無線手持產(chǎn)品過去的經(jīng)驗法則

　　3. 輸入端采用MOSFET的放大器比采用雙極型晶體管的放大器更不易受RFI的影響：將天線直接放置在die或電阻器上面，結(jié)果表明，采用MOSFET型輸入的放大器受到的干擾比采用雙極型晶體管輸入的小。這一觀測結(jié)果與以前Fiori所做的報告結(jié)果一致[4].

　　圖6. 遠(yuǎn)場天線和近場天線各自的信號傳遞路線

　　注：

　　l Far Field Antenna：遠(yuǎn)場天線

　　l Near Field Antenna：近場天線

　　圖7. 100kHz 至 6GHz載波頻率掃描

　　注：

　　l Coupling：耦合

　　l Invert：反相

　　l Off：關(guān)

　　l Bandwidth：帶寬

　　l Full：全部

　　l Fine Scale：精密標(biāo)度

　　l Position：位置

　　l Offset：偏移

　　l Probe Setup：選項設(shè)置

　　l Coupling & Impedance：耦合&阻抗

　　本文研究的主要結(jié)果表明，根據(jù)天線的定位情況，使用添加RFI電容這一過去的經(jīng)驗法則有可能導(dǎo)致干擾的增強。因此，建議系統(tǒng)設(shè)計者在使用RFI電容來增強設(shè)計的抗干擾能力之前，了解天線在產(chǎn)品上的位置。在近場條件下，使用較高阻值的反饋電阻器和采用MOSFET作輸入的運放仍是有效提高電路的抗RFI能力的技術(shù)。

　　參考文獻(xiàn)

　　[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.