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Dealing with platform interference

04 Jan 2016  | Kenneth Wyatt

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Nowadays, it seems as though every manufacturer wishes to add some form of wireless capability into new and existing mobile, household, industrial, scientific, and medical product. This trend towards the "Internet of Things" is in full swing and with it comes problems with EMI. That is, EMI from the product itself, that interferes with sensitive telephone, GPS/GNSS, and Wi-Fi/Bluetooth receivers. This is called "platform interference" and it's a big problem for manufacturers.

Most digital-based products create a host of on-board radio frequency "noise" (electromagnetic interference), that usually won't both the digital circuitry itself, but the harmonic energy from digital clocks, high-speed data buses, and on-board switch-mode power supplies can easily create interference well into the 700 to 950MHz mobile phone bands, causing receiver "desense" (reduced receiver sensitivity). In order to use the various mobile phone services (Verizon, ATT, Sprint, and others in the U.S.), manufacturers must pass very stringent compliance tests according to CTIA standards. This on-board digital noise often delays product introductions for weeks or months.

My last three clients all had platform interference issues that kept their products off the market until major redesigns were done. In this article, I'll show what this noise looks like, how to measure it and suggest some remediations.

First, let's take a look at a typical product. The details will remain general for confidentiality purposes. This board includes a USB port whose data ultimately gets transmitted via various mobile phone systems, depending upon the factory configuration.

Figure 1: The measurement setup for general PC board noise characterisation. The usual near field probes are used the most to narrow down the areas of noise generation and to characterise the type of noise produced. In this case, a high frequency current probe is being used to measure the USB cable harmonics.

Figure 2: There are two common types of high frequency harmonics; narrow band (in the aqua trace) and broadband (violet trace). The yellow trace is the ambient noise level of the measurement system and is always a good idea to document.

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