At 02:15 PM 6/11/00 +1200, Steve wrote: >There is one erroneous assumption though. >The clock frequency (and it's harmonics) aren't the main culprit you >are trying to supress. The RF noise etc is generated by the switching >transients which are of a much higher frequency and being close to an >impulse have lots of harmonics. The resulting spectrum is then >effectively modulated by the clock frequency. > >So the basis of your calculations should be 1 / (rise/fall time) >rather than the clock frequency. > >If you want some empirical proof (and have the gear to measure it), >build a counter using HC logic and then replace the logic with AC >parts. Keeping the clock frequency the same, compare the emissions of >the two. It's quite marked. The current impulses related to current surges at switching points is usually a much smaller effect in CMOS circuits. Taking the PIC microcontroller as the target, we want C = I x t / V If I=100mA and V=100 mV as before, and t = 50 ns, then C = 0.1x50ns/0.1 = 50 nF In addition, if we reason the impulses are going to be short duty cycle (not 100%), then a more reasonable value for C might be C = 50 nF x 10-20% = 5-10 nF So we don't need much capacitance to fix this problem. But you are correct that the fix to this problem would have to operate correctly at higher frequencies. However, this problem would show up easily as ringing on the signal transitions. So it's still true that if the signal looks clean and sharp in the time domain, the problem is solved. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================