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New Tool for calculating noise when using a Cartesian Feedback Loop

10:51 09 June in Blog, Featured, RF Building Blocks

    Many transmitters using non-constant envelope modulation schemes benefit from the improved efficiency and linearity associated with using a Cartesian feedback around the power amplifier. Though the concept of using feedback to linearize an amplifier is straightforward, there are many system elements and parameters that can influence the resulting transmitter noise performance in a real system. Having a means of estimating this at an early stage in the design process can help in the selection of both components and parameter settings within the loop.

    The CML Microcircuits CMX998 provides a highly integrated Cartesian feedback loop solution. However the loop noise performance will depend not only on the integrated functions but also on the local oscillator phase noise as well as external element choices in the power amplifier line-up and in the feedback path back to the device. The use of evaluation boards and prototyping can help to answer whether or not the overall noise performance will meet the target required by the relevant standard but this can be time consuming.

    A new means of calculating performance with the ability to enter both internal and external system parameters and local oscillator phase noise values is now available from CML Microcircuits. This calculator is provided free as a potentially useful tool, but please note that this is on the basis of no warranty. The calculator runs in Octave or which can be downloaded free from the Octave website ( and presents a graphical user interface when run. A version of the calculator is also available for Matlab but please note that this version requires the user to have access to a Matlab license.

    Parameter settings can be entered in tabular form or from a system level block diagram. This includes gain and noise figures for all the functional blocks within the loop. Similarly the local oscillator noise characteristics are entered with close in noise and noise floor values with the transition between them defined by two corner frequencies. The loop response is completed with values for the three time constants defining the error amplifier response. Two additional features allow the phase of the demodulated feedback signal to be adjusted and the delay time around the loop.

    CMX998 LNC block diagram
    CMX998 LNC block diagram

    The results for a calculation are presented graphically against selected frequency limits for both closed loop system noise, with the option of a specification mask superimposed, and separately the system open loop gain. If left open, the graphs will superimpose to allow easy comparison after any changes have been made.

    CMX998 LNC graph
    CMX998 LNC graph

    The CMX988 Loop Noise Calculator tool and instructions for use can be downloaded from the CMX998 product page, knowledge base here.

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