RU2519492C2 - Highly selective narrow-band lc-filter - Google Patents

Abstract

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

Description

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

In professional communication equipment for frequency signal selection, various types of LC bandpass filters are widely used. One of them is a filter containing an inductor, the first output of which is connected through a first capacitor to a common bus, its second output is connected to a second capacitor, the second output of which through a third capacitor is connected to a common bus. This scheme implements a first-class bandpass circuit for attenuation, contains a minimum number of inductances, which allows to obtain the highest transfer coefficient compared to many other strip chains, especially when implementing narrow passband. In addition, it is easy to manufacture. This filter is the closest in technical essence to the proposed device and is selected as a prototype. The disadvantage of the prototype filter is the small squareness of its amplitude-frequency characteristics, since it does not allow to realize the attenuation pole at finite frequencies.

The objective of the invention is to increase the selectivity of the filter while providing the most technologically advanced values of its elements.

The problem is solved in that in a filter consisting of two identical in structure links of bandpass filters, each of which contains a first inductor, the first output of which is connected to a common bus through the first capacitor, the second output of the first inductor is connected to the second capacitor, the second output which is connected to a third capacitor, the second terminal of which is connected to a common bus, in addition to the first terminal of the first inductor in each of the links connected fourth and fifth capacitor tori, with the second terminal of the fourth capacitor connected to the second terminal of the second capacitor, the second terminal of the fifth capacitor connected to the first terminal of the second inductor, and the first terminal of the sixth capacitor, the second terminal of the sixth capacitor connected to the second terminal of the second capacitor, and the second terminal of the second coil the inductance is connected to a common bus, in addition, the first output of the first inductance coil of the first link is connected to the seventh capacitor, the second output of which is connected to the input potential filter terminal and with an eighth capacitor, the second terminal of which is connected to a common bus, the second terminal of the second capacitor of the first link through the ninth capacitor is connected to the first terminal of the first inductor of the second link, the second terminal of the second capacitor of the second link is connected to the first terminal of the tenth capacitor, the second terminal which is connected to the output potential terminal of the filter and through the eleventh capacitor is connected to a common bus.

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

When comparing the claimed solution not only with the prototype, but also with other technical solutions known in science and technology, no solutions were found that have similar characteristics.

Figure 1 shows the electrical diagram of the proposed device.

The device consists of the first 1 and second 2 links of strip chains, identical in structure. Each link contains a first inductor 3, the first output of which is connected to a common bus through a first capacitor 4, its second output is connected to a second capacitor 5, and a third capacitor 6 is connected to its second output. A fourth 7 and a fifth 8 are connected to the first output of this coil capacitors, the second terminal of the fourth capacitor 7 is connected to the second terminal of the second capacitor 5, and a second inductor 9 and a sixth capacitor 10 are connected to the second terminal of the fifth capacitor 8, the second terminal of which is connected ene to the second terminal of the second capacitor 5. Input terminal device 11 via the seventh capacitor connected to the first terminal of the first inductor coil of the first link and the eighth through capacitor 12 to the common bus. The ninth capacitor 13 connects the second terminal of the second capacitor of the first link and the first terminal of the first inductor of the second link, The tenth capacitor 14 connects the second terminal of the second capacitor of the second link and the output potential terminal of the filter, which is connected to the common bus via the eleventh capacitor 15.

The device operates as follows.

This filter can be obtained by a number of equivalent transformations, while the initial circuit is a bridge circuit containing reactance Z _a and Z _b in each pair of branches. The electrical circuits of these two-terminal devices are shown in figure 2. If the frequency dependences of the resistances Z _a and Z _b are selected in accordance with Fig. 3, we obtain a second-class bandpass filter for attenuation, which can realize two poles of attenuation at finite frequencies. The passband of the filter is located between the frequencies ω ₁ and ω ₂ . The methodology for calculating the elements of such a filter is given in [2]. Elements of the circuit are determined from the relations:

$$\begin{array}{l}{L}_{\mathrm{one}}=\frac{M{Z}_m}{\Delta \omega }{C}_{01}=\frac{\mathrm{one}}{{\omega }_0{Z}_{m}M}{C}_{\mathrm{one}}=C_{01}\frac{\Delta \omega }{{\omega }_0}\\ {\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="00000009.png,00000011.png"\; state="\{\{state\}\}">L</figure-callout>}}_2=\frac{Z_m}{\Delta \omega M}{C}_{02}=\frac{M}{{\omega }_0{\mathrm{<figure-callout\; id="2"\; label="Z\; m\; C"\; filenames="00000009.png,00000011.png"\; state="\{\{state\}\}">Z\; </figure-callout>}}_m}{C}_{}{}_2=C_{02}\frac{\mathrm{<figure-callout\; id="0"\; label="\Delta \; \omega \; \omega "\; filenames="00000013.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}\omega }{{\omega }_{}}\\ \Delta \omega ={\omega }_2-{\omega }_{\mathrm{one}}M=\frac{\mathrm{one}+m_{\mathrm{one}}{m}_2}{m_{\mathrm{one}}+m_2}\left(\mathrm{one}\right)\end{array}$$

