CML Microcircuits
COMMUNICATION SEMICONDUCTORSCMX998Cartesian Feed-back Loop Transmitter IC. . . . . . Transmitter Linearization for High Speed Data Systems

INN/998/1

www.cmlmicro.com

The CMX998

••
••••
•••
•
•
•
•
•
•

Frequency Range: 100MHz to 1GHz
Wide-Band Noise: -148dBc/Hz
SPI Compatible C-BUS Serial Interface
Forward Path Up Converter
Reverse Path (Feedback) Down Converter
System and Path Gain-Controls
Error Ampliﬁers
Selectable Open and Closed Loop Functions
360° Loop Phase-Shift Control
Instability Detector
Linearization Gains of 30dB or More Achievable
Flexible Digital Interface (for CMX980A and CMX981)
Available in a Q1 (64 no-leads) VQFN Package
Low (3.0 to 3.6 volt) Power Requirement
EV9980 EvKit is Available for IC Evaluation and Demonstration

I
I and Q Inputs
fromTx Modulator

As a state-of-the-art design for an integrated CFBL, the CMX998 improves theefﬁciency and linearity of transmitters of non-constant envelope modulation by theuse of a ﬂexible Cartesian feed-back loop.
Consisting of an ‘I’ and ‘Q’ up-converter (modulator) in the forward (Tx) path and ‘I’and ‘Q’ mixers in the feedback (Rx) path, the CMX998 makes available a complete
collection of building-block type circuits on a single integrated circuit that the designer/user can select to perform all of the functions required to produce a clean, linear, Tx
output signal in a wide-ranging RF spectrum.
Covering a wide RF range, from 100MHz to 1GHz, it is usable with channel
bandwidths up to 150kHz.
The CMX998 offers high performance, low wide-band noise and a large degree oflinearization gain. Typically, 30dB of linearization gain is acheivable.
Available in a compact 64 no-leads VQFN package this product is an ideal companion
for the CMX981 advanced digital radio baseband codec IC in digital radio designs.

PAQCMX998

LO Generator
andPhase Control

CFBL Functions for:TETRA Terminals
TETRA 2 Terminals
APCO P25 Phase 2Aviation Systems
Mobile Satellite Terminals
Linear Modulation Schemes:
QPSK, pi/4 DQPSK, 8PSK, QAM, OFDM, F4FM
 Direct Interface to CMX981 and CMX980A
The CMX998

The Cartesian loop works to improve the linearity of the Tx Power Ampliﬁer (PA) by the action of a feedback loop.
The baseband I and Q format Tx input signal in the forward path and an I and Q demodulated feedback signal
in the reverse path, developed from the RF PA output, are each applied to an error ampliﬁer to form a negative
feedback, closed-loop system that corrects RF PA distortion. The feedback I and Q demodulator local oscillator
phase must be adjusted to align input and feedback I and Q signals.

Forward Path

FeatureProperty

Supply Requirement

3.0 to 3.6 Volts

Frequency Range

100MHz to 1GHzWideband Noise Figure-148dbc/HzControl

C-BUS - SPI Compatible
Serial Interface

Package Style

Q1 64 No-Leads VQFNThe I and Q forward path of the IC each consists of differential input ampliﬁer stages providing gain/attenuation, Evaluation Resource
EV9980 EvKit
ﬁltering and level translation via external components and for conversion to a single-ended format. I and Q
differential error ampliﬁers are employed to produce the CFBL error signal by comparing the feedback signal with the reference modulation signal. The ‘error’ signal
is a pre-distorted waveform that, when applied to the remainder of the forward path, will produce a perfect signal at the modulation output. The I and Q (up-converter)
modulator provides translation from baseband signals to a low noise modulated RF signal. This stage also provides gain control.

Input Ampliﬁers
 For input signal processing Two of each for I and Q Tx channels
 Differential inputs
 Flexible conﬁguration:• DC level translation• Filtering• Signal gain or attenuation

Error Ampliﬁers
 Compare reference signal with feedbacksignal
 Differential inputs
 Selectable functions via external components

Up Converter I and Q Modulators
 Low noise output
 Baseband translation to RF Gain/attenuation settings

Feedback (Reverse) Path

The feedback path comprises a variable attenuator, I and Q mixer down-converter and baseband ampliﬁers and switching operations. Low-noise baseband ampliﬁers
at the mixer outputs provide the correct gain to ensure that signal levels around the loop are optimised for noise and linearity. Various outputs are available at thisstage to permit monitoring. The loop switches provide the ability to break the loop and operate in open-loop mode whilst simultaneously reducing the gain of the error
ampliﬁers

Down Converter Low-noise, linear operation
 Variable attenuation input and output stages
 Broadband baseband path
 Minimum time-delay

