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RF Current Absorption Sheet Suppresses EMI
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Tokin Corp has marketed noise suppression sheets called Flex Suppressor for many years. Some customers have misunderstood the characteristics of the sheet, thus could not achieve effective countermeasure of electromagnetic interference (EMI) emission noise. So, we provide correct information of the Flex Suppressor sheet which plays a role of absorption of RF current, a source of the noise, instead of absorbing electromagnetic wave or shielding the wave.
What Is Wave Absorber?
An electromagnetic wave absorber is defined generally as an object separated by sufficient long distance in comparison to the wavelength of radiated wave, having a reflection coefficient S less than -20dB for a specific frequency band. In the broader definition, it is a material or structure, which dissipates a portion of incident electromagnetic energy into internal heat. In other words, this broad definition requires that (1) a portion of the electromagnetic energy penetrates into the interior, and (2) a portion of the electromagnetic energy penetrating into the interior is converted to thermal energy and consumed as heat.
Of these two conditions, (1) requires impedance matching. However, it is difficult to implement impedance matching in downsized equipments such as notebook personal computers or cellular phones, because in most cases an electromagnetic wave absorber cannot be located sufficiently far away from the noise source, with respect to the wavelength of the noise. As a result, the wave impedance at the interface of the absorber depends both on the radiation source impedance and distance (Fig 1).
To be effective as an absorber in the near field, the input impedance must be matched to materials design, taking into consideration of the boundary conditions. In the near field, the radiation source impedance will vary with the addition of equivalent resistance to the transmission path, making for a complex phenomenon.
In cases where countermeasures are addressed at the RF current source of the radiation noise rather than the radiated noise itself, positioning materials with magnetic loss properties close to the transmission line can fulfill the same role through inductive coupling.
Toward this end, we have proposed a method of suppressing RF noise by using composite magnetic materials with excellent magnetic loss characteristics at high frequencies1). The resulting noise suppression sheet has been commercialized under the names "Flex Suppressor" and "Film Impedor".
These noise suppression sheets consist of magnetic metal flakes arrayed and aligned in elastomer, which maintains them in an electrically insulated state. These are the two-dimensional noise countermeasure devices, with excellent magnetic noise suppression characteristics in the quasi-microwave band.
Resistance Suppresses Radiation
Let's give an example of suppressing RF noise from a signal line operated at higher frequency with a lossy magnetic material. When transmission lines for signals with conducted noise components are viewed as distributed-constant lines, the equivalent circuit for a line with length l is shown in Fig 2. When a lossy magnetic material is positioned on this line, it is possible to determine the amount of change in the equivalent circuit constants from the amount of change in the transmission characteristics (S parameter)2).
This analysis shows that when a lossy magnetic material is positioned directly above a microstrip line, the change in transmission characteristics is almost entirely due to the increase in series resistance component Rs and parallel conductance component Gp.
As a result, when the lossy material is positioned close to the transmission line, an increased equivalent resistance R (depending on frequency characteristics of an (imaginary part of the permeability) and the material dimensions) will operate to suppress the RF current in the transmission line.
If the target transmission line is the radiation source itself, it can be expected that the increased R will reduce the radiation noise by lowering the radiation efficiency, and the peak radiation by broadening the radiation wave (Q damping).
More Effective than Ferrite Plate
We show an experimental result of suppressing RF current when using a microstrip line.
We prepare two kinds of sheets, "Flex Suppressor" and ferrite plate (permeability = 700) with different characteristics of an imaginary part of the permeability as magnetic loss materials. These sheets are placed over a microstrip line with characteristic impedance of 50ohms (Fig 3). We measured the transmission characteristics (S parameter), and obtained equivalent circuit constants from the result. Table 1 shows typical values for equivalent circuit constants Ls, Rs, Cp and Gp (normalized for a 10mm test sample), along with several types of test sample. Also included with the cases for no test sample (air) and for a copper plate.
The positioning of the lossy material causes a drastic increase in loss components (Rs and Gp).
Next, we show examples of emission noise at near-and far-field applications. The noise comes from noise emitters in high frequency circuits including cellular phones and notebook computers.
Near Field Radiated Noises
On the scale of electronic equipment such as notebook PC and cellular phones, absorbers or shielding sheets designed to counter radiation source are almost always positioned close enough in comparison to the electromagnetic wavelength. The wave impedance at the boundary between the absorber and the equipment depends on the noise source impedance and distance. Therefore, maximizing the function of the wave absorber for the near field requires an exact input impedance based on the boundary conditions.
