USB 3.0* Radio Frequency Interference Impact on 2.
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Contents 1 Introduction ...................................................................................................... 6 2 Overview .......................................................................................................... 7 2.1 2.2 3 Impact of USB 3.0* Noise ................................................................................... 9 3.1 3.2 4 2.4 GHz Wireless Devices......................................................................... 7 USB 3.0* ................
Figures Figure Figure Figure Figure 2-1. 2-2. 3-1. 3-2. Figure 3-3. Figure 3-4. Figure 4-1. Figure 4-2. Figure 4-3. Figure 4-4. Figure 4-5. Figure 4-6. Figure 4-7. Sinc Function of USB 3.0* Data Rate ................................................... 7 Measured USB 3.0* Data Spectrum ..................................................... 8 Radiation from USB 3.0* Channels ...................................................... 9 Setup for Measurement of Noise from External USB 3.0* Hard Disk Drive .......
Revision History Revision Number 001 Description Revision Date • Initial release.
Introduction 1 Introduction The purpose of this document is to create an awareness of radio frequency interference to wireless devices operating in the 2.4 GHz ISM band as a result of certain USB 3.0* devices and cables. This is a guide to customers of the USB 3.0 RFI mitigation options that are available.
Overview 2 Overview 2.1 2.4 GHz Wireless Devices The 2.4 GHz ISM band is a widely used unlicensed radio frequency band for devices such as wireless routers, as well as wireless PC peripherals such as a mouse or keyboard. These devices may use standard protocols such as the IEEE 802.11b/g/n or Bluetooth, or they may use proprietary protocols. The radios may use frequency hopping, frequency agility, or may operate on a fixed frequency.
Overview As shown in Figure 2-1, the data spectrum is very broadband, ranging from DC to 5 GHz. Figure 2-2 shows the data spectrum measured by directly probing one of the USB 3.0 transmit-pair signals of a notebook computer. Figure 2-2. Measured USB 3.
Impact of USB 3.0* Noise 3 Impact of USB 3.0* Noise 3.1 Interoperability Issue As previously shown in Figure 2-2, the noise from USB 3.0 data spectrum can be high (in the 2.4–2.5 GHz range). This noise can radiate from the USB 3.0 connector on a PC platform, the USB 3.0 connector on the peripheral device or the USB 3.0 cable. If the antenna of a wireless device operating in this band is placed close to any of the above USB 3.0 radiation channels, it can pick up the broadband noise.
Impact of USB 3.0* Noise A near field probe was placed close to an external USB 3.0 hard disk drive (HDD) in an RF absorber chamber and a noise measurement was made using a spectrum analyzer, with and without the HDD connected. The results are shown in Figure 3-3. With the HDD connected, the noise floor in the 2.4 GHz band is raised by nearly 20 dB. This could impact wireless device sensitivity significantly. Figure 3-3. Noise from External USB 3.
Impact of USB 3.0* Noise 3.2 Impact to Wireless Device Performance In this paper, an example of a wireless mouse communicating to a notebook computer through an associated USB2 wireless dongle is used to demonstrate the impact of the USB 3.0 noise on wireless device performance. The test setup, shown in Figure 3-4, was used. An external USB 3.0 hard drive was connected to the USB 3.0 port on the notebook computer using a 3-foot USB 3.0 cable.
Impact of USB 3.0* Noise As shown in Table 3-1, there is degradation in the performance of the wireless mouse when a USB 3.0 device is connected to the notebook computer. At a distance of 3 feet and greater away from the notebook computer, a significant lag was experienced in the response of the mouse. The impact to mouse performance was found to be the same whether the USB 3.
Mitigation Methods 4 Mitigation Methods There are several areas where improvements can be made to minimize the impact of USB 3.0 noise on wireless device performance. Three areas where improvements can be made are: • Shielding on the USB 3.0 peripheral device • Shielding improvements on the USB 3.0 receptacle connector on the notebook • Wireless antenna placement, wireless receiver performance Each of these areas are studied in further detail in the following sections. 4.1 Shielding the USB 3.
Mitigation Methods Figure 4-1. Various Areas of USB 3.0* Peripheral Device Shielded Case 1: No shielding applied to HDD Case 2: Shielding on HDD Connector and small PCB area surrounding it Case 3: Shielding on HDD Connector and all exposed PCB area, body of the HDD partially shielded Case 4: Shielding on HDD Connector and all exposed PCB area, body of the HDD fully shielded In each of the above cases, the noise was measured near the USB 3.0 HDD connector. The results are shown in Figure 4-2.
Mitigation Methods Figure 4-2. Impact of Shielding the USB 3.0* Peripheral Device 4.2 Improving Shielding on the Notebook USB 3.0* Connector In addition to radiation from the USB 3.0 peripheral device, noise can radiate from the USB 3.0 receptacle connector on a PC platform. As an example, to measure the level of noise due to the USB 3.0 notebook connector, the test setup, shown in Figure 4-3, was used. A near field probe was placed near the USB 3.0 connector on Notebook ‘A’, in an RF absorber box.
Mitigation Methods As shown in Figure 4-4, noise radiated from the notebook USB 3.0 receptacle connector can raise the noise floor by about 25 dB, impacting the SNR of the wireless receiver. Figure 4-4. Noise Contribution Due to Notebook USB 3.0* Connector The USB 3.0 receptacle connector shield is designed to shield the signal pins. However, there are several shielding improvements that can be made to the connector.
Mitigation Methods Figure 4-5. Noise Contribution with Improved Shielding on Notebook USB 3.0* Connector An example of a receptacle connector with a fully enclosed back shield is shown in Figure 4-6. Figure 4-6.
Mitigation Methods 4.3 Wireless Antenna Placement To assess the impact of the location of the wireless receiver antenna with respect to the USB 3.0 port/device, the following changes were made to the test setup in Figure 3-4. For Case 2 in Figure 4-7, the wireless dongle was moved away from the USB 3.0 device and port using a USB2 extension cable, placing the dongle on the opposite side of the notebook computer. In Case 3, the wireless dongle was connected to a USB 2.
Mitigation Methods Another area in which antenna location could have a significant impact is on the transmitter side of the wireless device. To maintain a good SNR at the receiver, the level of the transmit signal should be maintained as high as possible. For example, transmitted signal level from the radio transmitter in the wireless mouse can be affected by attenuation due to the human hand. Table 4-2 shows a performance comparison of different wireless mouse models.
Performance Improvement 5 Performance Improvement Several noise reduction and mitigation strategies were described in the previous section. Noise emitted from the USB 3.0 devices can be reduced by applying shielding effectively and this helps maintain a good SNR at the wireless receiver. This section details the performance improvement gained by applying some of the mitigation strategies described in the previous section. Table 5-1 shows the response of Mouse ‘A’ when communicating with Notebook ‘A’.
Summary 6 Summary The noise generated due to the USB 3.0 data spectrum can have an impact on radio receivers whose antenna is placed close to a USB 3.0 device and/or USB 3.0 connector. The noise is a broadband noise that cannot be filtered out, since it falls within the band of operation of the wireless device (2.4–2.5 GHz). The noise degrades the signal-to-noise ratio that the wireless receiver sees and limits its sensitivity. This then reduces the operating wireless range of the device.
Acknowledgement 7 Acknowledgement The authors would like to acknowledge Nordic Semiconductor for their inputs on the design guidelines for a wireless receiver/transmitter.
