Noise filter circuit

Abstract

To provide a noise filter circuit that can reduce the effect of noise on a peripheral circuit while suppressing the propagation of noise emitted from a noise source to a transmission line that is farther from the noise source than a place where an inductor or a noise filter is placed.SOLUTION: A noise filter circuit 11 includes a capacitor 15, and a choke inductor or a noise filter 16. The capacitor 15 is shunt-connected between a transmission line 13 near a noise source 12 and a ground 14, which is a reference voltage. The choke inductor or the noise filter 16 is connected in series to the transmission line 13 at an arrangement point B farther from the noise source 12 than the connection point A of the capacitor 15 to the transmission line 13.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a noise filter circuit in which a capacitor is shunt-connected to a transmission line near a noise source, and an inductor or a noise filter is connected to the transmission line farther from the noise source than the connection point of the capacitor to the transmission line. [Background technology]

[0002] Conventionally, this type of noise filter circuit is used for communications in which the transmission frequency band and the reception frequency band are close to each other, such as the WCDMA (registered trademark) system disclosed in Patent Document 1, for example.

[0003] This noise filter circuit, for example, as shown in the circuit diagram in Figure 1, is composed of a band-stop filter 3 and is provided at the output section of a switching-type DC-DC converter 1. The DC-DC converter 1 supplies power to the transmitter / receiver circuit of the wireless device via a power amplifier 2. The band-stop filter 3 is composed of a chip capacitor C1 and an inductance L1 connected in parallel, and is connected to the DC-DC converter 1 via a power inductor 4. The side of the power inductor 4 facing the band-stop filter 3 is connected to ground via a current-smoothing capacitor 5.

[0004] When a power supply signal containing switching noise and ringing noise from DC-DC converter 1 and a communication signal in the wireless communication band are input to power amplifier 2, noise corresponding to the noise superimposed on the power supply signal occurs in the frequency bands on both sides of the communication signal in addition to the communication signal, due to the nonlinear characteristics (distortion) of power amplifier 2. However, by using band-stop filter 3 to block the passage of noise in the frequency range that is the difference between the receiving frequency band and the transmitting frequency band, the noise occurring in the frequency bands on both sides of the communication signal is reduced. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-187446 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the conventional noise filter circuit disclosed in Patent Document 1, noise N1 that emerges from DC-DC converter 1 and is conducted as indicated by the solid arrow, and reflected noise N2 that emerges from DC-DC converter 1, is reflected by bandstop filter 3, and is conducted as indicated by the dashed arrow, flow out to ground from current smoothing capacitor 5 that is shunt-connected to the output of DC-DC converter 1. For this reason, when the conventional noise filter circuit described above is used in a communication circuit that receives power from DC-DC converter 1, noise N1 and N2 that have flowed out to ground spread throughout the entire device, degrading communication quality. [Means for solving the problem]

[0007] The present invention has been made to solve such problems. a capacitor provided between the transmission line near the noise source and the reference voltage; The capacitor is provided on the transmission line at a location farther from the noise source than the connection point with the transmission line, and conducts noise from the noise source to the transmission line. 2400[MHz]~2500[MHz] frequency band an inductor or a noise filter that suppresses the propagation of noise to a transmission line that is farther from the noise source than the placement point, It is composed of , The phase at the connection point of a reflected noise wave, which is a noise that is transmitted from a noise source through a transmission line to an arrangement point and is reflected by an inductor or a noise filter and returns to the connection point, and the phase of an incident noise wave that is transmitted from the noise source through the transmission line and is incident on the connection point, are The noise that is transmitted from the noise source through the transmission line and reaches the placement location is a transmission line between the connection point and the placement point; Going The phase difference is set to 180°, which is the sum of the phase delay due to the return of the signal and the phase change due to the reflection of the noise wave at the inductor or noise filter. The noise filter circuit is configured such that the placement location is set at a position where a node of a standing wave of noise transmitted through the transmission line appears at the connection location.

