时钟抖动对射频脉宽调制性能的影响分析

Analysis of the effect of timing jitter on RF-PWM performance

摘要:

在全数字发信机系统中，射频脉宽调制(RF-PWM)将基带调制信号的幅度与相位信息编码为输出脉冲的宽度和位置。由于数字信号处理器件的非理想特性，其时钟信号的上升沿和下降沿存在抖动误差，影响RF-PWM的输出信号质量。基于3种RF-PWM实现方案，本文通过公式推导确定了时钟抖动引入的非线性失真项，并给出了时钟抖动影响下不同方案输出脉冲信号底噪的数学解析式。最后利用Matlab软件，对不同方案在时钟抖动条件下的基波、奇次谐波和底噪进行仿真验证，结果证明理论推导正确；同时对信号的矢量幅度误差(EVM)和邻信道功率比(ACPR)进行仿真，分析出时钟抖动对信号带内外性能的影响。结果表明，时钟抖动引入的非线性失真主要体现为底噪的抬高；不同RF-PWM实现方案时钟抖动的影响特性各有不同，其中五电平方案对时钟抖动影响具有抑制效果，且随时间分辨力的增大而增大。

Abstract:

In All-Digital Transmitter(ADTx) system, Radio Frequency Pulse Width Modulation(RF-PWM) encodes the amplitude and phase information of baseband modulation signal into the width and position of output pulse. Due to the non-ideal characteristics of digital signal processing devices, the rising edge and falling edge of the clock signal have jitter errors, which affects the output signal quality of RF-PWM. Based on three RF-PWM implementation schemes, the distortion term caused by timing jitter is determined by formula derivation, and the mathematical analytical expressions of the noise floor of different schemes' output pulse signals under the influence of timing jitter are given. Finally, Matlab is employed to simulate and verify the fundamental, odd harmonic and noise floor of different schemes under the condition of timing jitter, which proves the correctness of theoretical derivation. At the same time, the Error Vector Magnitude(EVM) and Adjacent Channel Power Ratio(ACPR) of signal are simulated, and the influence of timing jitter on the performance of signal in-band and out-of-band is analyzed. The results show that the nonlinear distortion caused by timing jitter is mainly reflected in the improvement of noise floor. For different RF-PWM implementation schemes, the influence characteristics of timing jitter are different, among which the five-level scheme has a restraining effect on timing jitter, and it increases with the increase of time resolution.