RU2501156C2 - Highly selective second-order lc bandpass filter - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: highly selective LC bandpass filter has two inductors and eight capacitors which are connected so as to form two links, the first link providing an attenuation terminal lower than the medium frequency and the second link providing an attenuation terminal higher than the medium frequency.

EFFECT: high selectivity of the filter by realising two attenuation terminals at end frequencies and improving squareness of the attenuation characteristics.

5 dwg

Description

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

There are a number of band-pass LC filters used in the radio communication technique [1], among which there is a filter containing a first capacitor, the first output of which is connected to the input potential terminal of the filter, its second output is connected to the first output of the second capacitor and the first output of the first inductor, the second terminal of which is connected to a common bus through the third capacitor, the second terminal of the second capacitor is connected to the first terminal of the second inductor [1]. This scheme allows you to implement a first-class bandpass filter for attenuation with a pole of infinite attenuation at a finite frequency and obtain the most technologically advanced values of the elements. This type of filter is the closest in technical essence to the proposed device and is selected as a prototype.

The disadvantage of the filter of the prototype is the lack of selectivity at frequencies from zero to the lower limit of the passband.

The objective of the invention is to increase the selectivity of the filter.

The problem is solved in that in the filter containing the first and second capacitors, the first output of the 1st capacitor is connected to the input potential terminal of the filter, its second output is connected to the first output of the second capacitor and the first inductor, the second output of which is connected through the third capacitor to common bus, and the second terminal of the second capacitor is connected to the first terminal of the second inductor, additionally introduced fourth, fifth, sixth, seventh and eighth capacitors, while the fourth capacitor connected to the input potential terminal of the filter, its second output is connected to a common bus, the fifth capacitor is connected to the first output of the second inductor, the second output of which is connected to the sixth capacitor, the second output of which is connected to the output potential terminal of the filter, as well as to the seventh and eighth capacitors , the second terminals of the fifth and seventh capacitors are connected to a common bus, the second terminal of the eighth capacitor is connected to the first terminal of the second inductor.

Comparative analysis shows that the claimed technical solution differs from the prototype in that the fourth, fifth, sixth, seventh and eighth capacitors are additionally introduced into the device, while the fourth capacitor is connected to the input potential terminal of the filter, its second output is connected to a common bus, the fifth capacitor connected to the first terminal of the second inductor, the sixth capacitor is connected to the second terminal, the second terminal of which is connected to the output potential terminal of the filter, as well as from the seventh and eighth capacitors, the second terminals of the fifth and seventh capacitors are connected to a common bus, a second terminal of the eighth capacitor connected to the first terminal of the second inductor.

When comparing the claimed solution, not only with the prototype, but also with other solutions known in science and technology, solutions with similar features were not found.

Figure 1 shows the electrical diagram of the proposed device. The device consists of a first capacitor 1, to the first output of which is connected a second capacitor 2 and a first inductor 3, which is connected through a third capacitor 4 to a common bus. The second output of the second capacitor 2 is connected to the second inductor 5, the fourth additional capacitor 6 is connected to the first capacitor 1 and the input potential terminal of the filter, its second output is connected to a common bus. The fifth additional capacitor 7 is connected to the first terminal of the second inductor, the sixth capacitor 8 is connected to the second terminal of the second, the second terminal of the sixth capacitor is connected to the output potential terminal of the filter, as well as to the seventh capacitor 9 and the eighth capacitor 10, the second terminal of which is connected to the first terminal second inductor. The second terminals of the fifth and seventh capacitors are connected to a common bus.

The device operates as follows.

The filter can be represented in the form of two links, the first link includes an inductor 3 and capacitors 1, 2, 4, 6 and a part of the capacitor 7. The remaining part of the circuit is a triple link. Each of these links is potentially equivalent to the symmetrical bridge circuit shown in figa.

In accordance with fig.2b, which shows the frequency dependence of the resistance of the branches Z ₁ and Z _{2 of the} circuit shown in figa, this circuit is a filter with a passband between frequencies ω ₁ and ω ₂ , one pole attenuation at a frequency ω _x .

Elements of this scheme can be calculated by the formulas [2]:

