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Back to Basics: The noise challenge for wireless devices

Posted: 02 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:machine-to-machine? embedded antennas? RF design?

In the past five years, connected and machine-to-machine (M2M) devices have become ever smaller. Many of these devices use embedded antennas and may include requirements for GPS, Wi-Fi, NFC, and/or 915Mhz.

Added to this, they incorporate more cellular bands than before for 3G and 4G, and customers are now looking for global coverage too. Network congestion also adds to the problem as we pump more data onto 3G and 4G pipes. The end result is that for devices that are as small as a cell phone, noise and emissions have become a significant problem.

The increased RF design complexity makes it almost impossible to fully contain noise and emissions that impact transmission and reception. Inband noise occurs when a device has emissions or harmonics of emissions present at the operating cellular frequencies. This noise can affect initial cellular reception or be reradiated by the cellular module. If the antenna efficiency and radiated power of the system is very high, it makes the problem worse.

What to do about it?
The key to solving this engineering challenge is to eliminate or suppress the noise that is causing performance compromises and most likely certification challenges. The first thing to do is to identify the noise source and then act to solve the problem by stopping the noise, filtering it or at least mitigating its affects on the system. Patching things up a little or using a less efficient antenna is not advisedit will only end up in a poor user experience in the field. Quite often the noise doesn't come from one single source; it is a combination of different factors. Ideally, you should look at the RF design at the beginning of the product design process to help minimize these emission effects and to keep them away from the antennas. Best practice RF design only comes with experience, that is, learning the hard way.

Steps to eliminating noisy devices
Select the right antenna: The first step is to team up with a wireless product designer to select and integrate the right antenna component selection. Size, position and mounting mechanisms are vital considerations. Avoid large components on the board itself, which are normally older types, and follow the component layout guidelines closely.
???Choose connectors carefully: Eliminate test points (at the end), connectors and wires. If this is not possible minimise them by using the latest powerful PCB design software to filter and bypass every integrated circuit.
???Use the right PCB: It is important for the PCB stack up configuration to be designed to contain emissions and to be optimized for RF. A four-layer board may be more expensive, but it more than pays for itself in performance when you compare it to a two-layer board.
???Consider traces: The routing of all power and signal traces and their thicknesses are critical for good RF performance. The distances between these traces and their impedances are also vital in order to achieve the best device performance
???Be aware of the ground plane: The size of the ground plane is important for antenna performance. For overall RF performance it is important to completely fill in the ground plane. This means ensuring that all unused areas of the PCB are filled with copper and not ground free. The ground plane should be one consistent, coherent and connected "pour" so you can see copper on all unused areas of the PCB, even between components.
???Review shielding: Implement shielding where necessary. In some cases, it may even be required to add shielding cans over active circuitry or potential sources of emissions like clocks or processors etc.

A quiet life
Two products can look similar to the eye, but one device could be noisy and the other one not, even though they are using the same components. It's all about the way the system is laid out and how certain components interact with each other. Partnering with an expert who has the right equipment, expertise and experience can help you design your product better and quicker. The end goal is to design products that are optimized from an RF perspective so you can enhance sensitivity and minimize the noise effects in your device.

About the author
Dermot O'Shea is co-founder and joint managing director of Taoglas Group Holdings. Taoglas has a Noise Control Division (NCD) in San Diego, California, where the company's on site engineering team is dedicated to helping M2M manufacturers design the best devices in the market and pass certification and approval processes easily and cost effectively.

To download the PDF version of this article, click here.

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