Preparing for Wireless Design Certification

Digi International
October 13, 2018

Knowledge is power, as they say. When building a wireless product, understanding and preparing for certification requirements can help optimize your design from the beginning so you can pass certifications on the first attempt. This article will cover the key certifications to prepare for as you begin planning your product design.

FCC certifications

Any product to be sold in the U.S. must pass FCC certifications, which include the following:

FCC Part 15, subpart B - Unintentional Radiator (EMI): Electronic devices with oscillations greater than 9 kHz that do not deliberately generate radio frequency emissions must comply with FCC Part 15, subpart B emission limits. Today, because devices have much faster clock speeds, this test can be difficult to pass. Faster clocks mean shorter wavelengths; as a result, smaller structures on your PCB can act as an antenna and propagate unwanted emissions. So you really need to minimize this noise in your electronics design.
Part 15C, 22, 24, 27 - Intentional Radiator: These tests assess output power and other signal characteristics for products with RF transmitters. Harmonics are the most common cause of test failure, and there are several typical reasons. Here are two common examples:
Non-linear power amplifiers in the transmitter chain can generate harmonics which are then radiated by the antenna.
Non-linear PCB components can pick up the fundamental frequency radiated from the antenna and then generate and radiate the harmonics of the fundamental frequency.
Specific Absorption Rate (SAR) -This test measures how much the products RF transmissions heat human tissue, and is designed to prevent health hazards. If the end product will be used within 20 cm of the human body, you must perform a maximum permissible exposure (MPE) calculation, at a minimum, to determine whether SAR testing is required.

Cellular certifications

Cellular over-the-air (OTA) tests are required for cellular designs with antennas located less than 20 cm from the radio, and are very challenging to pass. Cellular certification testing often includes the following, depending on carrier selection and geographic region:

Total Radiated Power (TRP) - Measures total power radiated from the device and is a function of the antenna radiation efficiency, impedance match, and radio output power.
Total Isotropic Sensitivity (TIS) - Measures radiated receiver sensitivity integrated over a sphere around the device. It is a function of antenna radiation efficiency, impedance match, and radio receiver sensitivity, but is often limited by noise radiated from host electronics.
Relative Sensitivity Intermediate Channel (RSIC) - tests for receiver sensitivity degradation on specific RF channels.
Radiated Spurious Emissions (RSE) - evaluates spurious emissions from the cellular transmitter as well as the host electronics

RF Engineering Manager, Kyle Sporre, summarizes PTCRB test requirements, and the regions where they are applicable in the simple whiteboard video session below.

It is very important to understand the design principles that affect certification testing early in your process and to apply best practices. Effectively controlling noise prior to performing radiated cellular tests such as TIS and RSIC can help you achieve the low EMI required to pass certification testing. TIS requires even quieter PCB emissions than FCC certifications, and failures caused by noise coming from the host electronics are common.

Note that products that do not include an antenna within 20 cm of the device are not subject to OTA tests. For example, this includes box products with an antenna port that requires the customer to supply the antenna, or products with cabled antennas that are more than 20 cm from the device. To ensure success, design your product up front with certification requirements in mind. If you need assistance, Digi's Wireless Design Services (WDS) team can help with your product design, or even correct design issues that can lead to certification failures.