Printed Circuit Board
Understanding RF PCBs
The PCB is the foundation of every electronic product. For RF circuits, the PCB is not just a mechanical carrier but an active participant in circuit performance. The substrate material, trace geometry, ground plane design, and stackup all directly affect signal integrity, impedance, loss, and coupling.
RF PCB Design Considerations
- Substrate: FR-4 for < 2 GHz. Rogers RO4003/4350 for 2-20 GHz. Rogers RT/Duroid for > 20 GHz.
- Impedance control: Trace width and substrate thickness set Z0. Typically 50 ohms +/- 10%.
- Ground plane: Continuous ground plane is essential. Avoid slots, gaps that disrupt return current.
- Via design: Via size, spacing, and ground via fencing critical at mmWave.
PCB Stackup
- 2-layer: Simplest, one signal layer. Adequate for many RF circuits.
- 4-layer: Two signal layers with internal ground and power. Standard for mixed RF/digital.
- 6-8 layer: Complex routing with multiple RF and digital layers. Careful isolation required.
Frequently Asked Questions
What makes an RF PCB different from digital?
RF PCBs require controlled-impedance traces, low-loss substrates, continuous ground planes, careful via design, and attention to coupling between traces. Standard digital PCBs on FR-4 are inadequate above 1-2 GHz.
What substrate should I use for an RF PCB?
Below 2 GHz: FR-4 is acceptable. 2-10 GHz: Rogers RO4003 or RO4350 (standard RF substrates). Above 10 GHz: Rogers RT/Duroid 5880, 6002, or specialized mmWave laminates like Taconic TLY.
Why is the ground plane so important?
The ground plane provides the return current path for all signals. Discontinuities in the ground plane (slots, gaps, insufficient vias) cause signal integrity problems, increased radiation, and impedance variations. A continuous ground plane is the single most important RF PCB rule.