Also known as: local oscillator, LO
A local oscillator (LO) is a tunable reference signal mixed with the incoming signal to shift a chosen band down toward baseband.1 Tuning a receiver is just changing the LO frequency — everything else in the analog chain stays fixed.
How it works
A mixer multiplies the incoming RF by the LO tone. Trigonometry says the product of two sinusoids contains their sum and difference frequencies, so a station at frequency f mixed with an LO at f_LO yields components at f + f_LO and |f − f_LO|. The receiver keeps the difference and discards the sum. By moving f_LO, the operator slides which slice of spectrum lands on the fixed intermediate frequency or at baseband — that motion is what a tuning knob or a frequency entry actually commands.
Two LO qualities dominate reception. Accuracy: if the LO sits slightly off its nominal value the whole band lands off-centre, appearing as a PPM frequency error the software must correct. Spectral purity: a real oscillator is not a single clean line but a tone smeared by phase noise. That skirt mixes nearby strong signals down onto the wanted one (reciprocal mixing) and blurs the demodulated constellation — a carrier-clean but modulation-degraded capture is the classic fingerprint of LO phase noise.
Variants
- Analog LO — a crystal-referenced PLL synthesiser or VCO driving a hardware mixer, as in a superhet front-end.
- Quadrature LO — two copies 90° apart feeding an I and a Q mixer, producing complex IQ directly; the basis of zero-IF direct-conversion receivers.
- Numerically controlled oscillator (NCO) — the fully digital LO: software generates a rotating complex tone and multiplies it against the sample stream inside a digital down-converter, with none of the drift or phase noise of an analog part.
In practice
An LO that is not locked to a stable reference drifts with temperature, which is why serious receivers use a TCXO or an OCXO and why cheap dongles need periodic PPM calibration against a known signal. The choice of LO frequency also sets where the image falls: high-side versus low-side injection place the image on opposite sides of the band, one of which may be easier to filter. In a purely digital second stage the NCO’s “LO” is exact by construction, so residual tuning error is entirely the analog front-end’s.
Relevance to SDR
The LO sets which part of the spectrum lands in the ADC’s window, so its accuracy and stability directly affect tuning and decode quality. GopherTrunk cannot fix a physically noisy or drifting hardware LO in software, but it does perform the final, exact frequency shift digitally with an NCO when it channelises each control and voice channel.
Sources
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Local oscillator — Wikipedia, on the tunable reference tone a mixer uses to shift a band down. ↩