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.

The local oscillator sets the tuned frequency: the mixer shifts the chosen band down for digitising.

How it works

In hardware the LO drives an analog mixer; in software a numerically controlled oscillator performs the same shift digitally inside a digital down-converter. LO inaccuracy shows up as a PPM frequency error.

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.

Sources

1. Local oscillator — Wikipedia, on the tunable reference tone a mixer uses to shift a band down. ↩

See also

• Superheterodyne receiver
• Digital down-converter (DDC)
• Frequency
• PPM frequency correction

Learn it step by step

• How an SDR receiver works