Pager

Overview

A pager is a one-way radio receiver designed to receive short numeric or alphanumeric messages transmitted on a dedicated frequency band. Unlike a two-way radio, the pager cannot transmit; it only listens to a broadcast channel, making it power-efficient and suited for devices carried for extended periods on a single battery charge.

The POCSAG (Post Office Code Standardisation Advisory Group) protocol is the international standard for pagers, permitting up to 4 call groups per frequency and message batches every two seconds. A pager automatically powers down its receiver when not receiving, waking periodically to check for incoming traffic to its assigned codes. When a message arrives, the device emits audio and/or vibration alerts.

Pagers remain in use in emergency services, manufacturing, hospitality, and healthcare, especially in environments where mobile networks are unreliable or where hands-free one-way notification is preferred.

How it works

The Antenna picks up radio waves on the 460–470 MHz band and feeds them into the RF Module. The POCSAG Decoder IC heterodyne mixes the incoming signal down to a fixed intermediate frequency, then the Discriminator demodulates the FM signal to recover the baseband FSK (frequency-shift keying) bit stream.

The POCSAG Decoder IC decodes the POCSAG frame format in hardware. POCSAG batches 16 addresses and four 32-bit code words per transmission. The decoder checks if any of those code words is addressed to one of the pager's preset address numbers. If a match is found, the Microcontroller is awakened and the 32-bit code word is placed in the Memory IC.

The Microcontroller extracts the message type (numeric vs. alphanumeric) and content from the code word. For numeric messages, a simple lookup table converts binary patterns to digits. For alphanumeric, the code word is interpreted as 5-bit characters. The text is stored and the Display Module is refreshed to show the incoming message.

Simultaneously, the Audio Module module's Tone Generator IC produces an alert beep and the Vibration Motor pulses on and off for 1–2 seconds. The user can acknowledge and scroll through stored messages via the Button Assembly membrane keypad.

In standby, the Microcontroller is halted and only a low-power wake circuit monitors the POCSAG Decoder IC. Every few seconds, the receiver synchronizes to the POCSAG frame boundary (using pilot tone or frame preamble) and checks for traffic before powering down again. This duty-cycle approach lets a single AAA battery cell power a pager for a month or longer.

Protocols & Standards

POCSAG is defined in ITU-T Recommendation X.25 and has been superseded for new deployments by more spectrally efficient protocols (DMR, P25, TETRA), but remains widely deployed in existing networks. Pagers are also manufactured to support legacy 1200 baud numeric-only protocols in some regions.

Frequency allocation varies: North America uses 929–930 MHz; Europe and Asia use 460–470 MHz. A pager hardware revision may be needed to operate on different bands.

Common Failure Modes

LCD contrast fades over time; weak reception occurs in buildings with poor RF penetration (especially at the band edge of allocated spectrum). Vibration motor bearings wear and may rattle or fail to spin after 5+ years of heavy use. Battery corrosion in the Battery Holder can prevent reliable contact. Antenna degradation (loosening, corrosion of threaded collar) reduces received signal strength.