Cordless Telephone System Basics

This page explains the fundamentals of cordless telephone systems and outlines the frequency allocations used by cordless phones.

As you probably know, most modern homes and apartments are equipped with telephone jacks in every room. This lets you move a single telephone around the house as needed. Cordless telephones offer even more convenience. A cordless telephone system is made up of two parts: a base unit and a portable handset.

The base unit connects to a standard telephone line and plugs into an AC power outlet. It serves as a charging station for the handset.

Fig.1 Cordless Telephone System

The diagram above illustrates the system. Both the base unit and the handset have two-way radio transmission and reception capabilities. This is achieved by incorporating an RF transceiver in each unit, enabling full duplex operation (simultaneous two-way communication).

To support full duplex communication, cordless telephones use two different frequencies: one for transmitting and one for receiving. The base unit transmits in the 43-46 MHz range and receives in the 49 MHz range. Conversely, the handset transmits in the 49 MHz range and receives in the 43-46 MHz range.

The FCC has allocated approximately 25 duplex channels for cordless telephone systems. Frequency modulation (FM) is the modulation technique used. Crystal control is implemented to precisely set these frequencies, allowing users to select a channel with minimal interference. The FCC limits the transmit power to a maximum of 500 mW.

The maximum usable range between the base unit and the handset is about 100 feet.

Cordless Telephone Frequency Allocations - New Channels

Original Cordless Frequency Channels

Advanced Cordless Systems

More advanced cordless systems have been developed that operate in the PCS band, ranging from 902 to 928 MHz. There are four main types of these advanced systems:

• The first uses FM on channels within the 900 MHz frequency range.
• The second operates in the 900 MHz range and utilizes digital technology with ADPCM (Adaptive Differential Pulse Code Modulation).
• The third employs spread spectrum technology and operates in the 902-928 MHz range.
• The fourth also uses spread spectrum but operates at 2.4 GHz.