CML Semiconductor Products
AN/Data/9x9B/1 December 1997
FX919B and FX929B Fade Management
and ‘A’ Version Compatibility
Introduction
This application note is to be read in conjunction with the current FX919B and FX929B Data Sheets.
This document is issued to clarify points in currently published documents and to present new information
that will enable the user to easily understand and implement this 4-Level FSK product.
The FX919B and FX929B are CMOS integrated circuits that contain all of the baseband signal
processing and Medium Access Control (MAC) protocol functions required for a high performance 4-level
FSK Wireless Packet Data Modems. They are designed to interface with the modem host processor and the
radio modulation/demodulation circuits to deliver reliable two-way transfer of the application data over the
wireless link. The FX919B and FX929B are individually backwards compatible with the FX919A and FX929A
but offer better performance during radio-link fading and have selectable Tx symbol shapes.
This document describes issues and strategies relating to fading applications. It also describes pertinent
FX919B and FX929B features and controls which may be used in an FX919B or 929B based system.
Fade Management
Data Decoding and Receiver Training
In order to accurately decode a received data stream, a modem must properly recover its clock and
signal components which are then interpreted to reconstruct the transmitted data.
Because transmit and receive modems are independent circuits (having separate and different local
clock frequencies and signal levels) a receiving modem must be trained or synchronised, immediately prior
to each data transfer, to establish a common and co-ordinated sense of bit timing.
In addition, the receiver must also be trained to establish a sense of nominal signal amplitude and DC
offset, against which the received data signal is compared to properly interpret its data bit values. Training is
generally performed by transmitting a specific data stream, the preamble, which provides evident and
consistent timing (zero crossing) and amplitude (maximum peak-to-peak range) characteristics.
During the training period, a receiving modem adjusts its local clock and level circuits until the known
preamble data stream is accurately reconstructed. Once this adjustment is complete, the transmit and
receive modems are synchronised and user data can be reliably transferred. (Note that data transfer must
immediately follow the preamble or synchronisation could be lost due to relative clock and level drift between
transmit and receive modems.). However both the FX919 and FX929 have been designed to lock onto the
received signal without any preamble, as is required in an application such as RD-LAP (Please refer to
section 1.6.3 of the relevant data sheet).
The Fading Problem
In mobile wireless applications, dynamic changes in the radio channel alter the phase and amplitude of
received signals. In order to maintain its synchronisation through these changes, a receiving FX919B and
FX929B uses feedback techniques to dynamically revise its local clock and level circuits according to the
changing nature of the received signal. However the receive-level measurement circuits on wireless data
modems such as the FX919 and FX929 can also be disturbed by short, large-amplitude noise bursts, as can
occur at the discriminator output during deep fades.
1
CMLMICRO [ CML MICROCIRCUITS ]
