PCB
Understanding RF PCBs
Unlike digital PCBs where signals are either high or low, RF PCBs carry analog signals whose amplitude, phase, and frequency must be preserved. This makes RF PCB design more demanding: every trace is a transmission line, every via is an impedance discontinuity, and every ground plane gap is a potential radiator.
RF PCB Design Rules
- Controlled impedance: All RF traces must be impedance-controlled (50 ohm typically). Requires specified stackup and trace width.
- Ground plane continuity: Unbroken ground plane beneath all RF traces. Never route across ground plane gaps.
- Via transitions: Every signal via must have adjacent ground vias (ground via fence) to maintain impedance.
- Component placement: Minimize trace length between RF components. Place decoupling capacitors immediately adjacent to power pins.
RF Substrate Selection
| Substrate | Freq Limit | Cost |
|---|---|---|
| FR-4 | < 3 GHz | $ |
| Rogers 4350B | < 30 GHz | |
| Rogers 5880 | < 77 GHz | $ |
| Alumina | < 100 GHz |
Frequently Asked Questions
What makes an RF PCB different from a digital PCB?
An RF PCB treats every trace as a transmission line that must maintain controlled impedance. Signal integrity depends on substrate material, trace geometry, ground plane continuity, and via design. Digital PCBs can tolerate imperfections that would be unacceptable at RF frequencies.
What substrate should I use for RF?
FR-4 is acceptable below 3 GHz for cost-sensitive designs. Rogers 4350B or similar is standard for 3-30 GHz. Rogers 5880 or similar low-loss PTFE substrates are needed above 30 GHz. The key factors are dielectric constant consistency and loss tangent.
Why is ground plane continuity so important?
RF return current flows directly beneath the signal trace on the ground plane. Any gap or slot forces the current to detour, creating an inductive loop that radiates, disrupts impedance, and causes crosstalk. An unbroken ground plane is the most critical element of RF PCB design.