Meaning
Unwanted radio frequency signals generated by the mixing of two or more transmitter frequencies in non-linear circuit elements can cause severe network interference. When these intermodulation spurious emissions occur, they can desensitize nearby receivers or violate regulatory emissions limits. This phenomenon usually happens when multiple transmitters share the same antenna or are placed very close together.
Generation Mechanism
When two signals pass through a non-linear component like a power amplifier or a dirty connector, new frequencies are produced. These products are mathematical combinations of the original frequencies, such as the sum or difference of their multiples. This non-linear behavior is aggravated when the input signals are driven at high power levels.
Suppression Design
Engineers use high-performance bandpass filters and linear amplifiers to prevent the generation of these unwanted mixing products. Isolators can also be placed after the amplifier to prevent reverse signals from entering the transmitter chain. These hardware modifications keep the spurious signals below the levels required by regulatory standards.
Radiated Limit
Regulatory bodies set strict limits on the power of any emitted spurious signal to protect the radio spectrum. These limits are measured in the far-field region of the device during the official compliance tests. If the device exceeds these limits, it cannot be certified for sale, which makes early testing with a spectrum analyzer during the prototyping phase essential for the design team.
