Hybrid RF and powerline communications for smart metering

Every deployed meter that cannot be reached is a truck roll, and a truck roll costs more than the meter. That single economic fact drives the whole design.

Why both links, on one board

Neither path is reliable everywhere.

Powerline carrier works where radio is blocked — deep in a building, in a basement, behind reinforced concrete. It fails where the line is electrically noisy, where the meter sits on a different phase from the concentrator, or where a transformer stands in the way.

Sub-GHz RF works where the line is hostile. It fails where the structure is.

Deployed at scale, both failure modes appear in the same estate. A hybrid gives each meter a fallback, which turns a communications problem into an installed-base problem you can actually manage remotely.

Where the difficulty is

Two front ends that dislike each other. A switching mains coupling network and a sensitive sub-GHz receiver on one board is a self-interference problem by construction. Physical separation, careful grounding, filtering on every boundary crossing, and a floorplan that treats the RF section as a keep-out — decided in the block diagram, not discovered at pre-compliance.

The mains side is a safety problem before it is a signal problem. The coupling network sits across live conductors. Isolation, creepage and clearance, surge withstand and fusing come first; the coupling behaviour comes second.

Measurement you can trust. Hall-effect current sensing gives isolation by construction, and its usefulness depends entirely on the analogue path around it — reference stability, layout away from switching nodes, and a sampling strategy that survives a noisy line.

Losing power gracefully. A meter that dies silently is a meter that has to be visited. Enough hold-up energy plus an early warning signal lets the firmware send a last message and close its filesystem cleanly. Dying-gasp is a hardware feature with an energy budget, not a firmware flag.

The part that is easy to underestimate

EMC. A board that deliberately injects signal onto the mains and simultaneously listens on a sensitive radio has to pass a chamber session with both active.

That is not a test you tune your way through afterwards. Return paths, plane strategy, filtering at every boundary and the physical separation between the two front ends all have to be right in the first layout — which is why the EMC work on a design like this starts at the block diagram.