In high-frequency circuit design, layer-based power supplies generally outperform bus-based ones because they allow the loop to consistently follow impedance paths. Moreover, the power supply board should facilitate a signal loop for all signals generated and received on the PCB. This practice reduces signal loops and noise, often overlooked in low-frequency circuit design.
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There are several ways to eliminate power supply noise in PCB board design.
1.1 Pay attention to through holes on the board: Through holes necessitate etched openings in the power plane to accommodate their passage. If these openings are too large, they affect the signal loop, forcing signals to detour and increasing loop area and noise. Additionally, signal lines concentrated near these openings share the loop, leading to crosstalk due to common impedance.
1.2 Ensure sufficient ground wires for connection lines: Each signal requires its dedicated loop, minimizing loop area to keep signal paths parallel.
1.3 Separate analog and digital power supplies: High-frequency devices are sensitive to digital noise, necessitating separate supply paths connected at the power entry point to minimize loop area.
1.4 Avoid overlap of separate power supplies between layers: Overlapping can couple circuit noise via parasitic capacitance.
1.5 Isolate sensitive components like PLLs.
1.6 Place power cables strategically: To reduce signal loops and noise, position power cables alongside signal cables.
2. Transmission lines
PCBs typically feature two types of transmission lines: strip lines and microwave lines. The primary issue with transmission lines is reflection, which introduces various challenges. For instance, reflected signals add to the original, complicating signal analysis. Reflections also cause return loss, adversely affecting signal integrity akin to additive noise interference.
2.1 Signal reflection back to the source increases system noise, hampering signal discernment by receivers.
2.2 Any reflected signal degrades signal quality and alters input signal shape. Impedance matching is crucial to mitigate these effects, although impedance calculations can be complex; specialized software aids in this regard.
2.3 Methods to eliminate transmission line interference in PCB design include:
– Avoiding impedance discontinuities (e.g., sharp corners, vias) that disrupt transmission line integrity.
– Opting for angled traces over sharp corners, minimizing vias, and segregating signal paths between PCB layers.
– Eschewing stub lines to prevent noise; short stubs terminate directly, while long stubs incorporate main transmission lines to reduce reflections.
3. Coupling
3.1 Common impedance coupling occurs when interference sources and affected devices share conductors (e.g., power supply, bus, ground).
3.2 Field common-mode coupling induces common-mode voltage due to radiating sources affecting disturbed circuit loops and shared reference planes.
3.3 Differential Mode Field Coupling involves direct radiation received by wire pairs or leads on the PCB, mitigated by twisting wires to reduce interference.
3.4 Crosstalk between lines degrades system performance, with capacitive crosstalk due to parasitic capacitance between lines and inductive crosstalk akin to signal coupling between unwanted transformer primary and secondary.
3.5 Power Line Coupling transmits electromagnetic interference from power lines to other devices.
3.6 Methods to mitigate crosstalk in PCB design:
– Proper termination for signal lines sensitive to crosstalk-induced interference.
– Maximizing distance between signal lines reduces capacitive crosstalk.
– Employing ground planes and managing trace spacing, especially critical at state transition points.
– Introducing ground wires between adjacent signal wires, connected to ground every 1/4 wavelength, effectively mitigates capacitive crosstalk.
– Minimizing loop area for inductive crosstalk and avoiding shared signal loops.
4. Electromagnetic Interference
As speeds increase, EMI becomes more pronounced, affecting high-speed devices more than low-speed counterparts. Techniques to reduce EMI in PCB design include:
4.1 Minimizing loop area to reduce the antenna effect and ensuring signals follow a single path between any two points.
4.2 Filtering on power and signal lines using decoupling capacitors, EMI filters, and magnetic components.
4.3 Shielding techniques, though detailed discussion is beyond this scope.
4.4 Lowering operating speeds of high-frequency devices.
4.5 Increasing PCB board dielectric constant and thickness to contain high-frequency components and prevent electromagnetic radiation.
5. High-frequency PCB design principles:
5.1 Ensure stable power supply and ground.
5.2 Careful wiring and termination to eliminate reflections.
5.3 Careful wiring and termination to minimize capacitive and inductive crosstalk.
5.4 Suppressing noise to meet EMC requirements is crucial.
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This revision aims to enhance clarity and readability while maintaining the technical depth required for PCB design considerations.