In addition, the effect of increased R on the transmission path, as described above, changes the line impedance, creating a complex phenomenon difficult to analyze extremely. So, we show our experimental result, instead of the analysis.
Micro-loop antennas with 1.5mm in diameter were used for the radiation source and detection element. Transmission level and coupling level for a sheet of "Flex Suppressor" lossy material (100mm x 100mm x 0.5mm), as a reference level of the free space coupling intensity level for antennas.
Compared with a rubber ferrite sheet, the K4E and R4 Flex Suppressor show lower coupling and transmissive levels.
Product Portfolio
Finally we introduce the products of the noise suppression sheets.
We refer to lossy materials used to suppress RF noise as EMI suppressor, and two of these are commercially available as sheet-form products: "Flex Suppressor" and "Film Impedor". "Flex Suppressor" is designed for the quasi-microwave band from 100 to 3,000MHz, available in a single-layer structure, and a triple-layer structure with internally sandwiched conductors. The internal conductive layer as an antenna caches electromagnetic waves, and a portion of the energy converted into current is then consumed in the magnetic loss layer. "Flex Suppressor" 3GF achieves a shielding characteristic of 20dB through electrical resistance adjustment in the shielding layer, but still provides coupling characteristics very close to those of single-layer types.
In addition to the "Flex Suppressor", we also offer the "Film Impedor" series, with thickness ranging from 0.05 to 0.1mm. The "Film Impedor" E50, for example, is an extremely thin EMI suppressor with a superior permeability of micron = 60, and is highly flame resistant (equivalent to UL94 V-0).
Table 2 indicates our customer's examples of noise suppression applications of "Flex Suppressor". Table 2a shows the examples of the applications for EMI suppression (compliant to various VCCI and FCC regulations), while Table 2b shows the applications to prevent operating (intra-system) faults. As shown in the table, "Flex Suppressor" is extensively used in mobile communications equipment, information terminals, AV equipment and other digital electronic systems.
by Shigeyoshi Yoshida, Manager, Products Development Division, Electronic Component Business Unit, Tokin Corp, Japan
References:
1)Yoshida, S, Sato, M, Sugawara, E and Shimada, Y, Journal of Applied Physics, vol 85, no 8, p 4643, 1999.
2)Tsuda F, Ono, H, Shinohara, S and Sato, R, 2000 IEEE EMC Symposium Recors, pp 867-870, 2000.
(April 2002 Issue, Nikkei Electronics Asia)
What Is Wave Absorber?
An electromagnetic wave absorber is defined generally as an object separated by sufficient long distance in comparison to the wavelength of radiated wave, having a reflection coefficient S less than -20dB for a specific frequency band. In the broader definition, it is a material or structure, which dissipates a portion of incident electromagnetic energy into internal heat. In other words, this broad definition requires that (1) a portion of the electromagnetic energy penetrates into the interior, and (2) a portion of the electromagnetic energy penetrating into the interior is converted to thermal energy and consumed as heat.
Of these two conditions, (1) requires impedance matching. However, it is difficult to implement impedance matching in downsized equipments such as notebook personal computers or cellular phones, because in most cases an electromagnetic wave absorber cannot be located sufficiently far away from the noise source, with respect to the wavelength of the noise. As a result, the wave impedance at the interface of the absorber depends both on the radiation source impedance and distance (Fig 1).
To be effective as an absorber in the near field, the input impedance must be matched to materials design, taking into consideration of the boundary conditions. In the near field, the radiation source impedance will vary with the addition of equivalent resistance to the transmission path, making for a complex phenomenon.
In cases where countermeasures are addressed at the RF current source of the radiation noise rather than the radiated noise itself, positioning materials with magnetic loss properties close to the transmission line can fulfill the same role through inductive coupling.
Toward this end, we have proposed a method of suppressing RF noise by using composite magnetic materials with excellent magnetic loss characteristics at high frequencies1). The resulting noise suppression sheet has been commercialized under the names "Flex Suppressor" and "Film Impedor".
These noise suppression sheets consist of magnetic metal flakes arrayed and aligned in elastomer, which maintains them in an electrically insulated state. These are the two-dimensional noise countermeasure devices, with excellent magnetic noise suppression characteristics in the quasi-microwave band.
Resistance Suppresses Radiation
Let's give an example of suppressing RF noise from a signal line operated at higher frequency with a lossy magnetic material. When transmission lines for signals with conducted noise components are viewed as distributed-constant lines, the equivalent circuit for a line with length l is shown in Fig 2. When a lossy magnetic material is positioned on this line, it is possible to determine the amount of change in the equivalent circuit constants from the amount of change in the transmission characteristics (S parameter)2).
