In order to meet the cost, power consumption and manufacturing process, we propose the following four transmission circuit architectures: Polar feedback "Lite", polar feedback, polar opening, direct modulation (zero).
Figure 1 shows the translation (or offset) architecture used in most GSM phones. The main advantage of this architecture is to utilize the low-pass filter in the phase locked loop (PLL) structure to function as a band pass filter, so that there is no need for any additional filter to provide excellent spectrum shielding performance, and to modulate There is no strict requirements of modules. The solution has been highly optimized through various generations of improvements, performance, integration, and cost.
Figure 1: GMSK modulation flat ring structure
Because GMSK (Gaussian minimum frequency shift keying) is constant enclosure modulation, the power amplifier (PA) can be operated in a saturated state to provide the highest efficiency. The modern PA module also integrates a CMOS power controller to provide a convenient interface for rising, falling edge control and power control DAC.
Polar feedback "Lite"
The concept of "Polar" modulator is to retain the flat-stranded ring structure, thus retaining all the above advantages. However, in order to support the EDGE mode, it is necessary to increase the modulation (AM) capability for the modulator: go to the AM modulation on the modulator output, fed to the amplitude controller, and feed it to the high dynamic range of VGA, thus reproducing the radio frequency AM and PM composite Signal. The signal is fed to a linear PA unit that is purely as an amplifier. Since PA is not compensated, its linearity and dynamic range must be very high to ensure signal quality. Of course, linear PA is usually less efficient as saturated PA.
Figure 2: Polar structure block diagram
To reduce the current in the GSM mode, the AM controller can be turned off and power control directly to PA. However, due to the design fold, even in the saturated state of the GSM, the efficiency of the PA is still lower than the pure saturated PA.
Polar feedback
The full polar architecture is shown in Figure 2, which is similar to the concept of polar Lite, and the advantage is that conventional GMSK saturated PA architectures can be configured to have the advantage of high power efficiency described above. This block diagram gives a feedback loop surrounding the PA in this solution. PA is "linear" using this feedback loop (removing AM to AM and AM to PM distortion). Based on PA feedback, the AM controller generates an AM error term. The system will adjust the PA gain accordingly to cancel the AM error. The AM-PM distortion caused by PA can be compensated using the PVD. The coupler used in this feedback path can be a stand-alone device or in the PA module.
Figure 3: Polar "Lite" Structure Box Diagram
Polar opening
The open loop architecture can be saturated PA, but the architecture does not include a feedback loop surrounding the Pa. Instead, the PA is characterized by power, temperature, voltage, and frequencies, and these data are stored in the lookup table (LUT). Select or insert the correction coefficients suitable for the working conditions with digital logic, and applied to the AM controller and IQ input when pre-distortion occurs. The AM composite signal is fed back to the PA amplitude controller, and the predistortion phase is fed back to the modulator and a flat ring, thereby eliminating the nonlinearity of the Pa. However, this method needs to consume a large amount of time calibration on the production line to compensate for the deviation between the components, and it is not easy to correct the system aging effect.
Figure 4: Polar opening structure
Direct modulation
Direct modulation is completely different from the architectural discussion, which does not use pan and intermediate frequency, but the modulator directs the IQ signal to the required RF channel. Sampling VGA method implements power control with output signals. Then, according to the noise performance of the modulator, an external filter may also be required before the signal is sent to the linear PA. The advantage of this architecture is simple, but from the perspective of noise and spurious performance, it is a severe challenge to design the RF modulator due to no integrated filtering. In addition, this architecture requires the use of linear PAs, which is not efficient with saturated PA.
Figure 5: Linear or zero difference structure block diagram
Table 1: Comparison of various modulation architecture
Summary
The direct modulation method looks very attractive, but there are some problems in the process, and the power efficiency is less efficient than the saturated PA used in polar modulation. In polar modulation design, the polar feedback method is highly efficient than polar "Lite" method, since the polar feedback method increases manufacturing robustness, and significantly reduces the calibration overhead associated with the open loop architecture.
Our other product:
