Local oscillator

Lore & Background

The local oscillator is a key component in the superheterodyne receiver, the most common type of radio receiver circuit. In this application, the LO frequency is chosen to be similar to the received radio frequency, such that the difference between them is much smaller than the RF. Either high-side injection (LO frequency greater than RF) or low-side injection (LO frequency less than RF) may be employed, and the difference frequency is filtered to extract the intermediate frequency (IF). In many receivers, the functions of local oscillator and mixer are combined into one stage called a "converter," reducing space, cost, and power consumption.

Local oscillators are also used in satellite television reception, where microwave frequencies from the satellite are converted to lower frequencies by a fixed-frequency LO and mixer mounted at the antenna, allowing signals to be sent over cable with acceptable loss. Performance requirements include ensuring no spurious signals are radiated (which could cause interference), producing a stable frequency with low harmonics, providing enough output power to drive subsequent stages, and maintaining low phase noise where timing is critical. In channelized systems, the precision of the frequency synthesizer must match the channel spacing.

During World War II, detection of local oscillator emissions could disclose the presence of a receiver. Allied soldiers were not allowed to have superheterodyne receivers because Axis equipment could detect LO emissions, leading to the creation of the foxhole radio, a simple improvised receiver with no local oscillator. Military receivers such as the RCA AR-88 were engineered to suppress LO emissions through excellent shielding and the use of tuned pentode RF stages with virtually zero reverse gain.

Reader's Guide

The local oscillator is fundamental to the superheterodyne receiver, the dominant radio receiver architecture, enabling improved performance by converting incoming signals to a fixed intermediate frequency. This frequency conversion process, heterodyning, produces sum and difference frequencies, allowing selective filtering and amplification at a stable IF. The LO's significance extends across numerous communications systems, including modems, cable television set top boxes, telephone trunklines, microwave relays, telemetry, atomic clocks, radio telescopes, and military electronic countermeasures. In satellite television, a fixed-frequency LO at the antenna converts microwave downlink signals to lower frequencies for transmission over cable, overcoming signal loss at the original frequency.

The choice of LO type involves trade-offs: crystal oscillators offer stability and low cost but fixed frequency; variable-frequency oscillators allow tuning but compromise stability. Modern systems use frequency synthesizers or phase-locked loops to generate stable, tunable LO frequencies while maintaining adequate noise characteristics. Performance requirements—low spurious emissions, stability against temperature/voltage/mechanical drift, sufficient output power, low phase noise, and precision compatible with channel spacing—are critical for system performance. The historical importance of LO emissions is illustrated by World War II countermeasures, where detection of LO radiation could reveal receiver presence, leading to designs that suppressed such emissions through shielding and low reverse gain stages.

Did You Know?

• In many receivers, the local oscillator and mixer are combined into one stage called a 'converter' to reduce space, cost, and power consumption.
• The RCA AR-88 military receiver used two tuned pentode RF stages with virtually zero reverse gain to prevent LO emissions from backing out through the antenna.