filter and the eighth capacitor, the second terminal of which is connected to the shared bus, the second terminal of the second capacitor of the first link through the ninth capacitor is connected to the first output of the first inductor of the second link, the second terminal of the second capacitor of the second link connected to the first output of the tenth capacitor, the second terminal of which is connected to the output potential terminal of the filter and through the eleventh capacitor connected to the common bus.