RU2544769C2 - Bandpass filter - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to radio electronics and can be used for partial selection of high-frequency signals in radio devices, television, communication systems and radio links for transmitting telecommunication data. The bandpass filter comprises first and second parallel circuits with partial inductive connection at the input and output of the filter, respectively, the resonance frequency f₀ of which is equal to the band centre f₀, a series communication circuit, also connected to the first and second parallel circuits with partial inductive connection; the input and output of the bandpass filter are connected in parallel to balancing capacitors, and the capacitance of the series communication circuit is equal to the capacitance of the parallel circuits with partial inductive connection.

EFFECT: reduced non-uniformity of frequency response in the filter band pass.

3 dwg

Description

The invention relates to electronics and can be used for frequency selection of high-frequency signals in radio devices, television, communication systems and radio channels for transmitting telecommunication data.

Known band-pass filter made on the basis of parallel oscillatory circuits with external inductive or external capacitive coupling (see book Alekseev L.V., Znamensky A.E., Lotkova E.D. Electric filters of meter and decimeter ranges. - M. : Communication, 1976, 280 p .; Fig. 3.1, a, b, p. 85). These filters contain several connected parallel oscillatory circuits and are widely used in the meter and decimeter frequency ranges (up to 1500 MHz). The main disadvantage of this filter is the poor physical feasibility of elements of parallel oscillatory circuits with a narrow passband. In accordance with the theory of filters, the ratio that determines the capacitance of the capacitors of parallel circuits has the form

$$C_k=\frac{{\mathrm{<figure-callout\; id="2"\; label="\alpha \; k"\; filenames="00000018.png"\; state="\{\{state\}\}">\alpha \; </figure-callout>}}_k}{2\pi \Delta f\cdot R},(\mathrm{one})$$

where k is the number of the parallel filter loop; α _k - k-th element of the low-frequency normalized prototype; Δf is the passband of the filter; R - load resistance for the filter at the input and output.

As follows from (1), in narrow-band filters, with a decrease in the passband Δf, the required capacitance of the parallel circuit capacitors C _k increases significantly, which at a constant resonant frequency of the circuits causes a decrease in their inductances. As a result, the quality factor of such parallel loops is low. This leads to a significant increase in direct losses in the passband of the filter.

A bandpass filter made on the basis of successive oscillatory circuits with internal capacitive or internal inductive couplings has somewhat lower direct losses (see the book Alekseev L.V., Znamensky A.E., Lotkova E.D. Electric filters of meter and decimeter ranges . - M .: Communication, 1976, 280 p .; fig. 3.1, c, d, p. 85). The magnitude of the inductance of successive oscillatory circuits is determined by the following well-known relation:

$$L_k=\frac{{\alpha }_k\cdot \mathrm{<figure-callout\; id="2"\; label="R"\; filenames="00000018.png"\; state="\{\{state\}\}">R</figure-callout>}}{2\pi \cdot \Delta f}.(2)$$

In accordance with (2), with a decrease in the passband Δf, the inductance L _k increases substantially, which leads to a decrease in the intrinsic Q factor of successive oscillatory circuits. This is due to the fact that successive oscillatory circuits with a large inductance (a large number of turns) and low capacitance due to the manifestation of the surface effect at high frequencies have a low Q factor and, accordingly, rather large direct losses in the filter passband.

Also known is a band-pass filter, which is the prototype of the invention (see the book Red E. Reference manual on high-frequency circuitry: Circuits, blocks, 50 ohm technology: Translated from German.- M .: 1990 - 256 p., P. 40, Fig. 1.45, diagram in the lower left corner) and containing the first and second parallel circuits with partial inductive switching, respectively, at the input and output, a serial communication circuit connected also to parallel circuits with partial inductive switching and configured as the first and second parallel loops with partial inductive switching on to the central frequency of the passband, while the inductive switching coefficient of the serial communication loop is selected so that the capacity of the serial communication loop is equal to the capacity of the parallel circuits with partial inductive switching.

A positive property of the prototype is the good physical realizability of the inductors and capacitors of the parallel circuits with partial inductive inclusion due to the fact that when partially switched on, the capacitance of the capacitors of the parallel circuits is significantly reduced and accordingly equal

$$C_k=\frac{{\alpha }_k\cdot {\mathrm{<figure-callout\; id="2"\; label="p\; L\; k"\; filenames="00000018.png"\; state="\{\{state\}\}">p\; </figure-callout>}}_{L_k}^{{}_{}}}{2\pi \Delta f\cdot R},(3)$$

- коэффициент индуктивного включения параллельного контура $$(p_{L_k}<<1)$$

.Where $$p_{L_k}$$

- coefficient of inductive inclusion of a parallel circuit $$(p_{L_k}<<\mathrm{one})$$

.

