无线手持产品过去的经验法则

　　3. 输入端采用MOSFET的放大器比采用双极型晶体管的放大器更不易受RFI的影响：将天线直接放置在die或电阻器上面，结果表明，采用MOSFET型输入的放大器受到的干扰比采用双极型晶体管输入的小。这一观测结果与以前Fiori所做的报告结果一致[4].

　　图6. 远场天线和近场天线各自的信号传递路线

　　注：

　　l Far Field Antenna：远场天线

　　l Near Field Antenna：近场天线

　　图7. 100kHz 至 6GHz载波频率扫描

　　注：

　　l Coupling：耦合

　　l Invert：反相

　　l Off：关

　　l Bandwidth：带宽

　　l Full：全部

　　l Fine Scale：精密标度

　　l Position：位置

　　l Offset：偏移

　　l Probe Setup：选项设置

　　l Coupling & Impedance：耦合&阻抗

　　本文研究的主要结果表明，根据天线的定位情况，使用添加RFI电容这一过去的经验法则有可能导致干扰的增强。因此，建议系统设计者在使用RFI电容来增强设计的抗干扰能力之前，了解天线在产品上的位置。在近场条件下，使用较高阻值的反馈电阻器和采用MOSFET作输入的运放仍是有效提高电路的抗RFI能力的技术。

　　参考文献

　　[1] Muhammad Taher Abuelma’atti “Radio interference by Demodulation Mechanisms Present in Bipolar Operational Amplifiers IEEE Transactions on Electromagnetic Compatibility, Vol 37. NO.2, May 1995.

　　[2] Robert E. Richardson, Jr. Modeling of Low-Level Rectification RFI in Bipolar Circuitry. IEEE Transactions on electromagnetic Compatibility, Vol.EMC-21, NO4, November 1979.

　　[3] Application Note AN1299 “Measuring RF Interference in Audio Circuits”. Authors Don LaFontaine and Bob Pospisil. www.intersil.com/data/an/AN1299.pdf

　　[4] Franco Fiori Compliance Engineering 2000 November, December issue “Integrated Circuit Susceptibility to Conducted RF Interference” www.ce-mag.com/archive/2000/novdec/fiori.html

　　[5] Hamid Ghadamabadi, James J. Whalen, R.Coslick, C. Hung, T. Johnson, W. Sitzman and J. Stevens Department of Electrical and Computer Engineering. “Comparison of Demodulation RFI in Inverting Operation Amplifier circuits of the same gain with different input and feedback resistors values”.www. ieeexplore.ieee.org/iel2/161/6451/00252748.pdf?arnumber=252748

　　[6] Hamid Ghadamabadi, James J. Whalen Department of Electrical and Computer Engineering. “Parasitic capacitances can cause demodulation RFI to differ in inverting and non-inverting operation amplifiers circuits” IEEE 1991 Electromagnetic compability, 1991, Symposium record.

　　[7] Robert E. Richardson, Vincent G. Puglielli and Robert A. Amadori. “Microwave Interference Effects in Bipolar Transistors” IEEE Transaction on Electromagnetic Compatibility, Vol. .EMC-17, NO.4, November 1975.