Citizens Band Radio - Technology

Technology

At the beginning of the CB radio service, transmitters and receivers used vacuum tubes; solid-state transmitters were not widely available until 1965, after the introduction of RF-power transistors. Walkie-talkie hand-held units became affordable with the use of transistors. Early receivers did not implement all channels of the service; channels were controlled by plug-in quartz crystals, with one of several operating frequencies selected by a panel control in more expensive units. Superheterodyne receivers (using one or two conversion stages) were the norm in good-quality equipment, although low-cost toy-type units used superregenerative receivers. With the earliest sets two quartz crystals were needed for transmitting and receiving on each channel, which was costly. By the mid-1960s "mixer" circuits made frequency-synthesized radios possible, which reduced cost and allowed full coverage of all 23 channels with a smaller number of crystals (typically 14). The next improvement came during the mid-1970s; crystal synthesis was replaced by PLL technology using ICs, enabling 40-channel sets with only one crystal (10.240 MHz). Almost all were AM-only, although there were a few single sideband sets.

Most CB radios sold in the United States have the following features:

• Automatic noise limiter or noise blanker: Reduces background noise (such as spark ignition)
• CB/WX switch: Selects weather-radio receiver
• Automatic level control (ALC): Limits the transmitter modulation level to reduce distortion
• PA: Some transceivers can drive an external speaker and act as a low-power public address system, or "bullhorn".
• RF gain: Adjusts the RF amplifier gain of the receiver; used to reduce received background noise, and to reduce "clipping" due to over-amplification of already-strong signals (for example, when the receiver is near the transmitter)
• NOR/9/19: Quickly tunes preset channels for calling or emergency use
• SWR: Meter used to monitor reflected power caused by mismatched antennas and antenna cables
• Volume control

Microphone choices include:

• Dynamic microphone: Uses magnetic coil and permanent magnet
• Ceramic mic: Uses a piezoelectric element; rugged, low-cost but high-impedance
• Echo mic: Deliberately introduces distortion and echo into transmitted audio
• Electret microphone: Uses an electrostatic method to convert sound to electrical signals
• Noise-canceling microphone: Uses two elements to reduce background noise
• Power mic: An amplified microphone