RF circuit design is unforgiving. At frequencies above 1 GHz, every trace becomes a transmission line, every via is an impedance discontinuity, and every material boundary affects signal propagation. Success depends on disciplined layout, appropriate material selection, and tight manufacturing tolerances.
Material Selection for RF
Standard FR-4 has a dielectric constant (Dk) of approximately 4.5 and a dissipation factor (Df) of 0.02. At frequencies above 2–3 GHz, FR-4's loss tangent causes unacceptable signal attenuation and its Dk variation with frequency distorts impedance matching. For RF circuits operating above 2 GHz, use low-loss materials:
- Rogers RO4003C: Dk 3.38, Df 0.0027 — good balance of performance and cost for WiFi, Bluetooth, LTE
- Rogers RO4350B: Dk 3.48, Df 0.0037 — processable like FR-4, popular for high-volume RF
- Rogers RT5880: Dk 2.2, Df 0.0009 — ultra-low loss for microwave and radar
- Hybrid stackups: Rogers outer layers with FR-4 inner layers — cost-effective for mixed RF/digital boards
Controlled Impedance
RF traces must maintain a specific characteristic impedance (typically 50Ω single-ended or 100Ω differential) along their entire length. Impedance mismatches cause signal reflections, standing waves, and power loss. Calculate trace width based on your stackup geometry (microstrip or stripline), dielectric constant, and copper thickness. Specify impedance tolerance (±10% for standard, ±5% for tight control) and request TDR measurement reports from your manufacturer.
Transmission Line Routing
- Microstrip: Trace on outer layer over a ground plane. Simple, but exposed to external interference.
- Stripline: Trace between two ground planes on inner layers. Better shielding, more consistent impedance, harder to debug.
- Coplanar waveguide: Trace with ground pour on the same layer, providing both lateral and vertical reference. Best for RF transition sections.
- Keep RF traces as short and straight as possible — no sharp bends (use curved corners with radius ≥ 3× trace width)
- Never route RF traces over plane splits or gaps
Component Placement
Place RF components in a signal-flow line from antenna to baseband with minimal trace lengths between stages. Keep the RF signal path as direct as possible. Group RF components together and separate them from digital circuits by at least 3mm. Place decoupling capacitors as close to RF IC power pins as physically possible — the loop inductance from IC pin to capacitor to ground determines effective decoupling bandwidth.
Grounding for RF
A solid ground plane beneath RF traces is non-negotiable. Do not route any signals on the ground plane layer under RF traces. Use via fences (ground vias spaced λ/20 apart) along RF traces to suppress parasitic modes. Connect component ground pads to the ground plane with multiple vias to minimize ground inductance.
Shielding
Sensitive RF sections benefit from metal shielding cans soldered to the board. Shielding prevents external interference from coupling into the RF path and contains local oscillator energy that might radiate to other circuits. Design shield footprints into your layout even if you are unsure whether shielding will be needed — it is easy to leave a shield off, but impossible to add one without designed-in mounting pads.
Antenna Integration
Board-edge antennas (patch, inverted-F, meander line) require a clear keep-out area with no ground plane, components, or metal nearby. Follow the antenna manufacturer's recommended keep-out dimensions precisely. Route the feed trace from the RF output to the antenna with matched impedance. Add a π-network or T-network of zero-ohm resistors and capacitors near the antenna feed point for impedance tuning during certification testing.
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