[0008] According to this configuration, at the connection point between the capacitor shunt-connected near the output part of the noise source and the transmission line, the phases of the reflected noise wave that comes from the noise source, is reflected at the location where the inductor or noise filter is disposed, and reaches the connection point, and the incident noise wave that comes from the noise source and is incident on the connection point are different from each other. 180° Therefore, the amplitude of the composite wave of the reflected noise wave and the incident noise wave is suppressed, and the magnitude of the noise flowing out from the connection point via the capacitor and the reference potential is reduced. Therefore, the inductor or noise filter can suppress the noise emitted from the noise source from propagating to a transmission line farther from the noise source than the location of the inductor or noise filter, while reducing the impact of the noise on peripheral circuits. [Effects of the Invention]

[0009] As a result, the present invention can provide a noise filter circuit that can reduce the effects of noise on surrounding circuits while suppressing the propagation of noise emitted from a noise source to a transmission line that is farther from the noise source than the location of the inductor or noise filter. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a circuit diagram showing a conventional noise filter circuit. [Figure 2] FIG. 1 is a circuit diagram showing a noise filter circuit according to a first embodiment of the present invention, and is a diagram for explaining the basic principle of the present invention. [Figure 3] 2 is a graph showing an example of the relationship between reflected noise waves and incident noise waves in the noise filter circuit shown in FIG. [Figure 4] 2 is a circuit diagram showing a state in which a load is connected to the output terminal of the noise filter circuit shown in FIG. 1, and is a diagram for explaining the mechanism of the present invention. FIG. [Figure 5] 5 is a graph showing the relationship between the path length and the phase difference between an incident noise wave and a reflected noise wave, and the relationship between the path length and the amplitude of a composite wave of the incident noise wave and the reflected noise wave in the noise filter circuit shown in FIG. [Figure 6] FIG. 4 is a circuit diagram showing a noise filter circuit according to a second embodiment of the present invention. [Figure 7] 7 is a graph showing the relationship between the amplitude of a composite wave of an incident noise wave and a reflected noise wave and the path length in the noise filter circuit shown in FIG. 6. [Figure 8] 7 is a graph comparing the spectrum of noise flowing through a capacitor in the noise filter circuit shown in FIG. 6 when the path length value is set to a suitable value and when the path length value is set to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, an embodiment of a noise filter circuit according to the present invention will be described.

[0012] FIG. 2 is a circuit diagram showing a schematic configuration of a noise filter circuit 11 according to a first embodiment of the present invention, and is a diagram for explaining the basic principle of the present invention.

[0013] The noise filter circuit 11 includes a capacitor 15 and a choke inductor or noise filter 16. The capacitor 15 is shunt-connected between a transmission line 13 near a noise source 12 and ground 14, which is a reference voltage. The choke inductor or noise filter 16 is connected in series to the transmission line 13 at an arrangement point B, which is farther from the noise source 12 than a connection point A of the capacitor 15 with the transmission line 13.

[0014] This choke inductor or noise filter 16 suppresses the propagation of noise conducted from noise source 12 through transmission line 13 to transmission line 13 that is farther from noise source 12 than arrangement point B. Furthermore, at the same time, noise N2 that is conducted from noise source 12 through transmission line 13 in the direction indicated by the dotted arrow and reaches arrangement point B is reflected to connection point A, and the phase of reflected noise wave N2 at connection point A is shifted by a predetermined phase from the phase of incident noise wave N1 that is conducted from noise source 12 through transmission line 13 in the direction indicated by the solid arrow and enters connection point A.

[0015] In the noise filter circuit 11 according to the present embodiment, at connection point A between the transmission line 13 and the capacitor 15, which is shunt-connected near the output section of the noise source 12, as described above, there is a predetermined phase shift between the phases of the reflected noise wave N2 that emerges from the noise source 12, is reflected at location B where the inductor or noise filter 16 is located, and reaches connection point A, and the incident noise wave N1, which is a noise traveling wave that emerges from the noise source 12 and is incident on connection point A.

[0016] 3 is a graph showing an example of the relationship between the reflected noise wave N2, which is a voltage noise, and the incident noise wave N1. The horizontal axis of the graph represents time, and the vertical axis represents the magnitude of the voltage noise at connection point A, i.e., at capacitor 15. The reflected noise wave N2 is represented by a dotted line, and the incident noise wave N1 is represented by a solid line. In this graph, the reflected noise wave N2 and the incident noise wave N1 are 180° out of phase with each other, and the reflected noise wave N2 is inverted relative to the incident noise wave N1.

[0017] Therefore, the noise filter circuit 11 according to this embodiment suppresses the magnitude of the amplitude of the composite wave of the reflected noise wave N2 and the incident noise wave N1, thereby reducing the magnitude of the noises N1 and N2 that flow out from the connection point A via the capacitor 15 and the ground 14. This makes it possible to reduce the effects of the noises N1 and N2 on the surrounding circuits while suppressing the propagation of the noise emitted from the noise source 12 to the transmission line 13 that is farther from the noise source 12 than the arrangement point B of the choke inductor or noise filter 16 by the choke inductor or noise filter 16.

[0018] 3, when the predetermined phase difference between the phases is 180°, the reflected noise wave N2 and the incident noise wave N1 cancel each other out, minimizing the amplitude of their composite wave. Therefore, when the predetermined phase difference is 180°, the impact of noise from noise source 12 on the peripheral circuits can be minimized.

