What is the difference between out-of-band and spurious emissions?

Out-of-band (OOB) emissions fall in the frequency range immediately adjacent to the necessary bandwidth, caused by the modulation process itself (spectral regrowth, sidelobes). They are measured as ACLR or spectrum emission mask compliance. Spurious emissions fall further from the carrier, in the spurious domain, and are caused by mechanisms unrelated to the modulation: harmonics (2f, 3f, etc.), intermodulation between the carrier and other signals in the PA, parasitic oscillations from feedback paths, and LO leakage from the upconversion mixer. The boundary between OOB and spurious domains is defined by the ITU Radio Regulations: typically 2.5 times the necessary bandwidth from the carrier center frequency. OOB limits are set by the air interface standard (3GPP, IEEE). Spurious limits are set by the regulatory authority (FCC Part 15/22/24/27, ETSI EN 301 908).

What are the typical regulatory limits for spurious emissions?

For cellular base stations, 3GPP specifies spurious emissions below minus 30 dBm measured in a 1 MHz bandwidth in the general spurious domain (30 MHz to 12.75 GHz), tightening to minus 96 dBm in certain protected bands (such as GPS L1 at 1575.42 MHz or radio astronomy bands). For mobile handsets, the limit is minus 36 dBm in a 100 kHz bandwidth. FCC Part 15 for unlicensed devices requires spurious emissions below minus 41.3 dBm/MHz (field strength of 500 microvolts per meter at 3 meters). These limits mean that a 46 dBm base station transmitter must suppress its second harmonic (at twice the carrier frequency) by at least 76 dB, typically requiring two or three stages of bandpass filtering at the antenna port.

How are spurious emissions measured?

Spurious emissions are measured with a spectrum analyzer connected to the transmitter output through a calibrated attenuator. The analyzer sweeps from 30 MHz to at least the 10th harmonic of the carrier (or to 12.75 GHz, whichever is less). Resolution bandwidth is set to the specified measurement bandwidth (typically 100 kHz or 1 MHz depending on the standard). At each frequency, the measured power is compared against the emission mask. For conducted measurements, the transmitter output connects directly to the analyzer. For radiated measurements, the device is placed in an anechoic chamber or on an open-area test site (OATS) and the field strength is measured with a calibrated antenna at the specified distance (typically 3 or 10 meters). Radiated measurements capture emissions from cables, connectors, and PCB traces that conducted measurements miss.

Regulatory

Spurious Emission

A 5G base station transmits at 3.5 GHz with +46 dBm output power. Its second harmonic at 7.0 GHz falls in a radio astronomy protected band. The regulatory limit in that band is −96 dBm per MHz. The transmitter must suppress the second harmonic by at least 142 dB (46 − (−96) = 142 dB). The PA alone generates the harmonic at about −30 dBc (30 dB below the carrier), giving 76 dB of natural suppression. The remaining 66 dB must come from output filtering: two cavity bandpass filters in series, each providing 35 dB rejection at 7 GHz. Without these filters, the base station would illegally interfere with radio telescopes attempting to detect signals at −200 dBm.

Category: Regulatory

Domain: >2.5× BW from carrier

Typical Limit: −30 dBm/MHz (general)

Spurious Emission Sources and Levels

Source	Typical Level	Frequency	Mitigation
2nd harmonic	−25 to −35 dBc	2×f_carrier	Lowpass/bandpass filter
3rd harmonic	−35 to −50 dBc	3×f_carrier	Lowpass/bandpass filter
LO leakage	−30 to −50 dBc	f_LO	Balanced mixer, filtering
Parasitic oscillation	−20 to −40 dBc	Unpredictable	Stability design, damping
IMD (collocated TX)	−30 to −60 dBc	mf₁ ± nf₂	Isolators, diplexers
Clock/digital coupling	−40 to −70 dBc	Clock harmonics	Shielding, layout

Required filter suppression:
S_filter = P_TX − P_spur − L_limit
46 dBm TX, −30 dBc harmonic, −96 dBm limit: S = 46 − 30 − (−96) = 112... wait no.
S_filter = (P_TX + P_spur,dBc) − L_limit
P_spur = 46 − 30 = 16 dBm. Need: 16 − (−96) = 112 dB suppression from filters!

ITU spurious domain boundary:
±2.5 × necessary bandwidth from carrier center

Common Questions

Frequently Asked Questions

OOB vs. spurious?

OOB: immediately adjacent, caused by modulation (spectral regrowth). Spurious: far from carrier, from harmonics/IMD/parasitics. Boundary: 2.5× BW from center. OOB limits: 3GPP/IEEE. Spurious limits: FCC/ETSI.

Regulatory limits?

BS general: −30 dBm/MHz. Protected bands (GPS, astronomy): −96 dBm/MHz. Handsets: −36 dBm/100 kHz. FCC Part 15 unlicensed: −41.3 dBm/MHz. A 46 dBm TX needs 76+ dB harmonic suppression for general, 142+ dB for protected.

How measured?

Spectrum analyzer sweep 30 MHz to 12.75 GHz (or 10th harmonic). RBW per standard (100 kHz or 1 MHz). Conducted: direct connection. Radiated: anechoic chamber at 3 or 10 m with calibrated antenna. Radiated catches PCB/cable emissions.

Related Terms

← Spectrum Analyzer Stability Circle →

Compliance

Spurious Filter Budget Tool

Enter transmit power, harmonic levels, and applicable regulatory limits. Compute the required filter suppression at each harmonic frequency to ensure compliance across all protected bands.

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