$$\begin{array}{ccc}{L}_{\mathrm{one}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="Z\; m\; m"\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">Z\; </figure-callout>}}_{m}m}{2\mathrm{<figure-callout\; id="2"\; label="\Delta \; \omega \; \omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}\omega }\sqrt{\frac{{\omega }_{}^{}}{}}\sqrt{\frac{{}_2^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}{{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2-{\omega }_{\mathrm{one}}^2}},& & C_{\mathrm{one}}=\frac{2\Delta \omega }{{\mathrm{<figure-callout\; id="0"\; label="Z\; m\; \omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">Z\; </figure-callout>}}_m{\omega }_{}^{}{}_0^2\mathrm{<figure-callout\; id="0"\; label="m\; \omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">m\; </figure-callout>}}\sqrt{\frac{{\omega }_{}^{}}{}}\sqrt{\frac{{}_0^2-{\omega }_{\mathrm{one}}^2}{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}},\\ {\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">C</figure-callout>}}_2=\frac{m}{{\mathrm{<figure-callout\; id="0"\; label="Z\; m\; \omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">Z\; </figure-callout>}}_m{\omega }_{}{}_0}\sqrt{\frac{{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2-{\omega }_{\mathrm{one}}^2}{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}},& & C_3=\frac{\mathrm{one}}{Z_m{\omega }_0\mathrm{<figure-callout\; id="0"\; label="m\; \omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">m\; </figure-callout>}}\sqrt{\frac{{\omega }_{}^{}}{}}\sqrt{\frac{{}_0^2-{\omega }_{\mathrm{one}}^2}{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}},\\ m_{\mathrm{one}}=\sqrt{\frac{C_2}{C_3}}=\sqrt{\frac{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\omega }_x^2}{{\omega }_{\mathrm{one}}^2-{\omega }_x^2}},& \Delta \omega ={\omega }_2-{\omega }_{\mathrm{one}},& {\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0=\sqrt{{\omega }_{\mathrm{one}}{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2},\\ Z_m=\frac{\mathrm{one}}{{\omega }_0\sqrt{C_2{C}_3}}\cdot \sqrt{\frac{{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2-{\omega }_{\mathrm{one}}^2}{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}},& {\omega }_{\mathrm{one}}^2=\frac{\mathrm{one}}{L_{\mathrm{one}}{C}_{\mathrm{one}}},& {\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_2^2=\frac{C_{\mathrm{one}}+C_3}{L_{\mathrm{one}}{C}_{\mathrm{one}}{C}_3}.\end{array}\left(\mathrm{one}\right)$$

Here Z _m is the nominal characteristic resistance;

ω ₀ is the average filter frequency;

Δω is the bandwidth.

. Емкость С₀ будет положительной, если С₁<С₂-С₃. Чтобы С₁ была меньше чем С₂-С₃ необходимо выполнить следующее условие:The value m characterizing the position of the frequency of infinite damping ω _x may be greater than unity for 0 <ω _x <ω ₁ , or m <1 for ω ₂ <ω _x <∞. From (1) it follows that C ₂ = C ₃ · m ² , therefore, in the first case C ₂ > C ₃ , in the second C ₂ <C ₃ . In the first case, we take C ₃ out of the bridge circuit and get the circuit shown in FIG. 3, in which the capacitance ΔC = C ₂ -C _{3 is} taken out as a sequential branch. In this case, the capacitance C ₁ can be represented by a series connection of the capacitance ΔC and $${\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">C</figure-callout>}}_0=\frac{C_{\mathrm{one}}\cdot \mathrm{<figure-callout\; id="2"\; label="\Delta \; C"\; filenames="00000007.png,00000008.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{C}_{}{}_2}{\Delta C-C_{\mathrm{one}}}$$

. The capacity C ₀ will be positive if C ₁ <C ₂ -C ₃ . In order for C _{1 to} be less than C ₂ -C ₃ , the following condition must be fulfilled:

,m ² > 1 + 2γ, where $$\gamma =\frac{\mathrm{<figure-callout\; id="0"\; label="\Delta \; \omega \; \omega "\; filenames="00000006.png,00000009.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}\omega }{{\omega }_{}}$$

,

which follows from relations (1). This means that this scheme is not implemented for all values of m. For filters with relative passbands greater than, for example, 20%, m must be greater than 1.1. This limitation is not critical for filters with a relative passband of less than 20%.

In the second case, when m <1 and C ₂ <C ₃ , we take out the capacitance C ₂ outside the bridge and come to the scheme shown in Fig. 4.

Including cascading circuits shown in figure 3 and figure 4, we obtain a diagram of the proposed device. Note that both links have the same characteristic impedances and their characteristic attenuation are summed. The first link provides the implementation of the attenuation pole below the average frequency, the second - above the average filter frequency.

The proposed bandpass filter circuit has the smallest possible number of inductive elements and allows two poles of attenuation to be realized at finite frequencies and, therefore, provides a better rectangularity of the attenuation characteristics.

Figure 5 shows the frequency response of the filter, the circuit of which corresponds to the proposed option, at a frequency of 15 MHz, with a passband of 370 kHz. When using inductors with a fraction of 200, the insertion loss in the passband is 2.5 dB. The coefficient of squareness at the levels of 20 dB / 3 dB is equal to 3.5.

Information sources

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

2. The Great Ya.I., Helmont Z.Ya., Zelyakh E.V. Piezoelectric filters. M .: Communication, 1966, 396 pp.

Claims (1)

A band-pass LC filter containing the first and second capacitors, the first output of the 1st capacitor is connected to the input potential terminal of the filter, its second output is connected to the first output of the second capacitor and the first inductor, the second output of which is connected to the common bus through the third capacitor, and the second terminal of the second capacitor is connected to the first terminal of the second inductor, characterized in that the fourth, fifth, sixth, seventh and eighth capacitors are additionally introduced into the circuit, while the fourth capacitor connected to the input potential terminal of the filter, its second output is connected to a common bus, the fifth capacitor is connected to the first output of the second inductor, the second output of which is connected to the sixth capacitor, the second output of which is connected to the output potential of the filter, as well as to the seventh and eighth capacitors , the second terminals of the fifth and seventh capacitors are connected to a common bus, the second terminal of the eighth capacitor is connected to the first terminal of the second inductor.

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