[0019] Figure 4 is a circuit diagram showing a state in which a load 17 is connected to the output terminal of the noise filter circuit 11 shown in Figure 2, and is a diagram for explaining the mechanism of the present invention. In Figure 4, parts that are the same as or correspond to those in Figure 2 are given the same reference numerals, and their explanation will be omitted. A predetermined phase difference occurs between the reflected noise wave N2 and the incident noise wave N1 on the shunt-connected capacitor 15 due to the following factors.

[0020] The first factor is that the phase of the reflected noise wave N2 is delayed by the time it takes for the reflected noise wave N2 to travel back and forth along the path length L of the transmission line 13 from connection point A where the capacitor 15 is mounted on the circuit board to location B where the choke inductor or noise filter 16 is located. The second factor is that when the reflected noise wave N2 is reflected at location B where the choke inductor or noise filter 16 is located, the phase of the reflected noise wave N2 changes by the phase angle of the reflection coefficient. As shown in Figure 4, the phase angle θ of this reflection coefficient is expressed by the following equation (1), where Z0 is the characteristic impedance of the transmission line 13, which is the path along which the noise propagates, Z1 is the impedance of the choke inductor or noise filter 16, and arg is the argument in the complex plane. θ=arg{(Z1-Z0) / (Z1+Z0)} [rad] …(1)

[0021] Therefore, by adjusting the impedance Z1 of the choke inductor or noise filter 16 and the location B where the choke inductor or noise filter 16 is disposed, the phase shift between the reflected noise wave N2 and the incident noise wave N1 can be set as desired. That is, the choke inductor or noise filter 16 has an impedance Z1 that shifts the phase of the reflected noise wave N2 at the connection point A by a predetermined phase relative to the phase of the incident noise wave N1, and the path length L, which is the length of the transmission line 13 between the connection point A and the location B, is provided at the location B that shifts the phase of the reflected noise wave N2 at the connection point A by a predetermined phase relative to the phase of the incident noise wave N1.

[0022] The graph shown in Fig. 5(a) shows the relationship between the phase difference between the incident noise wave N1 and the reflected noise wave N2 at the connection point A and the path length L for three types of impedance Z1 of the choke inductor or noise filter 16. The horizontal axis of the graph represents the path length L [m], and the vertical axis represents the phase difference [degree] between the incident noise wave N1 and the reflected noise wave N2. The characteristic line 21a shown by the solid line represents the case where the impedance Z1 of the choke inductor or noise filter 16 is approximately equal to the characteristic impedance Z0 of the transmission line 13 (Z1≈Z0), the characteristic line 22a shown by the short dashed line represents the case where the impedance Z1 is much smaller than the characteristic impedance Z0 (Z1<<Z0), and the characteristic line 23a shown by the long dashed line represents the characteristics in the case where the impedance Z1 is much larger than the characteristic impedance Z0 (Z1>>Z0).

[0023] Also, the graph shown in Fig. 5(b) represents the relationship between the amplitude of the combined wave of the incident noise wave N1 and the reflected noise wave N2 at the connection point A and the path length L for three types of impedances Z1 of the choke inductor or noise filter 16. The horizontal axis of the graph is the path length L [m], and the vertical axis indicates the amplitude of the combined wave of the incident noise wave N1 and the reflected noise wave N2. The characteristic line 21b shown by the solid line represents the case where the impedance Z1 of the choke inductor or noise filter 16 is approximately equal to the characteristic impedance Z0 of the transmission line 13 (Z1≒Z0), the characteristic line 22b shown by the short dashed line represents the case where the impedance Z1 is considerably smaller than the characteristic impedance Z0 (Z1<<Z0), and the characteristic line 23b shown by the long dashed line represents the characteristics in the case where the impedance Z1 is considerably larger than the characteristic impedance Z0 (Z1>>Z0).

[0024] From the graph shown in Fig. (a), it is understood that the phase difference between the incident noise wave N1 and the reflected noise wave N2 at the connection point A can be adjusted by selecting the impedance Z1 of the choke inductor or noise filter 16 and the path length L (placement location B). Also, from the graphs shown in Figs. (a) and (b), particularly, at the path lengths L1 at the impedance Z1 of the characteristic line 23a, the path length L2 at the impedance Z1 of the characteristic line 21a, and the path length L3 at the impedance Z1 of the characteristic line 22a where the phase difference between the incident noise wave N1 and the reflected noise wave N2 is 180°, it is understood that the incident noise wave N1 and the reflected noise wave N2 cancel each other out and the amplitude of the combined wave of the incident noise wave N1 and the reflected noise wave N2 becomes the smallest. Therefore, according to the noise filter circuit 11 according to the present embodiment, by adjusting the impedance Z1 of the choke inductor or noise filter 16 and the placement location B, the influence of the noise emitted from the noise source 12 on the peripheral circuit can be freely adjusted.

