PCB and EMC basics

Objectives

By the end of this lesson you will:

Explain the job of a ground plane and of decoupling capacitors placed close to a chip’s power pins
Name at least three factors that make a board emit interference or be sensitive to it
Explain why a product containing a radio or electronics must pass standards testing before it can be sold

Before you start

You already understand decoupling (lesson Basic components), a probe’s ground loop (lesson Oscilloscope basics), and can read a schematic (the previous lesson)
For the lab: a board, a magnifying glass or a phone camera, an oscilloscope with a probe
This lesson is only a foundation. Real PCB design and EMC testing are specialist subjects that go far deeper. Our goal is to be able to read a board and ask the right questions.

See it work first

Look closely at both sides of a board with a magnifying glass, or take close-up photos. Find these things:

Many small capacitors sitting right next to the main chip
Many small holes (vias), arranged in rows or spread across the board, especially around the edges and around any radio section
Metal covers over components (shield cans), and an area with no copper around an antenna
Small components right next to the USB connector (usually ESD protection parts or a ferrite bead)

Why does a designer add these things, when the schematic would still function without many of them? The answer is that a schematic tells you “what connects to what,” but not “which way the current actually flows” — and on a real board, the path current takes is everything.

Concepts

1. Current must return: ground planes and decoupling

Every bit of current leaving a chip’s pin along a trace must always flow all the way back to the chip through ground, completing a loop. At high frequency, the return current chooses the path with the least inductance, which is the path directly underneath the signal trace, because that makes the loop’s area as small as possible.

Left: a continuous ground plane — the return current runs directly beneath the signal trace, keeping the loop small. Right: a slot in the plane forces the current to detour, enlarging the loop, which emits and picks up more interference.

A ground plane is a full copper layer inside the board (a four-layer board is often stacked as signal / ground / power / signal). It lets return current run beneath every signal trace, keeping loops small and ground’s own resistance and inductance low. If the plane has a slot or is cut beneath a signal trace, the current must detour, and the loop grows immediately (right-hand picture). An easy rule to remember: never route a fast signal trace across a gap in the ground plane.

Decoupling must sit close. A real capacitor is not just capacitance — its package and the traces connecting it also carry inductance (ESL).

A 100 nF capacitor with 1 nH of total inductance resonates at f0 = 1 / (2π √(1 nH × 100 nF)) ≈ 15.9 MHz
At 100 MHz: the ideal capacitor part has an impedance of 1 / (2π × 100 MHz × 100 nF) ≈ 0.016 Ω
            but the inductive part has 2π × 100 MHz × 1 nH ≈ 0.63 Ω — almost 40 times larger

Above its resonant frequency, a capacitor starts behaving like an inductor, so at high frequency placement and loop length matter more than the capacitance value. An extra 5 mm of trace adds about 5 nH of inductance (a rough rule of about 1 nH per millimetre). If the current changes by 25 mA within 1 ns, the ripple voltage grows by 5 nH × 25 mA / 1 ns = 0.125 V.

A decoupling capacitor must sit close to a chip's power pin — the smaller the loop between the power pin, the capacitor, and ground, the less inductance it carries.

This is why a designer places a small capacitor right next to every power pin, drills the shortest possible vias down to the ground and power planes, and often combines several values together to cover multiple frequency ranges.

2. What makes a board emit or pick up interference

EMC (electromagnetic compatibility) is a device’s ability to work in an electromagnetic environment without disturbing other devices beyond a limit (emission), and without malfunctioning when disturbed within a specified level (immunity).

Factor	Why	Common fix
A large loop area	A loop is an antenna. For a small loop, the radiated field scales with current, loop area, and the square of frequency — and a large loop also picks up interference well	A continuous ground plane; keep the signal trace close to its return path
A fast signal edge	An edge rising within t_r spreads energy out to about 0.35 / t_r — for example, a 1 ns edge reaches about 350 MHz, even if the signal itself is only 1 MHz	Reduce output drive strength; add a small series resistor; use the slowest edge the application can tolerate
A clock signal and its harmonics	A 25 MHz square wave has odd harmonics at 75, 125, 175 MHz …	Keep the clock trace short, buried between planes; use a spread-spectrum clock if the chip supports one
A cable acting as an antenna	A cable about a quarter-wavelength long radiates and picks up well; a 1 m cable matches λ / 4 at 75 MHz	A ferrite bead or filter at the connector; a shielded cable grounded at the right point
An unprotected connector	Static electricity and interference enter through a connector first	A TVS diode, an RC filter, right at the connector
A high-impedance point or a floating pin	Easily picks up the ambient electric field (we already saw this in the floating-pin lesson)	A pull-up or pull-down; a filter at an ADC pin
Insufficient or distant decoupling	Supply ripple becomes interference for every chip sharing that rail	A capacitor close to the pin, several values, short vias

3. Why testing is required before sale

Radio spectrum is a resource everyone shares. A single device emitting beyond its limit can disturb aviation radio, medical equipment, or a whole building’s communication network. And a device that cannot withstand static or ambient fields might malfunction at a dangerous moment. Countries therefore require products to pass testing before they can be sold.