2Loop Switching Open/closed loop facilities
 Facilitates loop phase calibration

Instability Detector
 Detects out-of band energy
 Signal peak-hold facility
 Gain control and level shifterFunctions and Signal Flow
‘I’ Channel
Feedback InputA2IP

A1IO

EAIP

A2IO

EAIO

EAIN_++__A1IN‘Q’ ChannelFeedback Input

Input Amplifiers (A1)

Input Amplifiers (A2)
EAQP

EAQO

EAQN

A2QO

A2QP

A1QO

I and Q Channel Inputs
from Tx Modulator

Error Amplifier (‘I’)

MODQN‘I’ Channel Tx+MODIN

A1IP

DCMEAS

MODIP

MODQP

FORWARD PATH_A1QP‘Q’ Channel Tx+__A1QN

Tx Outputs to PA++MON

Error Amplifier (‘Q)

VCC

DC Null

4:1Balun

90

SDI

SDO

0/2

Level
Translation

CSN

Bus AccessandRegisters

LO Splitter
BufferDivide by 2

Feedback Phase
Shift Control

RESET

Up Converter

FORWARD POWER

C-BUS Serial Interface
SCKPA
REVERSE POWER

MOP

RF Power Detector

/2Phase
ShiftControl

Band
Gap

Band Gap
CircuitryRx Inputs fromRF Sensors

Loop Switches
Local Oscillator
Buffer+Instability Amplifier

DIP
1:1
Balun

Peak Detector_DIN

Down Converter

LOP

VREF

BVREF

LON
1:1
Balun

SIQ

PDI
PDO

IAO

IAI

IASIG

SOI SOQDOQ SIIFEEDBACK PATH

DOI

I and Q
Feedback Outputs
to Tx Error Amps

Local Oscillator
Input(a) Please note that with the exception of the band-gap circuitry, power supply inputs, outputs and components are not shown.
(b) Grey arrowheads shown on this diagram represent external, connecting circuitry, details of which are given in the product datasheet.

3

Evaluation, Experimentation and Design-in

CMX998 Performance

The EV9980 evaluation kit is available for evaluation andexperimentation of the CMX998 and to reduce design-intime.
••
•
•
•
•
•
•
•

Tx output spectrum in open loop and CMX998 closed loop operation

Allows Full Evaluation of the CMX998
Operational Frequency Range: 100MHz to 1GHz
Complete Demonstration of CFBL Functionality(at 400MHz and 800MHz)
Access to RF, Control and Baseband Signals
Support and Interfacing for Customer PAs
Differential or Single-ended I and Q Inputs
360° Loop Phase-Shift Control
Interfaces to CMX981 EvKit
Can Utilise PE0001 EvKit Interface

E V9980

Open loop (no linearization) gives poor performance

Member Companies

The CMX998, the CMX981 and TETRAThe CMX998 fulﬁls the TETRA requirement. The TETRA standard is a good benchmark for aCartesian feed-back loop as it calls for good linearity to achieve the required adjacent channel powers
of up-to -60dBc.
The EV9980 can be conﬁgured, via a dedicated interface, to work alongside the CMX981 Advanced
Digital Radio Baseband Processor or with a custom baseband system.
As an Advanced Digital Radio Processor IC compatible with the TETRA speciﬁcation, the CMX981
includes both I and Q and auxiliary codecs, is compatible with the CMX998 and provides a speciﬁc DCoffset correction register to calibrate out CMX998 DC offsets.

CML Microcircuits(UK)Ltd
COMMUNICATION SEMICONDUCTORSTel: +44 (0)1621 875500 Fax: +44 (0)1621 875600
sales@cmlmicro.com www.cmlmicro.com

CML Microcircuits(USA) Inc.

Your Local CML Distributor

COMMUNICATION SEMICONDUCTORS
Tel: +1 336 744 5050 and 800 638 5577 Fax: +1 336 744 5054
us.sales@cmlmicro.com www.cmlmicro.com

CML Microcircuits(Singapore)PteLtd
COMMUNICATION SEMICONDUCTORSSingapore
Tel: +65 67450426 Fax: +65 67452917
sg.sales@cmlmicro.com www.cmlmicro.com

ShanghaiTel: +86 21 63174107 and +86 21 63178916 Fax: +86 21 63170243
cn.sales@cmlmicro.com.cn www.cmlmicro.com
A CML Microsystems Plc Company© 2006 CML Microcircuits

Closed loop (CMX998 linearization) gives much improved performance

Design Resources
Design and application support is available from: www.cmlmicro.com/IC and EvKit Technical Datasheets
Function Image™ File Download
Application Notes
Frequently Asked Questions
Application Support via local ‘help’ desks