CB Radio

Overview

Citizens Band (CB) radio is an unlicensed two-way radio service on the 27 MHz band, popular in trucking, off-roading, and emergency communication. This transceiver provides 40 FCC-allocated channels with both amplitude modulation (AM) for maximum range and single-sideband (SSB) for better interference rejection and power efficiency. The compact form factor with a Hand Microphone allows mobile installation in vehicles or hand-carried operation.

The unit's signal path is straightforward: incoming RF from the antenna is routed to the Transceiver Mainboard, where it is mixed, amplified, and demodulated. Outgoing audio from the Hand Microphone modulates a carrier generated by the Synthesizer and PLL, which synthesizes any of 40 pre-programmed CB channels via a phase-locked loop. Output power is limited by regulation to 4 W in AM mode and 12 W in SSB mode. The Control Interface allows users to select channels and adjust squelch (noise gate) threshold.

How it works

The receiver path begins when RF energy arrives at the antenna and passes through the Antenna Connector to the Transceiver Mainboard. The RF Mixer IC combines the incoming CB signal with a local oscillator signal from the Synthesizer and PLL to produce an intermediate frequency around 455 kHz. This IF is amplified by IF Amplifier stages and passed through an AM detector (diode and filter) or an SSB product detector, depending on the selected mode.

The detected audio is filtered, amplified, and routed to the Speaker Assembly for monitoring. A squelch circuit (controlled by the Squelch Potentiometer) mutes the speaker when no signal is present, preventing background noise between transmissions.

For transmission, voice picked up by the Electret Capsule is preamplified and sent to the Modulator IC. In AM mode, the modulator varies the RF amplitude in proportion to the audio signal. In SSB mode, a balanced modulator generates only the lower or upper sideband, suppressing the carrier to save bandwidth and power. The modulated signal is then upconverted to the selected CB channel frequency (set by the Synthesizer and PLL) and amplified to the regulated 4 W (AM) or 12 W (SSB) output level before reaching the antenna.

The Synthesizer and PLL is the frequency-setting heart of the rig. A PLL Synthesizer IC phase-locked loop compares a reference frequency from a Reference Crystal (10.24 MHz) (10.24 MHz) with feedback from a Voltage-Controlled Oscillator. The error signal tunes the VCO until it reaches one of the 40 CB channel frequencies, where the loop locks. This design is far more stable and user-friendly than the manual tuning dials of vintage CB units.

The Power Supply converts incoming 12 V vehicle power to multiple regulated rails needed by the RF and audio circuits, with bulk filtering and fusing to protect against voltage spikes and provide emergency shutdown capability.

Design rationale

CB radio remains popular in mobile applications because of its legal unlicensed status (in most countries), long range due to the 27 MHz frequency (longer wavelength = better propagation than VHF), and simple operation. The choice of a PLL synthesizer over crystal-per-channel allows 40 channels in a compact unit; older CB radios required a separate crystal for each channel, making them much bulkier.

SSB modulation reduces occupied bandwidth from about 8 kHz (AM) to 3 kHz (SSB) and concentrates power into one sideband, doubling effective radiated power for the same input without violating FCC limits. This makes SSB the preferred mode for DX (long-distance) CB operation.

Squelch circuitry is essential in a small vehicle where the ambient noise can otherwise mask weak incoming signals; without squelch, every bit of electrical noise and atmospheric static would blast through the speaker.