Cartesian feedback around the power amplifier can improve efficiency in transmitters by using non-contact envelope modulation schemes. Using feedback to linearise an amplifier can be a simple process but there are many parameters around this that can influence the subsequent transmitter noise performance in a real system. By having the ability to estimate this early on in the design process, will allow you to select components and parameter settings within the loop more effectively.
Loop noise performance depends on the integrated functions, the local oscillator phase, and external element options in the power amplifier line-up and in the feedback path back to the device. Evaluation boards and prototyping can give you an indication of whether the overall noise performance will meet the standard requirements by this can be time consuming; the CML Microcircuits CMX998 can offer a highly integrated Cartesian feedback loop solution.
CML Microcircuits offers a new free calculator to estimate performance using internal and external system parameters, and local oscillator phase noise values. The calculator runs in Octave 126.96.36.199 or 188.8.131.52 (free to download here) or for Matlab (licence required).
Enter parameter settings in tabular form or form a system level block diagram including gain and noise figures for all the functional blocks within the loop. 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 done with values for the three-time constants defining the error amplifier response. The phase of the demodulated feedback signal and the delay time around the loop can be adjusted as part of two additional features.
CMX998 Cartesian Feedback Loop Transmitter
The CML CMX998 Cartesian Feedback Loop Transmitter increases efficiency and linearity of transmitters in non-constant envelope modulation systems. The CMX998 is able to linearise the Power Amplifier (PA) through feedback from the PA output by acting as a direct conversion quadrature mixer from I and Q to RF output. The CMX998 also features forward and feedback paths, local oscillator circuitry (loop phase control, integrated instability detector, and uncommitted op-amps for input signal conditioning.