Minimum separation between signal tracks is required to avoid crosstalk. Signal tracks in consecutive layers of a double-sided PCB should be run perpendicularly to avoid crosstalk.
Power supply decoupling
In Fig. 8, assume that the load is the switching load of a digital IC, the source is the power source for that IC and lines shown as transmission line are power supply rails for the IC containing switching load. Normally, the actual power source will be in another power supply card of the unit and voltages will be delivered through connector pins of the card to which this digital IC belongs. Voltages will be delivered to connector pins of the card either through wiring or through a motherboard. Power supply lines from connector pins to the digital IC will run through PCB power tracks. This total current loop will have some inductance because of lengthy power lines and the enclosed large loop area if care is not taken in the PCB layout.
Digital ICs source/receive large switching currents when logic gate circuits containing transistors switch states from high to low or vice versa. If rise/fall time is very low, this switching of current takes place in a very short time. This large change of switching current in a short time results in a voltage drop as per Eq. (3) and voltage spikes appear between actual power pins of the digital IC. If the total current loop area is large, this can result in EMI also.
Solution to this problem lies in putting power-supply decoupling capacitors between power and ground, one for every digital IC, and at the connector pins where power enters the board. These capacitors will provide the required switching current, without drop in voltage, from their stored charge acquired from power rails. They will regain through power rails the charge lost in supplying the switching current after switching is over. Decoupling capacitors also reduce the effective loop area of switching current since it is supplied by the nearest IC decoupling capacitor. Thus these reduce EMI also.
Usually, 0.1µF ceramic capacitors are used for digital ICs. 10-100µF tantalum capacitors in parallel with 0.1µF ceramic capacitors are used at connector pins where power enters the board. Tantalum capacitors exhibit more inductance at frequencies greater than a certain value, because of which ceramic capacitors are used in parallel to cover a higher frequency range. Simulation tools are available to fix location and values of decoupling capacitors, which need to be used for accurate design.
If you observe the spectrum of a digital signal on a spectrum analyser, you will find that most of the energy is contained in the frequency band up to a frequency called Knee frequency. The energy rolls off at the rate of -20dB/decade up to knee frequency. Beyond knee frequency, it rolls off much faster. Hence it is considered that most of the energy of a digital signal is contained in the frequency band from 0 to knee frequency, represented as Fknee. Value of Fknee is related to rise/fall time of digital edges, but not to clock rate.
Fknee is given by:
Fknee = 0.5/Tr
where Tr is the pulse rise time/fall time, whichever is lower.
Hence all digital circuits should have a flat frequency response up to Fknee to pass digital signal without distortion.