AN1998
An FM/IF system for DECT and other high-speed GFSK
applications
Rev. 3 — 24 July 2014
Application note
Document information
Info
Keywords
Abstract
Content
High-speed digital wireless PCS applications, Digital European Cordless
Telephone (DECT), FM/IF system
An NXP low-voltage high-performance monolithic FM/IF system, the
SA639 is introduced to meet the increasing demand for high-speed digital
wireless PCS applications. In order to assist the system design, a
SA639-based performance evaluation board has been developed
according to the Digital European Cordless Telephone (DECT)
specifications. This application note presents detailed descriptions of the
SA639 FM/IF system, evaluation board, and design information including
circuit diagram, component list, and the board layout. The experimental
performance evaluation procedures, measured bit error rate (BER),
sensitivity to frequency offset, and sensitivity to FM deviation variation of
this system are also presented. Results indicate that the low-voltage
SA639 FM/IF system provides superior performance for high-speed digital
wireless applications.
NXP Semiconductors
AN1998
An FM/IF system for DECT and other high-speed GFSK applications
Revision history
Rev
3.0
Date
20140724
Description
Application note; third release
•
•
2.0
1.0
20040114
19970820
The format of this application note has been redesigned to comply with the new identity
guidelines of NXP Semiconductors.
Legal texts have been adapted to the new company name where appropriate.
Application note; second release
Application note; initial release
Contact information
For more information, please visit:
http://www.nxp.com
For sales office addresses, please send an email to:
salesaddresses@nxp.com
AN1998
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 24 July 2014
2 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.
AN1998
All information provided in this document is subject to legal disclaimers.
© 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
Table 1.
Standard
Region
Summary of digital cordless standards
CT2/CT2+
Europe/Canada
CT2: 864 - 868
CT2+: 944 - 948
TDD
TDMA
40
100
1
GFSK
(FM dev: 14 kHz to 25 kHz)
72 kbit/s
32 kbit/s ADPCM
2 ms
10 mW
DECT
Europe
1880 - 1900
TDD
TDMA
10
1728
12
GFSK
(FM dev: 288 kHz)
1.152 Mbit/s
32 kbit/s ADPCM
10 ms
250 mW
PHS
Japan
1895 - 1918
TDD
TDMA
77
300
4
π/4-DQPSK
32 kbit/s ADPCM
32 kbit/s ADPCM
5 ms
80 mW
PACS
USA
Tx: 1850 - 1910
Rx: 1930 - 1990
FDD
TDMA
16 pairs
300
8/pair
π/4-DQPSK
32 kbit/s ADPCM
32 kbit/s ADPCM
2.5 ms
200 mW
Frequency band
(MHz)
Duplex
Multiple access
Number of channels
Channel spacing
(kHz)
Users/channel
Modulation
Bit rate
Speech coding
Frame duration
Peak power
frequency
Ch 10
1897 MHz
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
24-slot frame (10 ms: 11520 bits at 1.152 Mbit/s)
base-to-handsets
Ch 1
1881 MHz
1
2
3
4
5
6
7
8
handsets-to-base
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
time
one slot (417 μs: 480 bits at 1.152 Mbit/s)
32 bits
synchronization
64 bits
signaling
320 bits
information
4
error
control
60 bits
guard
space
aaa-014182
Fig 1.
DECT TDD/TDMA frame structure
AN1998
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 24 July 2014
4 of 20
NXP Semiconductors
AN1998
An FM/IF system for DECT and other high-speed GFSK applications
Gaussian filtered FSK (GFSK) modulation scheme is employed in DECT. GFSK is a
premodulation Gaussian filtered digital FM scheme.
Figure 2
shows the block diagram of
a GFSK modulator. The advantages of GFSK can be summarized as follows:
Constant envelope nature —
This allows GFSK modulated signal to be operated with
class-C power amplifier without introducing spectrum regeneration. Therefore, lower
power consumption and higher power efficiency can be achieved.
Narrow power spectrum —
Narrow main lobe and low spectral tails keep the adjacent
channel interference to low levels and achieve higher spectral efficiency.
Non-coherent detection —
GFSK modulated signal can be demodulated by the
limiter/discriminator receiver as shown in
Figure 3.
This simple structure leads to low-cost
GFSK receivers.
GAUSSIAN
LPF
f
b
= 1.152 Mbit/s
BT
b
= 0.5 Mbit/s
FM
MODULATOR
∆f = 288 kHz
GFSK modulated
signal for DECT
aaa-014203
Fig 2.
Block diagram of GFSK modulator
GFSK modulated
signal for DECT
LIMITER
FM
DISCRIMINATOR
f
b
= 1.152 Mbit/s
aaa-014204
Fig 3.
Block diagram of GFSK demodulator
AN1998
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 24 July 2014
5 of 20
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