In recent years, WLANs have been incorporated into many kinds of devices, and the services that use communication networks have been on the increase. However, sufficient communication speeds must be maintained in order for these services to be used efficiently and effectively. Under the IEEE 802.11n, 11ac, and other such specifications for implementing WLAN communication, WLAN communication speeds have been increasing, but if the reception sensitivity of the devices themselves is lacking, the benefits of these increased speeds cannot be enjoyed.
When there is noise in the same frequency bands as the WLAN communication bands inside a device, it may be picked up by the antenna of that device, causing the reception sensitivity to deteriorate. This phenomenon is referred to as reception sensitivity suppression. Since this suppression occurs in a narrow range inside the devices, even noise that is faint enough not to cause problems with radiated noise may in fact be enough to affect the reception sensitivity. As such, noise suppression methods that are more stringent than those used in the past are required.
In order to mitigate the deterioration in the reception sensitivity, it is necessary to reduce the noise that is picked up by the antenna, but no effects will be achieved by taking the suppression measures in a random way. The important aspects to pinpoint in this respect are the sources of the noise, the identification of the conduction mechanisms, the selection of the optimum EMI filters, and the placement of the parts in the optimum locations to block the paths along which the noise is transmitted (figure 1).
The interfaces, memories, and LSI chips involved in high-speed communication constitute the noise sources. It should be borne in mind that even when the operating frequency is slow, harmonic noise may be superimposed in a wide band onto the communication frequencies of the WLAN. Examples of noise conduction mechanisms are conductor conduction, spatial conduction, conductor conduction to spatial conduction, and spatial conduction to conductor conduction.
If spatial conduction serves as the mechanism, the use of shields consisting of metal plates and microwave absorbers is effective, while if conductor conduction serves as the mechanism, the use of EMI filters is effective.
In terms of EMI filters, we recommend ferrite beads, common-mode choke coils, π-type/T-type filters, and 3-terminal capacitors that support high frequencies and have an impressive noise suppression effect for the 2.4- and 5-GHz bands that comprise the communication bands. Even if noise is blocked in one place, it may pass along other conduction paths and leak out, so measures should be taken with awareness focused on blocking the noise to the greatest extent possible. The EMI filters should be placed in close proximity to the sources of noise.
Figure 2 shows an example where measures have been taken to suppress the noise picked up by an antenna in a BD recorder. In this particular case, the noise irradiated from the SATA cable and USB cable is picked up by the antenna, and by using the common-mode choke coils shown in figure 3 in the signal lines of each cable, noise was reduced by 3 dB or so across a wide band, as is shown in figure 4.
Figure 3a. Impedance curve (USB noise suppression: DLW21SN900HQ2)
Figure 3b. Impedance curve (SATA noise suppression: DLP0QSA070HL2)
If it is possible to verify that the noise picked up by an antenna has been reduced by the insertion of EMI filters, proceed to verifying whether the reception sensitivity has actually been improved.
If a reverberation chamber (one of the evaluation systems recommended by 3GPP) is used, it is possible to evaluate the reception sensitivity in the assumed multipath environment. In the example shown in figure 5, an improvement of approximately 2 dB has been achieved for both the reception sensitivity and throughput.
Department Responsible: Application Development Sec. Product Development Dept. EMI Filter Division, Murata Manufacturing Co., Ltd.