CN105320818A - Low pass filter and design method thereof - Google Patents

Abstract

The invention relates to a low pass filter and a design method thereof. The design method comprises the following steps: S0, the index is determined according to the design input condition of the low pass filter; S1, the normalized value of out-of-band specified frequency of the low pass filter is calculated; S2, the section number is determined according to an equal-ripple filter stop-band attenuation characteristic diagram; S3, a model diagram of the low pass filter is created according to the section number; S4, normalized values of all elements are determined according to a normalized element numerical table of an equal-ripple low pass prototype filter; S5, actual values of all elements are calculated according to the normalized values. According to the low pass filter designed with the design method, both input impedance and output impedance are 50 omega, the low pass filter can be matched with a front end antenna and rear end receiving equipment conveniently, about no more than 1 dB of effect is caused to sensitivity of downstream equipment, and the typical value is no more than 0.5 dB. At the same time, the low pass filter adopts a symmetric circuit, input and output can be exchanged, and the use is convenient.

Description

Low-pass filter and method for designing thereof

Technical field

The present invention relates to a kind of radio high frequency circuit engineering field, particularly relate to a kind of low-pass filter and method for designing thereof.

Background technology

In circuit and electronic system, wave filter is the important component part of automatic control, signal transacting and radio communication field, according to its attenuation characteristic, passes through selectively frequency, the signal needed is elected, and suppresses unwanted signal.

Existing short-wave receiver equipment, in its antenna input circuit, the quadravalence Low-pass Elliptic Filter be made up of inductance and capacity cell by, decays definitely by the signal outside receive frequency, improves the indexs such as picture frequency.But in actual use, due to the attenuation characteristic of Low-pass Elliptic Filter, its die-away curve there will be fluctuation of attenuation in the outer increase with frequency of passband, high-frequency interferencing signal particularly more than hundreds of megahertz to a Gigahertz can not be suppressed effectively, and the damping capacity of quadravalence Low-pass Elliptic Filter only has 30 to four ten points of shellfishes.

Summary of the invention

The object of the present invention is to provide a kind of simple and easy external low-pass filter that filter function is provided for external device.Utilize the present invention, can realize effectively suppressing to the high-frequency interferencing signal of a Gigahertz more than hundreds of megahertz.

In order to solve the problem, the invention provides a kind of method for designing of low-pass filter, comprising the following steps:

S0, the condition inputted according to low pass filter design, agriculture products;

The normalized value of the outer assigned frequency of S1, compute low pass filtered device band;

S2, according to equal-ripple filter stop band attenuation figure determination joint number;

S3, set up low-pass filter illustraton of model according to joint number;

S4, according to etc. ripple lowpass prototype filter normalization component values table determine the normalization numerical value of each element;

S5, actual value according to each element of normalization numerical evaluation.

As preferably, in step so), agriculture products comprises ripple, information source internal resistance and stopband attenuation in edge frequency, band.

In addition present invention also offers a kind of low-pass filter, designed by the method for designing of above-mentioned low-pass filter, comprise N joint; Each joint comprises inductive bank or capacitance group; N be greater than 1 odd number;

Inductive bank comprises M inductance, and capacitance group comprises K electric capacity; M and K be more than or equal to 1 integer;

Be connected in series between inductive bank, the common port of every two inductive bank is connected with one end of one group of capacitance group respectively; The other end ground connection of capacitance group.

As preferably, be with interior ripple α p≤0.2dB; Stopband attenuation α s >=60dB; N equals 9.

As preferably, described 9 joints comprise inductive bank L1, inductive bank L3, inductive bank L5, inductive bank L7, inductive bank L9, capacitance group C2, capacitance group C4, capacitance group C6, capacitance group C8;

Inductive bank L1, inductive bank L3, inductive bank L5, inductive bank L7, electric pole group L9 are sequentially connected in series; One end of capacitance group C2 is connected with the common port of inductive bank L3 with inductive bank L1, other end ground connection; One end of capacitance group C4 is connected with the common port of inductive bank L5 with inductive bank L3, other end ground connection; One end of capacitance group C6 is connected with the common port of inductive bank L7 with inductive bank L5, other end ground connection; One end of capacitance group C8 is connected with the common port of inductive bank L9 with inductive bank L7, other end ground connection;

The value of inductive bank L1 and inductive bank L9 is 0.368 μ H; The value of inductive bank L3 and inductive bank L7 is 0.613 μ H; The value of inductive bank L5 is 0.629 μ H; The value of capacitance group C2 and capacitance group C8 is 150pF; The value of capacitance group C4 and capacitance group C6 is 162pF.

