Characterizing & Debugging EMI Issues for Wireless and IoT Products

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Headings…
Characterizing & Debugging EMI Issues for Wireless and IoT Products
Wireless Self-Interference
Related Resources
Characterizing Platform (or Self-Generated) EMI
Types of Measurements
Step 1: Near Field Probes
Start With A Wide Frequency Span
Narrow The Span To The Downlink Band
Characterize DC-DC Converter Ringing and Rise-Times
Step 2: Using Current Probes

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While this digital switching will not usually bother the digital circuitry itself, that same harmonic energy from digital clocks, high-speed data buses, and especially on-board DC-DC switch-mode power supplies can easily create interference well into the 700 to 950 MHz (and higher) cellular phone bands, causing receiver “desense” (reduced receiver sensitivity).
On-board energy sources, such as DC-DC converters, ad- dress and data buses, and other fast-edged digital signals that can conduct or couple this EMI directly to wireless modules or their antennas.
A nearby antenna or TEM cell for measuring the actual near field emissions directly from the PC board or attached cables.
Three methods have been developed to diagnose self- interference for IoT-enabled devices: use of near field probes to help characterize the sources of harmonic energy on the board or system; a current probe to characterize the harmonic cable currents; or a nearby antenna to monitor the actual emissions while troubleshooting.
There are three useful measurements for characterizing board-level EMI, notably: a general examination over a wide frequency range; a narrower examination at just the receiver downlink band; and an oscilloscope measurement of the DC- DC converter switched waveform.
For example, measuring the Ethernet and DC-DC converter in a typical IoT device (see Figure 2) using the H-field probe reveals a very high level of broadband and narrow band EMI from 1 to 1500 MHz. The white circle indicates the approximate boundaries of the common U.S. cellular bands from 700 to 900 MHz, as well as the GPS frequency of 1575.42 MHz (general GNSS uses additional frequencies nearby).
Once the various noise sources on the board are identified and characterized, the second useful measurement is to narrow the span and look at just the receiver (downlink) band using the same near field probe in various locations

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