01
Start with the budget
Work out the current, the pins, the heat and the cost first. Choosing parts comes after that, never before.
01 / PRINCIPLEStart with the budget
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Let’s talkTopic 04 / The Art of Circuit DesignIn progress
A schematic is a claim.
The board is the proof.
The circuit that works on the bench and the circuit that ships are separated by a few dozen decisions nobody writes down. This is the set I keep coming back to.
Explore the ten principles ↓SIGNAL × STRUCTURE
Current does not read your schematic. It takes the path you actually built.
02
Every signal returns
Current that goes out has to come back. If you cannot say where the return path runs, that net is not routed yet.
02 / PRINCIPLEEvery signal returns
03
Decouple where the current is
Put the capacitor against the supply pin with its own via beside it — not wherever there happened to be room.
03 / PRINCIPLEDecouple where the current is
04
Impedance is geometry
The stack-up is decided before the first trace. It is a design decision, not the fabricator's default.
04 / PRINCIPLEImpedance is geometry
05
Separate, then bridge once
Keep analogue and digital ground apart, then join them at one point you chose on purpose. One point.
05 / PRINCIPLESeparate, then bridge once
06
A footprint is a contract
Check a new footprint against the datasheet, not against your last board. Getting it wrong costs the whole batch.
06 / PRINCIPLEA footprint is a contract
07
Design the test first
Test points, bring-up order and the jig — decided while the board can still change.
07 / PRINCIPLEDesign the test first
08
Name parts in a language
CAP, 120nF, 16V, 5%, X7R, 1206, SMD can be checked by a machine. “that capacitor” becomes a substitution error later.
08 / PRINCIPLEName parts in a language
09
EMC is architecture
Whether it passes is decided by placement, planes and cable entry — not by a filter added after the pre-scan fails.
09 / PRINCIPLEEMC is architecture
10
Assume it will be reworked
Somebody will cut a trace and add a wire on this board. Make that survivable now, while it is still cheap.
10 / PRINCIPLEAssume it will be reworked
02 / One capacitor, placed twiceTwo boards. One schematic.
Two boards. One schematic.
Only one of them works.
On a schematic the position of a decoupling capacitor means nothing — a schematic has no distance in it. On a board it means everything. Against the supply pin with its own via, or a few millimetres away around an obstruction, are two different products. Same netlist; supply noise an order of magnitude apart.
Cut the plane, and the current goes the long way round.
A trace is only half a circuit. The return current follows the path of least impedance, which at any real frequency means directly under the trace. Put a slot in the plane and it has to detour — and the loop it makes is now an antenna.
Slot in the planeThe return current cannot cross the slot, so it runs to the end of it and back. The loop area grows, the impedance rises, and the pair becomes an efficient radiator at exactly the frequency you cared about.
03 / Anatomy of a boardA board is not a surface.
A board is not a surface.
It is six layers of decision.
A 50 Ω trace: its width, its distance to the reference plane, and the dielectric in between — decided together, or not decided at all.
- 01Silkscreen
For people: polarity, pin one, revision. Assembly and rework both depend on it.
- 02Soldermask
Decides where solder goes and, more importantly, where it does not.
- 03Signal
The trace runs here. Its reference plane is the layer directly below.
- 04Prepreg
Dielectric thickness and constant. This is what actually sets the impedance.
- 05Plane
Power and ground. The return current for every signal above lives here.
- 06Core
The mechanical spine, and the reference for keeping the stack symmetric.
The library is infrastructure.
A parametric part name is a sentence anyone can check and any script can validate. A nickname is a purchasing decision waiting to go wrong at three in the morning during a rush build.
Design for the person who repairs it.
Test points, pin one in the silkscreen, room for a probe, an accessible rework path. The board will be modified — decide now whether that is possible or whether it means scrapping it.
Each gate wants a different artefact.
Architecture and budgets
Power budget, pin map, thermal path and the block diagram — before a single component is chosen.
Schematic, stack-up, bring-up
The stack-up and the impedance rules are released with the schematic, not discovered at fabrication.
Pre-scan, then formal
An informal EMC scan while the design can still move, then formal test and safety on a frozen configuration.
Schematic-first, or layout-aware?
Both produce a working schematic. Only one of them produces a board you can manufacture, test and pass EMC with — and the difference is when you start thinking about the physical.
Place it while you draw it.Slower to a review. Faster to a board.
Placement, stack-up and return paths are considered while the schematic is still soft. The schematic takes longer to freeze and then does not move again — which is the only part of the schedule that actually matters.
Before you release the gerbers
Answer five questions honestly.
- Does every high-speed net have an uninterrupted reference plane beneath it?
- Was every new footprint checked against the datasheet, not against a previous board?
- Can this board be brought up, probed and reworked once it is populated?
- Is the layer stack-up specified, with impedance rules that match it?
- Did you run the design rule check after the last change — the real last change?
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←All topicsRead topic 05 →Work in progress
Coming soon.
Topic 04 / The Art of Circuit Design
This topic is still being written. The structure below is in place, but the copy, figures and examples are not final — so they are blurred for now.