AN247 Datasheet | Philips Semiconductors

INTEGRATED CIRCUITS .. AN247 PCK2001 (CKBF) reduced EMI techniques Logic Products Group 1998 May 18 Philips Semiconductors Philips Semiconductors Application note PCK2001 (CKBF) reduced EMI techniques INTRODUCTION This application note describes the background, technique and results of EMI measurements on the PCK2001 (CKBF), showing reduced EMI emissions over standard design techniques. Through the use of circuit design, the high frequency EMI emissions from the CKBF were reduced from a standard design approach. These reduced emissions should help a board/systems manufacturer pass FCC and CISPR EMI requirements. BACKGROUND OF EMI, FCC AND CISPR STANDARDS EMI, Electro-Magnetic Interference, is any unwanted electromagnetic signal which may degrade the performance of an electronic device. It is generally thought of as one device creating electromagnetic radiation that interferes with the operation or performance of another device. PCs and PC motherboard designs have to meet standards set by regulatory agencies. Two standards are, FCC (Federal munications mission) regulations part 15B, and International Electrotechnical mission’s International Special mittee on Radio Interference (CISPR) 22B. These regulations specify limits: Most PC manufactures are required to meet these EMI testing requirements for system level design. Because of the regularity/repetition of the clock signals, the clock synthesis (CK100) and clock buffer (CKBF) in the 440BX (CK97 specification from Intel) designs create a significant amount of noise and EMI. The CK97 specification from Intel does not require spread spectrum techniques on the CK100 synthesis part or other reduced EMI techniques, but only specified spread spectrum as an option. There is also no reference to reduced EMI requirements on the CKBF. Early board builds and subsequent qualifications showed a serious problem with EMI, with a strong correlation to the CK100 and CKBF parts from all vendors. Spread spectrum on...