Mid-frequency simultaneous switching noise in computer systems

CMOS microprocessors operating in the hundreds of megahertz create significant current deltas due to the variation in switching activity from clock cycle to clock cycle. In addition to the high-frequency voltage variations more commonly discussed, a lower frequency noise component is also produced that lasts from 50 to 200 ns which we refer to as mid-frequency noise. In this paper, we discuss the design of IBM's CMOS S/390 computer for control of mid-frequency noise. This machine has a 10-way multiprocessor on a 127 mm by 127 mm MCM on a FR4 board. The chips on the MCM cause a current step of tens of Amps in a few cycles that can be sustained for many cycles. The power distribution and decoupling capacitors must supply that current without disturbing the voltage level at the circuits. The design of the system power distribution and modeling and verification of midfrequency noise in this system is presented.