$$$$

$$m_{\mathrm{one}}=\sqrt{\frac{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000009.png,00000011.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\omega }_{\infty \mathrm{one}}^2}{{\omega }_{\mathrm{one}}^2-{\omega }_{\infty \mathrm{one}}^2}}$$

$${\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="00000009.png,00000011.png"\; state="\{\{state\}\}">m</figure-callout>}}_2=\sqrt{\frac{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000009.png,00000011.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\omega }_{\infty 2}^2}{{\omega }_{\mathrm{one}}^2-{\omega }_{\infty 2}^2}}$$

$${\mathrm{<figure-callout\; id="0"\; label="m"\; filenames="00000013.png"\; state="\{\{state\}\}">m</figure-callout>}}_0=\frac{{\omega }_{\mathrm{one}}+{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000009.png,00000011.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2}{2}$$

With a symmetric (relative to the frequency ω ₀ ) arrangement of the poles of the attenuation, when m ₁ = 1 / m _{2, the} value of M <1. In this case, the capacitance C _{02 is} less than the capacitance C ₀₁ . We take the capacitance C ₀₂ outside the bridge circuit and denote it by C ₃ , and then select from the remaining part of the resistance Z _{a the} parallel capacitance C ₂ and simultaneously the serial capacitance C ₂ from the remaining part of the resistance Z _b , we obtain on the basis of the known equivalent transformation of the bridge circuit into overlapping T-shaped [2] circuit shown in figure 4, for which the values of its elements can be found from the relations:

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

Further, introducing an additional capacity C ₀ between the capacity C ₃ and the remaining fragment of the circuit of FIG. 4, we obtain the following modification of the circuit shown in FIG. 5, which will be equivalent to the circuit shown in FIG. 4 under the following conditions:

$$\begin{array}{l}{C}_{10}=\frac{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="00000013.png"\; state="\{\{state\}\}">C</figure-callout>}}_0^2{C}_{\mathrm{one}}}{(C_0-C_2-C_{\mathrm{four}})(C_0-C_2-C_{\mathrm{four}}-C_{\mathrm{one}})}\\ C_{\mathrm{twenty}}=\frac{C_0{C}_2}{C_0-C_2}\left(3\right)\\ L_{10}=L_{\mathrm{one}}{\left(\frac{C_0-C_2-C_{\mathrm{four}}}{C_0}\right)}^2\\ {\mathrm{<figure-callout\; id="40"\; label="C"\; filenames="00000013.png"\; state="\{\{state\}\}">C</figure-callout>}}_{40}=C_{\mathrm{four}}\frac{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="00000013.png"\; state="\{\{state\}\}">C</figure-callout>}}_0^2}{(C_0-C_2)(C_0-C_2-C_{\mathrm{four}})}\end{array}$$

Note that the introduction of the capacitance C ₀ allows you to reduce the value of L ₁ and increase the value of C ₁ , C ₂ , C ₄ to more convenient in practice. A pair of capacitors C ₀ and C ₃ at the input and output of the filter allows, using the well-known Norton transforms [1], to transform with the transformation coefficient n <1 all the values of the elements of the T-shaped part of the circuit.

After the cascade connection of two filters made according to the scheme of Fig. 5 and the corresponding equivalent conversion (star-delta) of the group of capacitors connecting both filters, we obtain the filter circuit shown in Fig. 1. Thus, the proposed filter scheme will correspond to a fourth-class band-pass filter according to attenuation, while, as the analysis of the circuit shows, the most technologically advanced values of the elements are obtained when relative bandwidths are implemented no more than 5-8%. Figure 6 shows the amplitude-frequency characteristic of a band-pass filter at a frequency of 80 MHz with a relative bandwidth of 2.5%. The circuit contains the smallest possible number of inductors, the values of which are 20 nH and which can be implemented in a miniature version with a quality factor of more than 500-600 units. The insertion attenuation of the filter is not more than 3 dB, with a coefficient of squareness at levels 60/3 dB less than 3 units. and attenuation in the delay band of more than 70 dB.

 Information sources

1. Cerne H.I. Inductive coupling and transformations in electric filters. - M .: Svyazizdat, 1962, 318 pp.

2. Velikin Y.I., Helmont Z.Ya., Zelyakh E.V. Piezoelectric filters. M. Communication, 1966, 396 pp.

Claims (1)

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

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