Also known as: phase noise, oscillator phase noise, SSB phase noise
Phase noise is the rapid, random fluctuation in the phase of an oscillator’s output, which smears an ideally pure carrier into a band of “skirts” around its nominal frequency.1 It is the frequency-domain face of timing jitter, and in a receiver it is the mechanism behind reciprocal mixing — the effect by which a noisy local oscillator lets a strong nearby signal raise the noise floor on the channel you are trying to hear.
How it works
An oscillator’s output can be written as cos(2πf₀t + φ(t)), where φ(t) is a small random phase perturbation. Because that phase wanders continuously, energy that would sit in a single line at f₀ is instead spread into a continuous pedestal on either side. The standard measure is £(f), single-sideband phase noise in dBc/Hz: the ratio of noise power in a 1 Hz bandwidth at a given offset from the carrier to the total carrier power. A quote like “−110 dBc/Hz at 10 kHz offset” means that 10 kHz away, each hertz of bandwidth holds 110 dB less power than the carrier. Phase noise is worst close in and improves as you move out, eventually flattening into the oscillator’s broadband noise.
Reciprocal mixing is why this matters on receive. In a mixer, the local oscillator multiplies with the incoming RF. If the LO carries phase-noise skirts, a strong off-channel signal mixes with those skirts and its energy lands in your IF passband as raised noise — even though the strong signal is nominally out of band. A clean LO keeps the skirts low so nearby strong signals stay out.
On transmit, the same phase noise degrades the signal directly: it rotates the constellation from symbol to symbol, inflating the error vector magnitude and limiting how high an order of modulation the link can carry.
In practice
Phase noise is set by oscillator design. A free-running crystal oscillator is good close in; a phase-locked loop synthesizer trades close-in noise (dominated by the reference and loop) against far-out noise (dominated by the VCO), and the loop bandwidth is chosen to blend the two. Higher output frequencies are worse: multiplying an oscillator up by N raises its phase noise by 20·log₁₀(N) dB. This is a distinct property from long-term frequency stability — phase noise is the short-term jitter around the carrier, while stability is the slow drift of the carrier’s average frequency; a source can be excellent at one and poor at the other.
Relevance to SDR
Reciprocal mixing is a real limit for SDR monitoring in crowded RF environments. A budget tuner with a noisy synthesizer can have its usable sensitivity destroyed near a strong pager or broadcast transmitter, because that signal reciprocal-mixes with the LO skirts and buries a weak trunking control channel. This is often mistaken for poor sensitivity when the real fault is LO spectral purity; better SDRs (and a preselecting band-pass filter to keep the strong signal out) are the fix.
GopherTrunk is a software decoder and does not generate any RF oscillator of its own; the phase noise that reaches its decode chain is whatever the front-end SDR contributed before sampling. The concept is diagnostic for its users: a signal that looks strong on the waterfall yet decodes with a stubbornly high error rate near a powerful neighbour is a classic reciprocal-mixing signature pointing at the receiver’s oscillator, not at GopherTrunk’s DSP.
Sources
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Phase noise — Wikipedia, definition of oscillator phase noise, dBc/Hz, and reciprocal mixing. ↩