[0025] 6 is a circuit diagram showing a schematic configuration of a noise filter circuit 31 according to a second embodiment of the present invention, and is a diagram for explaining an example in which the noise filter circuit 31 is used in a power supply circuit for a power amplifier for LTE (Long Term Evolution) communications. In Fig. 6, parts that are the same as or correspond to those in Fig. 4 are given the same reference numerals, and their description will be omitted.

[0026] In the second embodiment, compared to the first embodiment, the noise source 12 of the noise filter circuit 31 is a step-down DC / DC switching regulator IC12. The choke inductor or noise filter 16 constituting the noise filter circuit 31 is an RF choke inductor 16. While the present embodiment shows a configuration using an RF choke inductor 16, a noise filter such as a ferrite bead may also be used. A power inductor 18 is connected in series to a transmission line 13 at the output of the DC / DC switching regulator IC12, and a capacitor 15 is shunt-connected to the transmission line 13 for smoothing the output current of the DC / DC switching regulator IC12. The load 17 is a power amplifier 17 that receives power from the DC / DC switching regulator IC12.

[0027] To prevent noise from the DC / DC switching regulator IC12 from being conducted to the power amplifier 17, a choke inductor or a noise filter 16 is generally inserted between the output section of the DC / DC switching regulator IC12 and the power amplifier 17. However, this insertion may cause noise reflected by the choke inductor or the noise filter 16 to flow from the current smoothing capacitor 15 in the output section of the DC / DC switching regulator IC12 to the ground 14. For example, if noise in the range of 2400 MHz to 2500 MHz flows from the current smoothing capacitor 15 to the ground 14, this may lead to a deterioration in the sensitivity of wireless communications based on communication standards such as Wi-Fi (registered trademark) and Bluetooth (registered trademark).

[0028] However, by including the noise filter circuit 31 of this embodiment, it is possible to configure a power supply circuit for the power amplifier 17 that minimizes the outflow of noise in the range of 2400 MHz to 2500 MHz to the ground 14 via the current smoothing capacitor 15, while also including the RF choke inductor 16 that suppresses noise conduction to the power amplifier 17 due to the similar function of the noise filter circuit 31 as the noise filter circuit 11.

[0029] In this embodiment, the output section of DC / DC switching regulator IC12, which is composed of power inductor 18 and current smoothing capacitor 15, and the power supply input section of power amplifier 17 are connected by transmission line 13, such as a microstrip line or coplanar line, designed to have an inductance L0 per unit length of 28.8 μH and a capacitance C0 per unit length of 114 pF. A communication signal in the communication frequency band of 2400 MHz propagates through this transmission line 13. In this case, the characteristic impedance Z0 of transmission line 13 is expressed by the following equation (2), and the phase constant β of transmission line 13 at 2450 MHz is expressed by the following equation (3). Z0=(L0 / C0) 1 / 2 =50 [Ω] …(2) β≒ω·(L0·C0) 1 / 2 =88 [rad / m] …(3)

[0030] Furthermore, an RF choke inductor 16 for suppressing noise conduction in the above-mentioned communication frequency band is connected between the output of the DC / DC switching regulator IC12 and the power amplifier 17. The impedance Z1 at 2450 MHz of the RF choke inductor 16 is expressed by the following equation (4), where j represents the imaginary term of a complex number. Z1=j80 …(4)

[0031] In this case, the phase change θ of the reflected noise wave N2 due to reflection at the RF choke inductor 16 is calculated as shown in the following equation (5) by substituting equations (2) and (4) into the above equation (1). θ=arg{(Z1-Z0) / (Z1+Z0)} =arg{(j80-50) / (j80+50)} =1.12 [rad] …(5)

[0032] The phase difference between the reflected noise wave N2 and the incident noise wave N1 at the connection point A is a phase (θ-2βL) obtained by adding together the phase delay (-2βL) corresponding to the time it takes for the reflected noise wave N2 to travel back and forth along the path length L of the transmission line 13 between the connection point A and the arrangement point B, which is the first factor described above, and the phase change corresponding to the phase angle θ of the reflection coefficient, which is the second factor. Therefore, the condition under which this phase difference (θ-2βL) becomes 180°, which is the phase difference at which the reflected noise wave N2 and the incident noise wave N1 cancel each other out, is expressed by the following equation (6). Here, n is a natural number, and n = 1, 2, 3, ... (θ-2βL)=π(1-2n) …(6)

