GPS天线干扰与RAM振荡频率相关性的分析和优化

发布时间：2018-05-09 07:12

本文选题：全球定位系统 + 印刷电路板　；参考：《上海交通大学》2014年硕士论文

【摘要】：随着智能手机地兴起,包括GPS天线在内的多种天线被集成在了手机印刷电路板(后简称PCB)上,同时CPU主频达到1G赫兹,系统总线频率更加接近GPS频点,高频数字信号干扰射频信号的可能性大大增加,即信号完整性问题突出。本文研究了GPS天线被干扰的原因,发现了干扰源与双倍速同步动态随机存储器(DDR SDRAM,后简称DDR或RAM)总线振荡频率的相关性,对其做了详细系统的分析,并提出了减少干扰的软件优化方法,明显降低了对于GPS主频点的干扰,提高了耦合灵敏度。在研究中发现,RAM本身及其相关的频率的多次或高次谐波,通过接口线,或者通过其他走线,会泄露串扰到GPS天线的工作频点上。原因是噪声信号在PCB走线中振荡,会产生电磁场(带电体产生电场,带电体的定向移动产生磁场),耦合到GPS电路中时,GPS电路中会产生一定量的偶合电压,因为GPS信号本来就很弱,所以少量的串扰偶合电压就会影响到GPS性能。作者研究了几个有效的软件措施,包括天线附近测试点的处理;DDR频率的修改;液晶屏(后简称LCD)的移动工业处理器接口(后简称MIPI)传输时钟的优化;RAM驱动能力调整等手段。经过分析和试验,作者提出了对LCD控制器的动态存储器访问(后简称DMA)传输时的像素时钟(后简称PCLK)进行修改,此方法有效。这些方法使硬件不用改板而保证性能,节约了成本。作者在第四章重点阐述了发现干扰源的实验过程,经过层层排除和推断,查到干扰的原因。在PCB板卡设计完毕,结构堆叠和布局布线不能修改,而射频调试、天线匹配也无效时,修改驱动软件规避问题成为实用的办法。包括修改DDR的时钟和驱动能力、将空置的管脚配置为输出低电平、修改MIPI频率、修改LCDC传输频率等方法。其中控制谐波尽量错开干扰频点的思想得到了实验验证可行。在板卡级问题以外,当发现串扰和芯片内部数字信号相关时,我们找到了封装导致的问题源头,说明信号完整性问题和芯片后端设计有关系,是个系统性全方位问题,需要各方面的考虑和妥协。通过本次课题的分析和实践发现其他天线干扰原因也有类似的情况。参考此文中的分析和实践,芯片设计者可以对此类问题有本质的认识,未雨绸缪,提前做好芯片布局和后端设计;嵌入式系统开发软硬件人员可以在类似问题中找到规避解决办法。
[Abstract]:With the rise of smart phones, a variety of antennas, including GPS antennas, have been integrated on the printed circuit board of the mobile phone. At the same time, the main frequency of the CPU has reached 1G Hz, and the frequency of the system bus is closer to the GPS frequency. The possibility of interference with RF signals by high frequency digital signals is greatly increased, that is, the problem of signal integrity is prominent. In this paper, the reason of the interference of GPS antenna is studied, and the correlation between the interference source and the oscillation frequency of DDR SDRAM (or DDR or RAM) bus is found, and the systematic analysis of the interference source is made. A software optimization method to reduce the interference is proposed, which reduces the interference to the main frequency point of GPS and improves the coupling sensitivity. In the study, it is found that multiple or high order harmonics of the RAM itself and its associated frequencies may leak crosstalk to the working frequency of the GPS antenna through the interface line or other lines. The reason is that the noise signal oscillates in the PCB circuit, which will produce electromagnetic field (electric field produced by charged body, magnetic field produced by directional movement of charged body, coupling voltage of PCB circuit when coupled to GPS circuit. Because the GPS signal is weak, a small crosstalk coupling voltage will affect the GPS performance. Several effective software measures are studied, including the modification of DDR frequency at the test point near the antenna, the optimization of RAM drive capacity of the mobile industrial processor interface (MIPI) of liquid crystal screen (LCD) and so on. After analysis and experiment, the author proposes to modify the pixel clock (PCLK) in the dynamic memory access (DMA) transmission of the LCD controller. This method is effective. These methods ensure the performance of the hardware without changing the board and save the cost. In the fourth chapter, the author focuses on the experimental process of finding the interference source, and finds out the reason of the interference by removing and inferring it layer by layer. When the design of PCB card is finished, the structure stack and layout and wiring can not be modified, and the RF debugging and antenna matching are invalid, it becomes a practical method to modify the driving software to avoid the problem. It includes modifying the clock and driving ability of DDR, configuring the vacant pin to output low level, modifying MIPI frequency, modifying LCDC transmission frequency and so on. The idea of controlling harmonics to stagger the interference frequency as far as possible has been proved feasible by experiment. In addition to the card level problem, when the crosstalk is found to be related to the internal digital signal of the chip, we find the source of the problem caused by the encapsulation, which shows that the signal integrity problem is related to the design of the back end of the chip, and it is a systemic and omnidirectional problem. All aspects need to be considered and compromised. Through the analysis and practice of this subject, it is found that other antenna interference causes have similar situation. Referring to the analysis and practice in this paper, the chip designer can have the essential understanding to this kind of problem, make the plan in advance, do well the chip layout and the back-end design in advance; Embedded system development software and hardware personnel can find a solution to similar problems.
【学位授予单位】：上海交通大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TN820

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