As preferably, capacitance group C2 and capacitance group C8 comprises a 150pF electric capacity respectively; Capacitance group C4 and capacitance group C6 comprises a 150pF electric capacity and 12pF electric capacity respectively; 150pF electric capacity is connected with 12pF Capacitance parallel connection.As preferably, each inductance in inductive bank shields with radome respectively.

As preferably, also comprise PCB;

PCB is the double-deck copper foil circuit board of thickness 2mm, conductor thickness 0.5mm;

Inductive bank comprises J inductance, is positioned at the wherein side of PCB; Orthogonal between adjacent two inductance; J be more than or equal to 1 integer;

Radome comprises pin pad over the ground, and the spacing of adjacent two pins over the ground between pad is 5mm.

As preferably, the inductive bank being positioned at two ends is connected with radio frequency connector respectively; The radio frequency connector at two ends matches.

As preferably, radio frequency connector is connected with external unit by concentric cable.

The low-pass filter gone out designed by low pass filter design method provided by the invention, low-pass filter compared to existing technology, has following characteristics:

1, the impedance of input and output is 50 Ω, conveniently mates with front end antenna and rear end receiving apparatus.

2, the present invention selects the ripple LC low-pass filters such as employing one 9 joint, and the inside and outside flatness of band is better, and attenuation outside a channel 60dB has redundancy.

3, this passive low ventilating filter allows a small amount of insertion loss, requirement≤0.8dB.About have on the sensitivity of follow-up equipment the impact being not more than 1dB through actual measurement, representative value is not more than 0.5dB.

4, low-pass filter is symmetric circuit, input, output interchangeable, easy to use.

5, survey instrument range frequency and be 3GHz to the maximum, within the scope of maximum range, attenuation outside a channel 60dB has redundancy.

Accompanying drawing explanation

Fig. 1 is 2dB equal-ripple filter stop band attenuation figure in one embodiment of the invention.

Fig. 2 is that in one embodiment of the invention, joint number N is the low-pass filter electricity principle model figure of 9.

Fig. 3 is normalized 9 joint low-pass filter electricity principle model figure in one embodiment of the invention.

Fig. 4 is 9 joint low-pass filter true value electrical schematic diagrams in one embodiment of the invention.

Fig. 5 is 9 joint low-pass filter engineering circuit figure in one embodiment of the invention.

Fig. 6 is the low-pass filter engineering circuit figure in one embodiment of the invention after parameters revision.

Embodiment

For making the object, technical solutions and advantages of the present invention clearly, below in conjunction with accompanying drawing, the embodiments of the present invention are explained in detail.

For solving the problems of the technologies described above, the first embodiment of the present invention provides a kind of method for designing of low-pass filter, comprises the following steps:

S0, the condition inputted according to low pass filter design, agriculture products;

The normalized value of the outer assigned frequency of S1, compute low pass filtered device band;

S2, according to equal-ripple filter stop band attenuation figure determination joint number;

S3, set up low-pass filter illustraton of model according to joint number;

S4, according to etc. ripple lowpass prototype filter normalization component values table determine the normalization numerical value of each element;

S5, actual value according to each element of normalization numerical evaluation.

In the present embodiment, first according to the condition that required low pass filter design inputs, setting leading indicator.Four indexs that the present embodiment provides are respectively:

Edge frequency f _p=30MHz; Ripple α in band _p≤ 0.2dB;

Information source internal resistance R _o=50 Ω; Stopband attenuation α _s>=60dB.

The ripple type wave filter such as to select according to above-mentioned four indexs.

Specific design step is as follows:

S1, calculating f _snormalized value (f _srefer to the outer a certain assigned frequency of low-pass filter band):

Work as f _s=60MHz, then ${{\mathrm{\ω}}^{\′}}_s=\frac{{\mathrm{\ω}}_s}{{\mathrm{\ω}}_p}=\frac{60}{30}=2,$ Therefore $\frac{{{\mathrm{\ω}}^{\′}}_s}{{{\mathrm{\ω}}^{\′}}_p}-1=1.$

S2, determine required joint number N:

Check according to Fig. 1, when time, if α _s>=60dB, then N should get>=7.

In the present embodiment, N gets 9.

S3, set up low-pass filter electricity principle model figure:

When joint number N gets 9, obtain low-pass filter electricity principle model figure as shown in Figure 2.