This analysis shows that when a lossy magnetic material is positioned directly above a microstrip line, the change in transmission characteristics is almost entirely due to the increase in series resistance component Rs and parallel conductance component Gp.
As a result, when the lossy material is positioned close to the transmission line, an increased equivalent resistance R (depending on frequency characteristics of an (imaginary part of the permeability) and the material dimensions) will operate to suppress the RF current in the transmission line.
If the target transmission line is the radiation source itself, it can be expected that the increased R will reduce the radiation noise by lowering the radiation efficiency, and the peak radiation by broadening the radiation wave (Q damping).
More Effective than Ferrite Plate
We show an experimental result of suppressing RF current when using a microstrip line.
We prepare two kinds of sheets, "Flex Suppressor" and ferrite plate (permeability = 700) with different characteristics of an imaginary part of the permeability as magnetic loss materials. These sheets are placed over a microstrip line with characteristic impedance of 50ohms (Fig 3). We measured the transmission characteristics (S parameter), and obtained equivalent circuit constants from the result. Table 1 shows typical values for equivalent circuit constants Ls, Rs, Cp and Gp (normalized for a 10mm test sample), along with several types of test sample. Also included with the cases for no test sample (air) and for a copper plate.
The positioning of the lossy material causes a drastic increase in loss components (Rs and Gp).
Next, we show examples of emission noise at near-and far-field applications. The noise comes from noise emitters in high frequency circuits including cellular phones and notebook computers.
Near Field Radiated Noises
On the scale of electronic equipment such as notebook PC and cellular phones, absorbers or shielding sheets designed to counter radiation source are almost always positioned close enough in comparison to the electromagnetic wavelength. The wave impedance at the boundary between the absorber and the equipment depends on the noise source impedance and distance. Therefore, maximizing the function of the wave absorber for the near field requires an exact input impedance based on the boundary conditions.
In addition, the effect of increased R on the transmission path, as described above, changes the line impedance, creating a complex phenomenon difficult to analyze extremely. So, we show our experimental result, instead of the analysis.
Micro-loop antennas with 1.5mm in diameter were used for the radiation source and detection element. Transmission level and coupling level for a sheet of "Flex Suppressor" lossy material (100mm x 100mm x 0.5mm), as a reference level of the free space coupling intensity level for antennas.
Compared with a rubber ferrite sheet, the K4E and R4 Flex Suppressor show lower coupling and transmissive levels.
Product Portfolio
Finally we introduce the products of the noise suppression sheets.
We refer to lossy materials used to suppress RF noise as EMI suppressor, and two of these are commercially available as sheet-form products: "Flex Suppressor" and "Film Impedor". "Flex Suppressor" is designed for the quasi-microwave band from 100 to 3,000MHz, available in a single-layer structure, and a triple-layer structure with internally sandwiched conductors. The internal conductive layer as an antenna caches electromagnetic waves, and a portion of the energy converted into current is then consumed in the magnetic loss layer. "Flex Suppressor" 3GF achieves a shielding characteristic of 20dB through electrical resistance adjustment in the shielding layer, but still provides coupling characteristics very close to those of single-layer types.
In addition to the "Flex Suppressor", we also offer the "Film Impedor" series, with thickness ranging from 0.05 to 0.1mm. The "Film Impedor" E50, for example, is an extremely thin EMI suppressor with a superior permeability of micron = 60, and is highly flame resistant (equivalent to UL94 V-0).
Table 2 indicates our customer's examples of noise suppression applications of "Flex Suppressor". Table 2a shows the examples of the applications for EMI suppression (compliant to various VCCI and FCC regulations), while Table 2b shows the applications to prevent operating (intra-system) faults. As shown in the table, "Flex Suppressor" is extensively used in mobile communications equipment, information terminals, AV equipment and other digital electronic systems.
by Shigeyoshi Yoshida, Manager, Products Development Division, Electronic Component Business Unit, Tokin Corp, Japan
References:
1)Yoshida, S, Sato, M, Sugawara, E and Shimada, Y, Journal of Applied Physics, vol 85, no 8, p 4643, 1999.
2)Tsuda F, Ono, H, Shinohara, S and Sato, R, 2000 IEEE EMC Symposium Recors, pp 867-870, 2000.
(April 2002 Issue, Nikkei Electronics Asia)
