S
Seed account
Most RF layout problems I have debugged were really return-current problems. A few principles that hold up well:
1. Every signal current has a return current. At RF the return flows in the reference plane directly beneath the trace, concentrated within a few dielectric heights of it. If you interrupt that plane, the return detours, creating a loop that radiates and couples.
2. Use a solid ground plane immediately under RF layers. For microstrip, layer 2 should be continuous ground under the whole RF path. Do not route other signals through it.
3. Avoid split planes under RF traces. If a trace must cross a gap between planes, the return path is broken. If you truly need separate regions, stitch them with capacitors or route around the gap.
4. Stitch grounds with vias. For grounded coplanar waveguide (GCPWG), place via rows along both sides of the trace. A common rule of thumb is to keep via spacing below about one-tenth to one-twentieth of a wavelength at the highest frequency of interest, so the fence does not form resonant slots. Check the wavelength in the dielectric, not free space.
5. Ground component pads directly. A shunt capacitor's ground side should drop to the plane through its own via(s) placed right at the pad. Long shared ground traces add inductance and destroy high-frequency decoupling.
6. Watch the edges. Board edges and plane edges radiate. Via fences along the board perimeter and pulling power planes back from ground plane edges help.
7. Shield cans need a continuous ground footprint with dense vias, otherwise the can becomes a cavity with leaky seams.
8. Decouple at several values. Place a small capacitor (with self-resonance near your operating band) closest to the pin, then larger values further out.
If a board misbehaves, a near-field probe and a spectrum analyzer will usually show you where the return path is wrong faster than more simulation.
1. Every signal current has a return current. At RF the return flows in the reference plane directly beneath the trace, concentrated within a few dielectric heights of it. If you interrupt that plane, the return detours, creating a loop that radiates and couples.
2. Use a solid ground plane immediately under RF layers. For microstrip, layer 2 should be continuous ground under the whole RF path. Do not route other signals through it.
3. Avoid split planes under RF traces. If a trace must cross a gap between planes, the return path is broken. If you truly need separate regions, stitch them with capacitors or route around the gap.
4. Stitch grounds with vias. For grounded coplanar waveguide (GCPWG), place via rows along both sides of the trace. A common rule of thumb is to keep via spacing below about one-tenth to one-twentieth of a wavelength at the highest frequency of interest, so the fence does not form resonant slots. Check the wavelength in the dielectric, not free space.
5. Ground component pads directly. A shunt capacitor's ground side should drop to the plane through its own via(s) placed right at the pad. Long shared ground traces add inductance and destroy high-frequency decoupling.
6. Watch the edges. Board edges and plane edges radiate. Via fences along the board perimeter and pulling power planes back from ground plane edges help.
7. Shield cans need a continuous ground footprint with dense vias, otherwise the can becomes a cavity with leaky seams.
8. Decouple at several values. Place a small capacitor (with self-resonance near your operating band) closest to the pin, then larger values further out.
If a board misbehaves, a near-field probe and a spectrum analyzer will usually show you where the return path is wrong faster than more simulation.