Fig. 1 is when joint number N is 9, low-pass filter electricity principle model figure.Wherein, R _oinformation source internal resistance, R _1oit is loaded impedance.L1, C2, L3, C4, L5, C6, L7, C8, L9 are the inductive bank and the capacitance group that form wave filter.Each is inductive bank or capacitance group independently, is called a joint.

S4, tabling look-up obtains the normalized value of filter element

Checked in by ripple lowpass prototype filter normalization component values tables such as 0.2dB, during n=9, the normalization numerical value of each element is:

g ₁＝1.3860g ₁＝1.3938g ₃＝2.3093g ₄＝1.5340

g ₅＝2.3728g ₆＝1.5340g ₇＝2.3093g ₈＝1.3938

g ₉＝1.3860g ₁₀＝1.0000

Wave filter normalization is exactly impedance and frequency normalization.

L _ar=0.2dB, g ₀=1, ω ' _pthe table such as component values such as normalization such as lowpass prototype filter such as ripple such as grade of=1 is as follows:

n	g 1	g 2	g 3	g 4	g 5	g 6	g 7	g 8	g 9	g 10	g 11
												1	0.4342	1.0000
2	1.0378	0.6745	1.5386
												3	1.2275	1.1525	1.2275	1.0000
4	1.3028	1.2844	1.9761	0.8648	1.5386
												5	1.3394	1.3370	2.1660	1.3370	1.3394	1.0000
6	1.3598	1.3632	2.2394	1.4555	2.0974	0.8838	1.5386
												7	1.3722	1.3781	2.2756	1.5001	2.2756	1.3781	1.3722	1.0000
8	1.3804	1.3875	2.2963	1.5217	2.3413	1.4925	2.1349	0.8972	1.5386
												9	1.3860	1.3938	2.3093	1.5340	2.3728	1.5340	2.3093	1.3938	1.3860	1.0000
10	1.3901	1.3983	2.3181	1.5417	2.3904	1.5536	2.3702	1.5066	2.1514	0.9034	1.5386

Frequency normalization is with edge frequency ω _pfor reference, by the frequency of reality all divided by frequencies omega _p.Normalized like this edge frequency ω ' _p=1.

Impedance normalization is divided by characteristic impedance R each element impedance ₀( g ₀for input conductance, and G ₀=1).Like this, the component value of normalization wave filter and the component value of practical filter just have following relation:

$\begin{array}{ccc}{R}^{\′}=\frac{R}{R_0}& L^{\′}=\frac{{\mathrm{\ω}}_{p}L}{R_0}& C^{\′}={\mathrm{\ω}}_P{R}_0\end{array}$ (formula 1)

Normalized 9 joint low-pass filter electricity principle model figure as shown in Figure 3.

Wherein, G' ₀for input conductance normalized value, G' ₁₀for load conductance normalized value.

The true value of each element of S5, calculating filter

According to formula 1, the true value of each element of calculating filter:

$L_1=\frac{R_0{L^{\′}}_1}{{\mathrm{\ω}}_p}=\frac{R_0{g}_1}{{\mathrm{\ω}}_p}=\frac{50\×1.3860}{2\mathrm{\π}\×30\×{10}^6}=0.36765\mathrm{\μ}H$

$C_2=\frac{{C^{\′}}_2}{{\mathrm{\ω}}_p{R}_0}=\frac{g_2}{{\mathrm{\ω}}_p{R}_0}=\frac{1.3938}{2\mathrm{\π}\×30\×{10}^6\×50}=147.89pF$

$L_3=\frac{R_0{L^{\′}}_3}{{\mathrm{\ω}}_p}=\frac{R_0{g}_3}{{\mathrm{\ω}}_p}=\frac{50\×2.3093}{2\mathrm{\π}\×30\×{10}^6}=0.61256\mathrm{\μ}H$

$C_4=\frac{{C^{\′}}_4}{{\mathrm{\ω}}_p{R}_0}=\frac{g_4}{{\mathrm{\ω}}_p{R}_0}=\frac{1.5340}{2\mathrm{\π}\×30\×{10}^6\×50}=162.76pF$

$L_5=\frac{R_0{L^{\′}}_5}{{\mathrm{\ω}}_p}=\frac{R_0{g}_5}{{\mathrm{\ω}}_p}=\frac{50\×2.3728}{2\mathrm{\π}\×30\×{10}^6}=0.62941\mathrm{\μ}H$

C ₆＝C ₄＝162.76pF

L ₇＝L ₃＝0.61256μH

C ₈＝C ₂＝147.89pF

L ₉＝L ₁＝0.36765μH

$R_{10}=R_0{R^{\′}}_{10}=\frac{R_0}{{G^{\′}}_{10}}=\frac{R_0}{g_{10}}=\frac{50}{1}=50\mathrm{\Ω}$

By above-mentioned Synthesis designing low-pass filter circuit figure out and element true value as shown in Figure 4.