, здесь f₀ - центральная частота полосы пропускания фильтра) выбор малого значения коэффициента включения $$p_{L_k}$$

позволяет за счет трансформирующих свойств параллельных контуров существенно уменьшить емкость конденсаторов C_k и получить высокую добротность параллельных контуров с частичным индуктивным включением в метровом и дециметровом диапазоне длин волн. Это в свою очередь обеспечивает малую величину прямых потерь фильтра. Однако частичное включение параллельных контуров обладает существенной частотной дисперсией трансформирующих свойств. Идеальная трансформация сопротивлений осуществляется только на резонансной частоте параллельного колебательного контура с частичным как индуктивным, так и емкостным включением. Поэтому в полосно-пропускающем фильтре (прототипе), в котором используется частичное индуктивное включение параллельных контуров по входу и выходу и частичное индуктивное включение контура связи, существенно искажается форма амплитудно-частотной характеристики (АЧХ). При этом, чем больше полоса пропускания фильтра, тем сильнее искажается форма АЧХ. Таким образом, основным недостатком прототипа является большая неравномерность АЧХ в полосе пропускания.An analysis of relations (1) and (3) shows that in narrow-band filters ( $$\frac{\mathrm{<figure-callout\; id="0"\; label="\Delta \; f\; f"\; filenames="00000017.png,00000018.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}f}{f_{}}\approx 0,\mathrm{one}÷0,01$$

, here f ₀ is the central frequency of the filter passband) selection of a small value of the switching coefficient $$p_{L_k}$$

due to the transforming properties of the parallel loops, it is possible to significantly reduce the capacitance C _k and to obtain a high quality factor of the parallel loops with partial inductive inclusion in the meter and decimeter wavelength ranges. This in turn provides a small amount of direct filter loss. However, the partial inclusion of parallel circuits has a significant frequency dispersion of transforming properties. An ideal transformation of resistances is carried out only at the resonant frequency of a parallel oscillatory circuit with partial inductive and capacitive switching. Therefore, in the band-pass filter (prototype), which uses a partial inductive connection of parallel circuits at the input and output and a partial inductive connection of the communication circuit, the shape of the amplitude-frequency characteristic (AFC) is significantly distorted. In this case, the greater the passband of the filter, the more distorted the shape of the frequency response. Thus, the main disadvantage of the prototype is the large non-uniformity of the frequency response in the passband.

The task of the invention is to reduce the unevenness of the amplitude-frequency characteristics in the passband of the filter.

The problem is achieved by the fact that in the known filter parallel to the input and output connected additional correcting capacitors, the capacity of which is

, $$C=\frac{\sqrt{\frac{R_0}{R}}-\mathrm{one}}{R_0\cdot R}\cdot L$$

,

where R is the value of the load resistance for the bandpass filter,

R ₀ is the resonant characteristic impedance of the bandpass filter,

L is the inductance of the parallel circuits with a partial inductive inclusion, while the inductance of the series coupling circuit is chosen equal to

, $$L\text{'}\text{'}=L\cdot \frac{{\left(Q_f-\mathrm{one}\right)}^2+\mathrm{one}}{{\mathrm{<figure-callout\; id="2"\; label="Q\; f"\; filenames="00000018.png"\; state="\{\{state\}\}">Q\; </figure-callout>}}_f^{}}$$

,

- добротность полосно-пропускающего фильтра.Where $$Q_f=\frac{{\mathrm{<figure-callout\; id="0"\; label="f"\; filenames="00000017.png,00000018.png"\; state="\{\{state\}\}">f</figure-callout>}}_0}{\Delta f}$$

- Q-factor of the band-pass filter.

Figure 1 shows the electrical circuit diagram of the proposed filter. Figure 2 shows the frequency response of the filter with additional corrective capacitors (solid curve) and prototype (dashed curve) with the full inclusion of the communication loop. Figure 3 shows the frequency response of the proposed filter with additional correcting capacitors for two values of the inductance of the serial communication circuit: 1) the value specified in the claims (solid curve), 2) the value equal to the inductance of the parallel circuits with a partial inductive inclusion (dashed curve).

