Analysis And Optimization Of The Correlation Between GPS Antenna Interference And RAM Oscillation Frequency

With the rise of smart phone, many kinds of antennas including GPS antenna are integrated on the PCB of mobile phone. Meanwhile, CPU frequency increases to one giga-Hz,and system bus frequency also significantly increases up to GPS main frequency, The possibility that high frequency digital signal interferes radio frequency signal greatly increases,which means the signal integrity issues become evident..The paper summarizes the causes of GPS interference.The oscillation frequency of DDR SDRAM bus is closely related to the interference and crosstalk. And we proposed a software method for reducing interference.It significantly optimized the frequency interference of the GPS,improved coupling sensitivity. In the study we found the repeated or high frequency harmonics of RAM itself and its associated frequency.Crosstalk will reveal the GPS antenna on the working frequency through the interface lines or by other lines.Because these noise signals oscillate in the PCB traces and so generated electromagnetic field(The charged body generates electric field.The directed moving of the charged body generates a magnetic field).When the circuit is coupled to the GPS,the GPS circuit will produce a certain amount of coupling voltage.Because the GPS signal is already so weak that a small amount of crosstalk coupling voltage will affect the GPS performance.The writer researched several effective software methods, including the handling of the antenna test point nearby; the changes of DDR frequency; the optimization of LCD MIPI transmit clock; the adjustment of RAM drive capacity. The author proposed a method about LCDC PCLK modify and tested it effective. Through these means, the hardware on the board need not to change, but the performance is guaranteed, and the cost is saved.In the fourth chapter the author focuses on the experiment which found the source of interference, and found the cause of the interference through many exclusion and inference.But when the PCB board design was completed,and the stack structure and layout could not be modified,meanwhile RF testing and antenna matching were also invalid,to modify driver software was a practical way to circumvent the problem. Including modifying DDR clock and driving ability, configuring vacant pin as output low level,modifying MIPI and LCDC transmission frequency.The idea which control the harmonic to stagger interference frequency was feasible by experimental verification.Apart from the PCB-level issues,when we saw the crosstalk was related to digital signal inside the chip, we found the source of the problem was caused by the package,indicating signal integrity problems and back-end chip design have a relationship.This is a comprehensive systemic problem that requires each considerations and compromises.Through the practice and analysis of this subject,we found that other antenna interference have the similar reasons.As reference,this article is good for chip designers essentially understanding such issues by the analysis and practice,and taking precautions for the chip layout and back-end design.The embedded system developers can find software and hardware circumvent solutions about similar problems.