9 joint low pass filter element true value electrical schematic diagrams are converted into engineering circuit figure, as shown in Figure 5.

Wherein:

L ₁＝L ₉＝0.36765μH≈0.368μH

L ₃＝L ₇＝0.61256μH≈0.613μH

L ₅＝0.62941μH≈0.629μH

C ₂＝C ₈＝147.89pF≈148pF

C ₄＝C ₆＝162.76pF≈163pF

J1 is signal input part opening connector, and J2 is signal output part opening connector.

R _oinformation source internal resistance, R _o=50 Ω; Or the impedance of antenna output equivalent, its impedance is 50 Ω.Connected by J1, as the input impedance of this low-pass filter during actual use.

R _1oloaded impedance, by calculating R _1o=50 Ω, namely the signal input port equiva lent impedance of this low-pass filter rear end receiving apparatus, its impedance is 50 Ω.As the output impedance of this low-pass filter during actual use.

In the present embodiment, electric capacity adopts withstand voltage high radioceramic medium chip capacitor, and precision is ± 1%, need not draw pin capacitor its objective is that the distribution parameter removed and draw pin generation is on the impact of circuit with chip capacitor.

Ceramic dielectric capacitor, its electric property is stablized, and does not substantially change with the change of temperature, time, voltage; High frequency characteristics is good, can be operated in high frequency, superfrequency, very high frequency band well.

C ₂=C ₈=148pF=100pF+48pF, adopts these two electric capacity of 100pF and 48pF and the requirement of the next equivalent 148pF capacitance of the mode connect.

Equally, C ₄=C ₆=163pF=150pF+12pF+1pF, adopts these three electric capacity of 150pF, 12pF and 1pF and the requirement of the next equivalent 163pF capacitance of the mode connect.

Inductance, employing high q-factor, diameter are the polyester enameled round copper wire coiling voluntarily of φ 1.0mm, and inductance value can accomplish the numerical values recited of theory calculate, and deviation controls in very little scope.

L ₁=L ₉=0.368 μ H, error range: ± 0.002 μ H.

L ₃=L ₇=0.613 μ H, error range: ± 0.003 μ H.

L ₅=0.629 μ H, error range: ± 0.003 μ H.

Connector J1, J2, adopt L16 type radio frequency (RF) coaxial connector, 50 Ω are selected in characteristic impedance, are widely used in 0 ~ 10GHz microwave coaxial device and radio instrument.This connector, by concentric cable and other equipment connections.

Printed Circuit Board Design, the double-deck copper foil circuit board that employing thickness is 2mm, conductor thickness is 0.5mm.

Flaky electric capacity device placed by bottom (weld layer), and one end of electric capacity cabling short as far as possible with the pin of telefault is connected, and the other end is directly connected with ground wire; Top layer (device layer) lays telefault, orthogonal between adjacent two coils, to reduce the mutual inductance between them; Double-deck copper foil circuit board, except device, remainder adopts large area floor file mode to arrange plate, to guarantee the other end good earth of electric capacity; PCB goes back the Isolated Shield cover that cloth is placed with isolation telefault, and this radome comprises pin pad over the ground, and the spacing of pin over the ground between pad is 5mm.

During installation, with radome, each telefault shielding is got up, between coil and radome, have certain spacing, to reduce coil distribution parameter over the ground, also reduce the induced current in radome simultaneously, make the Q value of coil not by too much influence.

Device housings adopts high-strength aluminum alloy material, the ground pad of printed circuit board is directly connected with enclosure interior by screw, the shell of connector J1, J2 is directly fixed by screws in the left and right sides of device housings, the input and output heart yearn of connector J1, J2 is connected with the corresponding pad on printed circuit board by the silver-coated copper wire of 1mm wire diameter, the inner shell of connector fixes weld tabs by screw, nut, with the silver-coated copper wire of 1mm wire diameter, weld tabs is connected with the ground pad on printed circuit board again, requires that the cabling of silver-coated copper wire is the shortest.