The proposed filter (figure 1) contains the first and second parallel circuits with a partial inductive inclusion 1, 2, a serial communication circuit 3, in which the beginning of the inductor 4 is connected to a common housing, and its end is connected to the beginning of the inductor 5, the end of which is connected with a bandpass filter input. The common point of connection of the terminals of the inductor 4 and the inductor 5 is connected to the beginning of the inductor 8 of the serial communication loop 3. The beginning of the inductor 6 of the first parallel circuit with a partial inductance 1 is connected to the input of the bandpass filter, and its end is connected to one of the terminals capacitor 7 of the first parallel circuit with a partial inductive inclusion 1, while the other output of the capacitor 7 is connected to a common housing. To the end of the inductance coil 8 of the serial communication circuit 3, one terminal of the capacitor 9 of the serial communication circuit 3 is connected, the other terminal of which is connected to the end of the inductance coil 13 of the second parallel circuit with a partial inductance 2, and the beginning of the inductor 13 is connected to a common housing. The beginning of the inductor 11 of the second parallel circuit with a partial inductive switch 2 is connected to the output of the pass-pass filter, and its end is connected to one of the terminals of the capacitor 10 of the second parallel circuit with a partial inductive switch 2, while the other terminal of the capacitor 10 is connected to a common housing. The beginning of the inductance coil 12 of the second parallel circuit with a partial inductive switch 2 is connected to the common point of the capacitor 9 of the serial communication circuit 3 and the inductor 10 of the second parallel circuit with a partial inductive switch 2, and its end is connected to the output of the bandpass filter. Additional correction capacitors 14 and 15 are included respectively between the input and output of the bandpass filter and the common housing.

Note that the connection with a common housing in the proposed device refers to a common bus with zero potential.

,The proposed bandpass filter operates as follows. As can be seen from the consideration of the structure shown in Fig. 1, the band-pass filter is a classic third-order polynomial filter in which a partial inductive switching of the first and second parallel circuits 1, 2 is used, the resonant frequencies of which are the same and equal $$f_{}$$$${}_0=\frac{\mathrm{one}}{2\pi \sqrt{(L\mathrm{one}+\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="00000018.png"\; state="\{\{state\}\}">L</figure-callout>}2+L3)\cdot C\mathrm{one}}}=\frac{\mathrm{one}}{2\pi \sqrt{(L\mathrm{four}+\mathrm{<figure-callout\; id="5"\; label="L"\; filenames="00000018.png"\; state="\{\{state\}\}">L</figure-callout>}5+L6)\cdot \mathrm{<figure-callout\; id="2"\; label="C"\; filenames="00000018.png"\; state="\{\{state\}\}">C</figure-callout>}2}}=\frac{\mathrm{one}}{2\pi \sqrt{L\cdot C\mathrm{one}}}=\frac{\mathrm{one}}{2\pi \sqrt{L\cdot \mathrm{<figure-callout\; id="2"\; label="C"\; filenames="00000018.png"\; state="\{\{state\}\}">C</figure-callout>}2}}$$

,

where L = L1 + L2 + L3 = L4 + L5 + L6 are the inductances of parallel circuits with a partial inductive inclusion 1, 2.

индуктивность последовательного контура связи 3 оказывается равной индуктивности параллельных контуров с частичным индуктивным включением 1, 2, то есть L'=L.As you know, parallel circuits with partial inductive as well as capacitive switching have the ability to transform the connected loads R at the resonant frequency into the value of the resonant characteristic resistance R ₀ , that is, to provide a given transformation coefficient. However, at other frequencies, the value of the transformation coefficient changes. In the proposed filter, the influence of the frequency of the transformation coefficient dependence, which is called the frequency dispersion, is compensated by connecting additional correction capacitors 14, 15 and a decrease in the inductance of the series coupling circuit 3 in accordance with the mathematical expression given in the claims. From this expression it follows that for Q _f > 1 the condition L '<L is satisfied. With a decrease in inductance L ', the resonant frequency of the series coupling circuit 2 increases. Thus, in the proposed filter, the parallel circuits with partial inductance 1, 2 and the series coupling circuit 3 have different resonant frequencies, which together with additional correction capacitors 14, 15 reduces unevenness Frequency response in the passband. Note that under the condition R ₀ = R, that is, when parallel circuits 1, 2 are fully turned on at the input and output, in accordance with the ratio given in the claims, the capacitance value of the additional correction capacitors 14, 15 is zero. In addition, with a wide bandwidth $$(Q_f=\frac{{\mathrm{<figure-callout\; id="0"\; label="f"\; filenames="00000017.png,00000018.png"\; state="\{\{state\}\}">f</figure-callout>}}_0}{\Delta f}=\mathrm{one})$$

the inductance of the series coupling circuit 3 is equal to the inductance of the parallel circuits with a partial inductive inclusion 1, 2, that is, L '= L.

