ctim are further apart then both electric and a ground plane construction. magnetic fields are involved in the coupling, and the conductors 4. Partition sensitive and noisy circuits SO as to be able to control must be considered as antennas. For conductors whose dimensions the interfaces between them and to the external environment. are much smaller than the wavelength then the maximum electric Ensure that internal and external ground noise cannot couple field component in the far field at a distance d metres due to a out of or into the system, by careful design of the cable ground current I at frequency f flowing in the conductor is (Ott, 1985) given connections. by Equation 23.5 for a loop and 23.6 for a monopole against a ground plane, where A is the area of the loop, and L is the length of 23.12.1 Choice of logic the conductor. A careful choice of logic family will help to reduce high frequency 131.6x10 (FAI) (23.5) volts per metre emissions from digital equipment and may also improve r.f. and transient immunity. The harmonic spectrum of a trapezoidal wave, which approximates to a digital clock waveform, shows a roll off of Circuit design and layout 23/17 amplitude with frequency which depends on the risetime (Figure laid out to avoid this. Check them at the prototype stage with a high 23.23). Using the slowest risetime compatible with reliable oper- frequency oscilloscope or spectrum analyser, even if nothing ap- ation of the circuit will minimise the amplitude of the higher order pears to be wrong with the circuit function. Ringing on pulses harmonics where radiation is more efficient. transmitted along un-terminated transmission lines will generate Therefore, the slowest logic family that will do the job should frequencies which are related only to the length of the lines with always be used. Where parts of the circuit must operate at high perhaps sufficient amplitude to be troublesome. All long lines speed, fast logic should only be used for those parts and the clocks should be terminated, particularly if