The upper end cover of device housings, its end face is embedded with open circles silk screen shielding strip, plays the effect contacting put in place completely when attaching together with bottom end cover, to meet design of Electromagnetic Shielding demand.

During debugging, not change the inductance value of telefault, and change or adjustment capacitance meet the specification.Through debugging, capacitance parameters revision is as follows:

C ₂=C ₈=150pF; C ₄=C ₆=162pF=150pF+12pF, adopts 150pF and 12pF bis-electric capacity and the requirement of the next equivalent 162pF capacitance of the mode connect.

After tuning parameter correction, low-pass filter engineering circuit figure as shown in Figure 6.

The respective embodiments described above realize specific embodiments of the invention, persons of ordinary skill in the art may appreciate that and in actual applications, can do various change in the form and details to it, and without departing from the spirit and scope of the present invention.

Claims (10)

1. a method for designing for low-pass filter, is characterized in that, comprises the following steps:

S0, the condition inputted according to low pass filter design, agriculture products;

The normalized value of the outer assigned frequency of S1, compute low pass filtered device band;

S2, according to equal-ripple filter stop band attenuation figure determination joint number;

S3, set up low-pass filter illustraton of model according to described joint number;

S4, according to etc. ripple lowpass prototype filter normalization component values table determine the normalization numerical value of each element;

S5, actual value according to each element of described normalization numerical evaluation.

2. the method for designing of low-pass filter according to claim 1, is characterized in that, in described step S0, described agriculture products comprises ripple, information source internal resistance and stopband attenuation in edge frequency, band.

3. a low-pass filter, is characterized in that, comprises N joint; Each joint comprises inductive bank or capacitance group; Described N be greater than 1 odd number;

Described inductive bank comprises M inductance, and described capacitance group comprises K electric capacity; Described M and described K be more than or equal to 1 integer;

Be connected in series between described inductive bank, the common port of every two described inductive bank is connected with one end of capacitance group described in a group respectively; The other end ground connection of described capacitance group.

4. low-pass filter according to claim 3, is characterized in that, ripple α in described band _p≤ 0.2dB; Stopband attenuation α _s>=60dB; Described N equals 9.

5. low-pass filter according to claim 4, is characterized in that, described 9 joints comprise inductive bank L1, inductive bank L3, inductive bank L5, inductive bank L7, inductive bank L9, capacitance group C2, capacitance group C4, capacitance group C6, capacitance group C8;

Described inductive bank L1, inductive bank L3, inductive bank L5, inductive bank L7, electric pole group L9 are sequentially connected in series; One end of described capacitance group C2 is connected with the common port of described inductive bank L3 with described inductive bank L1, other end ground connection; One end of described capacitance group C4 is connected with the common port of described inductive bank L5 with described inductive bank L3, other end ground connection; One end of described capacitance group C6 is connected with the common port of described inductive bank L7 with described inductive bank L5, other end ground connection; One end of described capacitance group C8 is connected with the common port of described inductive bank L9 with described inductive bank L7, other end ground connection;

The value of described inductive bank L1 and described inductive bank L9 is 0.368 μ H; The value of described inductive bank L3 and described inductive bank L7 is 0.613 μ H; The value of described inductive bank L5 is 0.629 μ H; The value of described capacitance group C2 and described capacitance group C8 is 150pF; The value of described capacitance group C4 and described capacitance group C6 is 162pF.

6. low-pass filter according to claim 5, is characterized in that, described capacitance group C2 and described capacitance group C8 comprises a 150pF electric capacity respectively; Described capacitance group C4 and described capacitance group C6 comprises a 150pF electric capacity and 12pF electric capacity respectively; Described 150pF electric capacity is connected with described 12pF Capacitance parallel connection.

7. low-pass filter according to claim 3, is characterized in that, each inductance in described inductive bank shields with radome respectively.

8. low-pass filter according to claim 7, is characterized in that, also comprises PCB;

Described PCB is the double-deck copper foil circuit board of thickness 2mm, conductor thickness 0.5mm;

Described inductive bank comprises J inductance, is positioned at the wherein side of described PCB; Orthogonal between adjacent two described inductance; Described J be more than or equal to 1 integer;

Described radome comprises pin pad over the ground, and the spacing of adjacent two described pins over the ground between pad is 5mm.

9. low-pass filter according to claim 3, is characterized in that, the described inductive bank being positioned at two ends is connected with radio frequency connector respectively; The described radio frequency connector at two ends matches.

10. low-pass filter according to claim 9, is characterized in that, described radio frequency connector is connected with external unit by concentric cable.