The Research Of Wideband Phase Modulation Technique Used In Polar Transmitter

Polar transmitter is a kind of transmitter which can meet the requirements of high linearity and high efficiency simultaneously. Since the data rate of baseband signal is increasing rapidly, the traditional PLL-based phase modulation technique can’t meet the requirement of the bandwidth specifications because of its intrinsic limited closed-loop bandwidth. More and more attentions have been paid to the development of newly phase modulation technique used in polar transmitter.This paper studies a new wideband phase modulation system based on the newly definition of the digital clock signal, which is Fractional-N direct digital frequency synthesizer. This paper’s main work and innovation points are shown as follows:1. The digital clock signal is redefined. The main difference between the traditional digital clock signal and the newly defined digital clock signal is that the traditional digital clock is made up of infinite base digital clock signal with the same period; however, the newly defined digital clock signal is composed of several base digital clock signals with different periods. In this way, the hardware implementation of the newly defined digital clock signal can be much easier than the hardware implementation of the traditional digital clock signal.2. The Fractional-N direct digital frequency synthesizer is studied in this paper. This frequency synthesizer’s output signal is the newly defined digital clock signal, namely the output frequency is combination of several frequencies rather than a single frequency. Besides, the output signal is rectangular signal and the rectangular signal is beneficial for integration in digital circuits.3. The Fractional-N direct digital frequency synthesizer is an open-loop system. Since the system is an open-loop system, the date rate of baseband signal is no longer restricted by the closed-loop bandwidth of the traditional PLL-based phase modulator. It is a doable scheme for wideband phase modulation.4. The non-spurs quantization block, successive requantizer, is studied. This block adopt the idea of statistics, in this way; the deterministic spurs can be transformed into non-deterministic spurs, whose magnitude of the power spectral density is small enough that can’t be seen on the top of the phase noise floor. This approach can guarantee thepower spectral of the output signal can be spur-free. Besides, in the design of the successive requantizer, the nonlinearity of the circuit is considered so that the power spectral density of the second power, third power, fourth power, fifth power of the output signal are spur-free.The results of hardware tests show that when the reference clock is 1GHz, Fractional-N direct digital frequency synthesizer can effectively work at the rate of 20 Mbps with 4.54% EVM. Theoretically, EVM can be better with higher reference clock. Therefore, The Fractional-N direct digital frequency synthesizer can be used as a wideband phase modulator in polar transmitter.