RF Layout Guidelines
Understanding RF Layout
RF layout is where the design meets reality. Even perfect schematics will fail with poor layout because parasitic inductance, capacitance, coupling, and radiation from the physical structure dominate at microwave frequencies.
Key RF Layout Rules
- Controlled impedance: All RF traces must be impedance-controlled (50 ohm microstrip or stripline, verified by stackup calculation).
- Ground plane: Continuous, unbroken ground plane directly beneath all RF traces. No splits, gaps, or routing underneath.
- Component placement: Minimize trace length between RF components. Group related stages. Isolate TX from RX.
- Via stitching: Ground vias around RF cavities at < lambda/10 spacing to prevent cavity resonance.
- Thermal: Multiple ground vias under power devices for thermal and electrical ground.
Frequently Asked Questions
What are the critical RF layout rules?
Controlled impedance traces, continuous ground plane, shortest possible RF paths, adequate via stitching (< lambda/10), proper component orientation, and thermal management. Poor layout can degrade performance by 10-20 dB.
Why is ground plane continuity so important?
The ground plane provides the RF return current path. Any break forces current to detour, creating a slot antenna that radiates, increases crosstalk, and changes trace impedance. Never split the ground under RF traces.
How close should via stitching be?
Via spacing < lambda/10 at the highest frequency of interest. At 10 GHz (lambda = 30 mm): vias every 3 mm. At 28 GHz: every 1 mm. This prevents the ground plane from acting as a parallel-plate waveguide.