Quantitative evidence that the proposed bandpass filter provides a reduction in frequency response unevenness is performed using computer simulation in the frequency domain for the following source data: R ₀ = 1500 Ohm, R = 50 Ohm, Δf = 8 MHz, f ₀ = 210 MHz . For this example, according to the methodology described in the book by Red E. Reference manual on high-frequency circuitry: Circuits, blocks, 50 ohm technology: Per. with him. - M .: 1990 - 256 S., and according to the ratios indicated in the claims, the values of the elements of the proposed filter, shown in figure 1, were calculated:

C1 = C2 = C3 = 11.32 pF; C4 = C5 = 3.031 pF; L1 = L10 = 1.993 nH;

L2 = L9 = 7.277 nG; L3 = L6 = 41.50 nH; L = L1 + L2 + L3 = L4 + L5 + L6 = 50.77 nG.

When calculating the above values of the filter elements, the following values of the elements of the low-frequency normalized prototype were used: α ₁ = 0.8533; α ₂ = 1,1036; α ₃ = 0.8533.

Since the first and second parallel circuits 1, 2 of the proposed filter carry out partial inductive switching for both input (output) and for the serial communication circuit 3, we consider separately the effect of each partial inductive switch on the shape of the frequency response.

The results of computer simulation of the frequency response of the proposed filter with the full inclusion of the serial communication loop 3 and for the prototype are shown in figure 2 (the frequency response of the prototype is shown by a dashed line, and the frequency response of the proposed filter with additional correction capacitors 14, 15 is shown by a solid line). For modeling, S-parameters were used, which are called scattering parameters. In particular, the parameter S ₂₁ corresponds to the transmission coefficient and describes the frequency response of the filter. As can be seen from the consideration of the graphs of figure 2, the introduction of additional correction capacitors 14, 15, the capacitance of which is selected according to the ratio given in the claims, reduces the frequency response unevenness in the passband.

The effect of reducing the inductance of the serial communication loop 3 on the frequency response form is shown in the graphs of Fig. 3, obtained also by computer simulation for the above parameters of the proposed filter. In Fig. 3, the dashed line shows the frequency response for the same values of the inductance of the parallel circuits with a partial inductance 1, 2 and the inductance of the serial communication circuit 3. As can be seen from the consideration of figure 3, the frequency response in this case has an unacceptably large non-uniformity. When choosing the inductance of the serial communication loop 3 according to the ratio given in the claims, it is possible to reduce the unevenness of the frequency response in the passband (solid curve in figure 3). At the same time, computer simulation showed that the frequency response of the filter received a slight shift down the frequency axis by (0.5-1)%, which is eliminated by an increase in the initial data for calculating the parameters of the filter elements by (0.5-1)% of the central frequency bandwidth f ₀ .

Thus, the proposed band-pass filter has a significantly lower non-uniformity of the frequency response compared to the prototype.

In addition, in the meter and decimeter wavelength ranges, the proposed filter with a passband of less than 5% can reduce direct losses due to the high quality factor of parallel circuits with partial inductive switching 1, 2. Note also that additional correction capacitors 14, 15 can be performed in in the form of contact pads on which surface-mounted induction coils and band-pass filter capacitors are installed, which improves the design of the filter. In the microwave range, the bandpass filter is technologically tuned by changing the inductance of the frameless coils, and constant capacitors are used as filter capacitors. This tuning method allows you to shift the operating frequency band within ± 10% along the frequency axis and to ensure fine tuning of the filter to a given passband.

Claims (1)

A band-pass filter containing first and second parallel circuits with partial inductive switching, respectively, at the input and output, the resonant frequency of which is equal to the central frequency of the passband, a serial communication circuit connected also to the first and second parallel circuits with partial inductive switching, while the coefficient partial inductive inclusion of the serial communication circuit is selected so that the capacity of the serial communication circuit is equal to the capacitance of the parallel circuit s with partial inclusion of the induction, characterized in that the parallel input and output band-pass filter connected corrective additional capacitors whose capacitance is equal to

,
where R is the value of the load resistance for the bandpass filter, R ₀ is the resonant characteristic resistance of the bandpass filter, L = L1 + L2 + L3 = L4 + L5 + L6 is the resulting inductance of the first and second parallel circuits with partial inductive switching, respectively in this case, the inductance of the serial communication loop is chosen equal to

,
Where

- Q-factor of the band-pass filter.

Images