A radio clock is an electronic combination of a digital alarm clock and AM/FM radio. It is commonly a stationary, plug-in device used as a bedside alarm clock but may be battery-powered for use during travel or overnight stays. Also known as a clock radio, a radio clock sometimes contains a receiver that receives radio signals from an atomic clock or other time standard, which helps make it a reliable, accurate timepiece.
Atomic, radio-controlled clocks house a device that keeps time based on a coded radio signal it receives from a time standard such as certain radio stations. These clocks synchronize once daily either from one or multiple time transmitters, such as GPS, to maintain extreme accuracy. Weather conditions or bad atmospheric conditions may prevent an atomic clock from being synchronized correctly. As a result, some models include an indicator that alerts the radio clock’s owner to possible inaccuracy.
A radio clock usually contains a built-in alarm that can be set to sound at a specific time. Some allow the device’s radio to activate instead of an alarm’s beep at the designated time. For people who require multiple alarms, a radio clock known as a progressive alarm clock allows different alarms like the radio to sound at different intervals throughout the day. A “snooze” button on a radio clock resets the alarm to sound at a later time, usually 5 to 10 minutes, as determined by the user. This function allows a person to sleep a few minutes later than the set alarm time without the risk of sleeping later than intended.
Modern radio clocks generally have more features than just a digital clock and AM/FM radio. Some travel radio clocks might include built-in compasses, lanterns, wireless thermometers, and other outdoor-related gadgets. Another popular feature is an integrated CD player or MP3 player interface.
Digital radio clocks may feature a battery backup to help keep the correct time in the event of a power outage or surge. This is an important addition for radio clock owners who regularly rely on the clock's alarm. Radio clocks that do not have battery backup risk being reset to midnight, or another incorrect time, thereby causing the alarm not to trigger on time.
Technologies like the computer alarm clock and cellphone alarms have begun to replace stationary radio clocks for their convenience, ease of use, and portability. Both computer and cellphone alarms can be set to various tones as determined by the owner, which lets one wake up to a different alarm each day. Music may also be downloaded to either device, and may be used in place of a tone as the wake-up alarm.