Controlled Impedance: RF PCB Layout: Impedance, Grounding and Antenna Interfaces

Radio frequency design on a printed circuit board is governed by a small number of rules that are easy to state and difficult to satisfy, because the layout must control an impedance, contain a field and reject a coupling, all at the same time and in the same area often occupied by digital circuitry.

Controlled Impedance as the Starting Point

A radio frequency trace is a transmission line, and its impedance is set by the width of the conductor, the thickness of the dielectric beneath it and the dielectric constant of the material. The trace is only a transmission line if the reference plane is continuous beneath it.

Once the geometry is fixed, the width becomes non-negotiable. A designer who narrows a radio trace to squeeze past an obstacle has changed its impedance, and the change appears as a reflection at that point. Where a taper is unavoidable, it should be gradual and short, and its effect should be simulated. Our high frequency laminate notes describe the material choices.

Ground Planes Under Radio Sections

A radio section needs a solid, uninterrupted ground plane directly beneath it, and that plane has to be connected to the rest of the ground in a way that does not create a loop. The usual arrangement is a local plane that is stitched to the main ground at the point where the radio’s return current actually flows.

The stitching is what prevents the local plane from becoming an antenna. A plane that is isolated except for one connection forms a resonant structure at some frequency, and if that frequency falls inside the band of interest the section becomes an unintentional radiator.

Keeping the Radio Away From Digital Noise

The sensitivity of a receiver is limited by the noise that reaches its input, and the largest source of noise on a mixed board is usually the digital circuitry. The separation is achieved by distance, by shielding and by keeping the returns separate, in that order of effectiveness.

Distance is the cheapest and the most reliable. Where the board does not allow distance, a shield can is used, and where a shield is used the layout must provide a clean ground ring for it to attach to.

Filters and Matching Networks

A matching network transforms the impedance of the device to the impedance of the line, and its components are placed as close to the device as the layout allows. The distance between them is part of the circuit, not an approximation of it.

Grounding for those components matters as much as their values. A shunt component that is grounded through a long thin trace has an inductance in series with it, and that inductance changes the response of the network. Our ferrite bead notes describe the related filtering choices.

Antenna Interfaces and Feed Lines

Where the antenna is on the board, the feed line is part of the antenna system and its impedance must be controlled from the radio output to the antenna feed point. A feed line that runs over a split plane or through a connector with a poor ground return will radiate and detune.

Where the antenna is external, the connector becomes the critical item, and its ground pins should be bonded to the plane with multiple vias placed close to the pins rather than at the edge of the pad. Our antenna layout notes describe the interface rules.

Vias and Their Electrical Effect

A via introduces a discontinuity in the impedance of the line, and the size of the discontinuity depends on the via’s capacitance and on the inductance of the return path. Grounding vias placed adjacent to the signal via reduce the loop and therefore reduce the discontinuity.

At higher frequencies the unused portion of the via barrel acts as a stub and creates a notch in the response at a frequency determined by its length. The remedies are a thinner board, a blind via that does not pass through the whole stack, or backdrilling.

Materials and the Loss Budget

The dielectric loss of the laminate becomes significant above a few gigahertz, and it adds to the conductor loss to produce the total insertion loss of the line. A design with a tight loss budget therefore constrains the material as well as the geometry.

The copper surface profile also contributes, because the current crowds into the surface at high frequency and a rough surface lengthens its path. Both effects are specified in the material datasheet and both should be part of the material selection rather than an afterthought.

Verification Before Release

The radio section is verified by simulation first and by measurement on the first article, and the measurement has to be made on a board with the same stack and the same finish as production. A prototype built on a different stack proves nothing about the impedance.

The measurements worth taking early are the impedance of the lines, the isolation between the radio and the digital section, and the sensitivity in the presence of a representative digital load. Each corresponds to a decision made in the layout. Our test coupon notes describe how the impedance is verified.

Process Control and Verification

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

FAQ

Can a radio section work without a controlled impedance stack? It can at low frequency and over short distances, where the electrical length is a small fraction of a wavelength. As the frequency rises or the trace lengthens, the lack of a defined impedance becomes a measurable loss and eventually a functional failure.

How much separation is needed between radio and digital? There is no single figure, because the answer depends on the edge rates and on the sensitivity of the receiver. A practical approach is to put as much distance as the board allows, then add a shield if the measurement shows that the distance is not enough.

What does gopcb provide for an RF design? We provide the stack-up with the dielectric constant at the frequency of interest, the impedance calculation for each line width, test coupons that carry the same geometry as the product and a coupon measurement with the shipment. We also confirm the finish, since it affects the conductor loss.