Also known as: spurious emission, spurs, spurious output, unwanted emissions
Spurious emissions are any radio-frequency energy a transmitter radiates outside the bandwidth it needs for its wanted signal, that could be removed without harming the information being sent.1 They include harmonics, mixer and intermodulation products, parasitic oscillations, and local-oscillator leakage — and because they interfere with other services, national and international regulators cap them with hard limits.
How it works
Emissions are split by frequency offset. Out-of-band emissions fall in the shoulders immediately adjacent to the channel and come mostly from modulation sidebands and imperfect pulse shaping; they are controlled through the occupied-bandwidth and adjacent-channel-power specifications. Spurious emissions proper are the discrete products that land farther away — at harmonics, at sums and differences of the various oscillators inside the radio, and at parasitic resonances.
The physical origins mirror those of harmonics: every nonlinear stage (power amplifier, mixer, frequency multiplier) creates new frequencies, and any oscillator that leaks past its intended stage can radiate directly. A superheterodyne transmitter with several conversion stages has many possible spur frequencies, computed from integer combinations m·f₁ ± n·f₂ of its internal signals.
Because most spurs sit well away from the wanted channel, the primary defence is filtering — low-pass filters after the final amplifier for harmonics, and band-pass / cavity filters to reject conversion products — combined with good shielding and careful gain staging so that no stage is driven hard enough to generate strong products in the first place.
In practice
Regulators specify a spurious-emission mask: a curve of maximum allowed power versus frequency offset that a transmitter must sit beneath. In the United States the FCC sets these limits in its rules for each radio service; in Europe ETSI harmonised standards do the same, both anchored to ITU-R recommendations. Limits are typically quoted as an attenuation relative to the mean transmit power (for example, “−60 dBc or better”) or as an absolute power in a reference bandwidth. Land-mobile equipment must pass type approval against these masks before it can be sold, and repair or modification that degrades a transmitter’s spurious performance makes it non-compliant.
Relevance to SDR
Spurious-emission compliance is a transmitter concern, and every P25, DMR, and TETRA base station or subscriber unit is engineered and tested against the applicable mask. A poorly maintained transmitter that develops a spur can appear as a phantom carrier on a monitored band.
GopherTrunk does not transmit and therefore has no spurious-emission obligations of its own. The concept still helps its users interpret a waterfall: SDR receivers generate internal spurs and images too — a spike that stays put as you retune, or that appears at a predictable offset, is usually a receiver-generated spur rather than a real signal. Recognising these avoids chasing spurious “transmissions” that are artefacts of the front-end, and pairs naturally with an understanding of image frequency and receiver overload.
Sources
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Spurious emission — Wikipedia, definition and regulatory framing of unwanted transmitter output. ↩