AN1998

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© NXP Semiconductors N.V. 2014. All rights reserved.

Application note

Rev. 3 — 24 July 2014

3 of 20

NXP Semiconductors

AN1998

An FM/IF system for DECT and other high-speed GFSK applications

1.

Introduction

To achieve the goal of wireless personal communications, allowing users access to the

capabilities of the global communications network at any time without regard to location

and mobility, cellular and cordless telephony have been taken as two major approaches.

Cellular systems are evolving towards smaller cells (micro cells) and lower power levels to

provide higher overall capacity. Cordless telephones have evolved from home appliances

towards widespread ‘universal’ low-power personal communications systems. With the

advent of digital cordless telephony, cordless systems with enhanced functionality have

been developed that can support higher data rates and more sophisticated applications

such as wireless private branch exchanges (WPBX) and public-access Telepoint systems.

One of the first digital cordless standards is the Digital European Cordless

Telecommunications (DECT) system, a pan-European standard designed to connect all of

Europe with a common digital cordless system. DECT is also a flexible standard for

providing a wide range of services in small cells.

In this application note, the SA639, an NXP low-voltage FM/IF system with several

important features such as post filter amplifier and active data switch, is proposed for

DECT and other high-speed digital wireless applications. A SA639-based DECT receiver

evaluation board has been developed. Detailed description of the SA639 FM/IF system,

structure of the evaluation board, design information, and experimental evaluation results

are presented.

2.

Review of DECT standard

DECT is designed as a flexible interface to provide cost-effective communications

services to high user densities in small cells. This standard is intended for the applications

such as domestic cordless telephony, Telepoint, cordless PBXs, and Radio Local Loop

(RLL). It supports multiple bearer channels for speech and data transmission (which can

be set up and released during a call), hand over, location registration, and paging.

Functionally, DECT is closer to a cellular system than to a classical cordless telephone.

However, the interface to PSTN or ISDN remains the same as for a PBX or corded

telephone. Table 1 is a summary of the key specifications of DECT and other digital

cordless telephone systems.

DECT is based on Time Division Duplex (TDD) and Time Division Multiple Access

(TDMA) with 10 carriers in the 1880 MHz to 1900 MHz band. Figure 1 illustrates the

DECT TDD/TDMA frame structure.

The completed frame is 10 ms in duration with 24 time slots. The first 12 slots are

allocated for the transmission from base station to handsets, and the other 12 slots are for

the transmission from handsets to base station. Each slot is 417

μs long with 480 bits. The

first 32 bits is a ‘1010...’ sequence for synchronization. The 32 kbit/s ADPCM CODEC is

used for speech coding in DECT, which provides 320 bits during each 10 ms frame. When

a call is made, two slots (one is in the first 12 slots, the other is in the last 12 slots) are

assigned to the user for transmit and receive.