CMX998

– Cartesian Feedback Loop Transmitter

RF Building Blocks, Transmitters

CMX998

– Cartesian Feedback Loop Transmitter

A Cartesian Feedback Loop (CFBL) Transmitter improves the efficiency and linearity of transmitters for non-constant envelope modulation systems. The CMX998 is an integrated solution for a linear based Cartesian Feedback Loop (CFBL) transmitter. Acting as a direct conversion quadrature mixer from I and Q to RF output, it provides the capability to linearise the Power Amplifier (PA) via feedback from the PA’s output. Included are forward and feedback paths; local oscillator circuitry including loop phase control; an instability detector and uncommitted op-amps for input signal conditioning. The differential inputs are ideal for direct connection to standard modems and interface ICs such as CML’s CMX7163 QAM Data Modem, CMX983 Analogue Front End (AFE) for Digital Radio, CMX7364 Multi-mode High Performance Modem and more!

Devices are available through our Distributor Network.

Download Datasheet

Detailed block diagram of the CMX998 chip, showing phase shift control, LO splitter/buffer/divide-by-2, power detector, DC null, baluns and PA stages, labelled with pins including VREF, BVREF, VDDIO, SCK, SDI, SDO, RESET, CSN, A1IN/A1IP, A1QN/A1QP, EAIN/EAIP, EAQN/EAQP, MODQP/MODQN/MODIP/MODIN, MON, VCC, MOP and DIN/DIP

CMX998-block-image

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CMX998

What is the specification for the accuracy of the CMX998 transmitter in terms of I/Q phase and amplitude balance?

A. The modulation accuracy of a CMX998 transmit is determined by the down-converter (feedback path).

The I/Q amplitude and phase balance of the down-converter is therefore the key factor in determining the contribution the CMX998 makes to a transmitters modulation accuracy.

In practice it doesn’t matter if the I/Q error is an amplitude imbalance or a phase imbalance, it is the total error that is important.

For this reason the CMX998 specifies the I/Q phase and amplitude balance as a composite in terms of ‘Feedback path Image Suppression’.

A worst-case value of –30dB is guaranteed.

The parameter can be calculated from I/Q phase and amplitude balance using the following formula:

Can the CMX998 be used for TETRA 2?

A. Yes, the CMX998 meets the linearity and wideband noise requirements of TETRA QAM modes for 25kHz, 50kHz, 100kHz and 150kHz channels.

Note that a different loop-filter might be required for the 100kHz and 150kHz channels.

How do I control the CMX994 or CMX998 on the DE9941 board?

The DE9941 includes the CMX7164 Multi-Mode Modem, the CMX998 Cartesian Feedback Loop Transmitter, and the CMX994 Direct Conversion Receiver. When the PE0003 Ev Kit Interface Card is used as the host controller for DE9941, the PE0003 communicates with the CMX998 and CMX994 through the CMX7164’s SPI Thru-Port. The CMX7164 SPI Thru-Port is beneficial because it allows control of peripherals with fewer uC pins and PCB traces.

Can you help explain how the instability detector on the CMX998 operates and how it can be used?

A. The instability detector found on the CMX998 is a relatively simple mechanism that measures the level of out-of band energy of the feedback loop.

If the loop becomes unstable due to phase inaccuracies the feedback loop will tend to spill energy out-of band.

The instability detector comprises a high-pass filter followed by a peak-detector circuit and this combination indicates the presence of this high frequency energy.