[0033] Equation (6) is calculated for the path length L, and the values ​​of equations (3) and (5) are substituted into the calculated equation for the path length L, and the path length L is calculated as shown in equation (7) below. L={θ+(2n-1)π} / 2β ={1.12+(2n-1)π} / (2×88) [m] …(7)

[0034] 7 shows the relationship between the amplitude of the composite wave of the reflected noise wave N2 and the incident noise wave N1 at connection point A and the path length L in this embodiment. The horizontal axis of the graph represents path length L [mm], and the vertical axis represents the amplitude of the composite wave. From the graph, it can be seen that if the path length L is 24 [mm] when n = 1, 60 [mm] when n = 2, 96 [mm] when n = 3, and so on, then the amplitude of the composite wave will be minimized by setting location B of RF choke inductor 16 at a position where a node of the standing wave appears on current smoothing capacitor 15.

[0035] In other words, when an RF choke inductor 16 having an impedance Z1 at 2450 [MHz] expressed by equation (4) is used, by designing the noise filter circuit 31 so that the path length L of the transmission line 13 between the connection point A and the placement point B is the distance expressed by equation (7), it is possible to suppress noise conduction from the DC / DC switching regulator IC 12 to the power amplifier 17 while minimizing the outflow of noise in the range of 2400 [MHz] to 2500 [MHz] to the ground 14 via the current smoothing capacitor 15.

[0036] Note that the numerical values ​​used here are merely examples, and the specific numerical values ​​of the inductance L0 per unit length of the transmission line 13, the capacitance C0, the impedance Z1 of the RF choke inductor 16, etc. may be different from the above numerical values.

[0037] 8 is a graph comparing the spectrum of noise N (see FIG. 6) flowing through the current smoothing capacitor 15 when the path length L is set to the preferred value (=24 mm) shown in FIG. 7 for the noise filter circuit 31 according to the second embodiment and when the path length L is set to a comparative example value (=40 mm). The horizontal axis of the graph represents the noise frequency (MHz), and the vertical axis represents the noise level (dBμA). A solid characteristic line 41 represents the characteristic when the path length L is 24 mm, and a dashed characteristic line 42 represents the characteristic when the path length L is 40 mm.

[0038] From this graph, it can be seen that at noise frequencies between 2400 MHz and 2500 MHz, characteristic line 41 with a path length L of 24 mm has a lower noise level than characteristic line 42 with a path length L of 40 mm, demonstrating favorable results. The noise spectrum shown in this graph was derived through simulation using LTSPICE simulation software from ANALOG DEVICES.

[0039] As described above, the noise filter circuit 31 according to the second embodiment can reduce the effects of noise on peripheral circuits while suppressing the propagation of noise emitted from the DC / DC switching regulator IC12 to the transmission line 13 that is farther from the noise source than the arrangement position B of the RF choke inductor 16. Therefore, when this noise filter circuit 31 is used in a communication circuit that receives power from the DC / DC switching regulator IC12, it is possible to prevent noise in the frequency band of 2400 MHz to 2500 MHz from leaking to the ground 14, thereby preventing a decrease in communication quality. [Explanation of symbols]

[0040] 11,31...Noise filter circuit 12...Noise source (DC switching regulator IC) 13...Transmission line 14...Ground (reference potential) 15...Capacitor 16...Choke inductor or noise filter (RF choke inductor) 17...Load (power amplifier) 18...Power inductor

Claims

[Claim 1] a capacitor provided between the transmission line near the noise source and the reference voltage; an inductor or a noise filter that is provided on the transmission line at a location farther from the noise source than a connection point between the capacitor and the transmission line, and that suppresses propagation of noise in a frequency band of 2400 MHz to 2500 MHz that is conducted from the noise source through the transmission line to the transmission line that is farther from the noise source than the location; a phase difference between a phase at the connection point of a reflected noise wave that is generated by noise that is transmitted from the noise source through the transmission line to the arrangement point and is reflected by the inductor or noise filter and returns to the connection point, and a phase of an incident noise wave that is transmitted from the noise source through the transmission line and is incident on the connection point, is set so that the sum of a phase delay caused by the noise that is transmitted from the noise source through the transmission line to the arrangement point traveling back and forth on the transmission line between the connection point and the arrangement point, and a phase change caused by the reflection of the reflected noise wave by the inductor or noise filter, is 180°; The noise filter circuit is arranged at a position where a node of a standing wave of noise transmitted through the transmission line appears at the connection point.

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