Polar loop architecture uses separate amplitude modulation and phase modulation feedback control of the output signal. By using closed loop control of the transmitted phase as well as the amplitude modulation, the limitations of the open loop polar architecture can be overcome almost entirely. By taking the feedback signal for the phase loop from the output of the PA, the AM to PM conversion in the PA can be corrected to a high degree and non-linearities of the PA control curve are corrected at the same time. Most important, this removes the requirements for extensive characterization, individual calibration and predistortion.

Feedback is also important because open loop systems rely on linear behavior or “blind” corrections of the non-linear parameters in the system. For example, when the PA parameters change as a function of temperature, supply voltage, signal level or component variation, the performance of the open loop system will rapidly degrade. By applying feedback control of amplitude as well as phase modulation, the system becomes much more robust to variations in PA and system parameters.

Polar loop architecture puts a unique set of requirements on the PA. While these requirements are slightly different from the requirements of, for example, the open-loop polar transmitter, it is the case that any PA suitable for an (open loop) polar transmitter can also be used for a (closed loop) polar loop transmitter, while the opposite is not necessarily true. The polar loop transmitter requires a PA with the same power control range as an open loop architecture, but the closed loop control is able to correct for a significant degree of deviation and variation of the PA control characteristic. The loss in the coupler is usually about 0.1 dB. Noise and output power requirements are quite similar for the two architectures, and a similar level of PAE can be obtained from both.

Figure 5 shows a simplified block diagram of Skyworks' polar loop transmitter used in many generations of Helios EDGE chipsets. The modulation is done at an IF with a conventional quadrature mixer and analog Cartesian vector input voltages (I and Q). The output of this mixer is used as the reference input for both the amplitude loop and the phase loop. Also, the feedback signal for both loops is taken from the same point, at the output of the PA, and the IF path (with downconversion, filtering and amplification of the IF signal) is common for the two loops. This architecture ensures good synchronization of the amplitude and phase components within the bandwidth of the two loops.

The dual feedback loop ensures robust performance even under voltage-standing wave ratio (VSWR) variations without using an isolator. No external PA filtering is required to meet the transmitter noise in the receive band. To minimize integration difficulty, polar loop transmitters use a standard in-phase and quadrature (I/Q) interface and do not require the extraction of AM and PM signals in the digital domain. There is no mode change between GMSK and 8-PSK modulation, and the transmitter operates seamlessly in multislot enhanced general packet radio service (EGPRS). The polar loop transmitters meet or exceed GSM-type approval requirements for EDGE and GSM/GPRS in quad-band operation (850 MHz, 900 MHz, 1800 MHz and 1900 MHz) with little to no calibration.

The polar loop architecture offers excellent and robust system performance with minimum sensitivity to changes in test conditions, PA or other system parameters. It offers closed loop power-level control, high PA efficiency by use of saturated operation, and provides the lowest overall system cost (no SAW filter or isolator is required). Additionally, the polar loop architecture features a standard I/Q interface and no mode change between GMSK and EDGE.

Jennifer Chou is the RF marketing director for Skyworks Solutions Inc. and is responsible for leading the GSM/EDGE/WEDGE RF marketing team in generating product business plans, defining market positioning, and creating the product line roadmap. In June 2002, Conexant Systems' wireless business merged with Alpha Industries to create Skyworks. She originally joined the business as a product applications engineer, GSM at Rockwell International before that unit became part of Conexant, and later as a product line manager, GSM RF subsystems for Conexant. She holds a BSEE and MSEE from Columbia University.

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