Feature: RF
f1
f2
Band 20 cellular modem (832-862 MHz) 868 MHz short range device
f2
- f1 2f1 - f2 2f2 - f1
f2 2f1
+ f1 2f2 2f1 + f2 3f1 2f2 + f1 3f2
OEM instructions. For example, the instructions may state that that a specific module can only be used with a particular antenna design, so the designer and manufacturer must comply with that specification. Only antennas that are covered by an FCC grant or in the Module Test Report for the EU can be used. While the use of an antenna of the same type with less of equal gain is allowed, one of a different type will require full spectrum testing.
Frequency 1st order (fundamentals) 2nd order (products) Figure 1: Harmonics and intermodulation products
EMC While RF modules are typically shielded to limit unintended emissions and increase EMC immunity, when they are integrated into a larger electronic or electrical system, the integration itself could alter the EMC immunity as different systems interfere with each other. For example, this might be the case if the radio module is co-located with another and capable of simultaneous transmission. For products with multiple radios, there is an EU requirement for an intermodulation investigation in accordance with Clause 6.2 from EG 203 367. This is intended to ensure there are no spurious emissions caused by the interaction of these transmitters. In Europe the EMC requirements
for a radio product are somewhat greater than that for a non-radio product. The EMC for radio standards (EN 301 489’s) require that immunity testing is performed to ensure that communication remains robust in the presence of interference (i.e., test the product with it performing a communication function) and then additionally to check that external interference does not cause the product to unintentionally transmit. Many radio products transmit only sporadically and hence are in a ‘not transmitting’ state for extended periods. This test is checking to see that, if the radio is not
supported to be transmitting, that the presence of interference does not cause a transmission to happen. While Europe has an emissions and
immunity requirement for EMC, the FCC is purely interested in emissions coming from non-radio intentional transmitters. One key consideration when buying a radio module is whether it can meet the levels for immunity that the product is going to be subject to when it’s in use. Generally, modules will be assessed to 3V/m, according to ESTI requirements, but industrial products usually require more stringent immunity tests, for example 10V/m.
Antennas Manufacturers must be careful to maintain compliance with the module’s
3rd order (products)
Co-location If you have your module in the same product as another transmitter, having two radios in the same product that are physically transmitting at the same time can cause intermodulation products due to two signals being present in the spectrum. This means that co-location and simultaneous transmission must be considered. The FCC requires the simultaneous
transmission of separate modules to be evaluated using the FCC multi- transmitter procedures. Intermodulation products can occur
from the sum and difference of multiple transmitters, as well as multiples of those frequencies. Figure 1 shows the formula for first, second and third order intermodulation products. Table 1 gives an example of how this formula can be applied for a product containing both an LTE cellular module operating on band 20, as well as an 868MHz short range device.
Table 1: Example of how to apply the formula for intermodulation products
16 July/August 2025
www.electronicsworld.co.uk
Amplitude
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