EMC emissions, such as the CISPR family of standards (CISPR 32 for multimedia and information technology equipment)
EMC immunity, such as CISPR 35 and the IEC 61000-4-x test series (IEC 61000-4-2 is the electrostatic discharge test)
Radio. Using radio spectrum must follow each country’s own rules, such as ETSI EN 300 328 for the 2.4 GHz band in Europe, and FCC Part 15 in the United States. In Thailand, radio communication equipment falls under the NBTC (National Broadcasting and Telecommunications Commission)
Electrical safety, such as IEC 62368-1 for audio, video and information technology equipment. In Thailand, some product categories must also meet mandatory TISI (Thai Industrial Standards Institute) standards

The exact list of standards a product must pass depends on the product category and the country of sale — always check with the relevant authority or an accredited test lab. A commonly misunderstood point: using a pre-certified radio module does not automatically certify the whole product. The main board, wiring, power supply, and enclosure can all change the emission result on their own. Experienced teams therefore run their own pre-compliance testing, with a near-field probe and a spectrum analyzer, starting from the earliest prototypes, because fixing a problem during design is far cheaper than fixing it after failing a formal test.

Worked example

Problem: a team sends a prototype temperature sensor that reports over WiFi for testing. Emissions exceed the limit around 75 MHz and 125 MHz. Reviewing the design, they find:

A two-layer board, with the external memory’s 25 MHz clock trace 6 cm long, routed across a slot cut into the ground plane to make room for a power trace
The main chip’s decoupling capacitor sits about 15 mm from the power pin, connected by a thin trace
A 1 m USB cable connects directly to the connector, with no filter

Analysis

75 and 125 MHz are the 3rd and 5th harmonics of the 25 MHz clock, so the clock trace is the likely source
The clock trace crosses the ground plane’s slot, forcing the return current to detour — a large loop (the loop-area factor)
A 1 m USB cable has λ / 4 landing exactly at 75 MHz, so any interference leaking onto the cable radiates well (the cable-antenna factor)
Decoupling 15 mm away adds about 15 nH of inductance (the 1 nH-per-millimetre rule); above a few MHz, the capacitor barely helps anymore

Fixes, in order of value for effort

Re-route the clock trace to be short and never cross the slot, or close the slot and move to a four-layer board with a full ground plane
Add a 22 to 33 Ω series resistor right at the clock’s source pin, or reduce drive strength, to slow the edge down
Move the decoupling capacitor right next to the power pin, using short vias down to the plane
Add a ferrite bead or a filter for USB, right at the connector

Then run pre-compliance with a near-field probe, comparing before and after the fix, before sending it for another round of formal testing.

Practice

A 10 nF capacitor has 0.8 nH of total inductance. At what frequency does it resonate? Above that frequency, how does it behave?
A signal edge rises in 2 ns. To about what frequency does its energy spread?
A 0.5 m cable has λ / 4 landing at what frequency (using the speed of light, 3 × 10⁸ m/s)?
A 16 MHz clock is a square wave. At what frequencies do the three strongest harmonics fall?
Name three factors that make a board sensitive to interference (immunity), with a fix for each.
A company uses a pre-certified WiFi module and concludes the product needs no further EMC testing. How would you respond?

Solution

f0 = 1 / (2π √(0.8 nH × 10 nF)) ≈ 56.3 MHz. Above this, its impedance is dominated by inductance — it behaves like an inductor, filtering less and less as frequency rises.
0.35 / 2 ns = 175 MHz.
λ = 4 × 0.5 m = 2 m, so f = 3 × 10⁸ / 2 = 150 MHz.
A square wave (50% duty) carries only odd harmonics. The strongest is the fundamental, 16 MHz, followed by 48 MHz and 80 MHz.
For example: a floating pin or a high-impedance point (add a pull-up or pull-down); a long, unfiltered sensor cable (add an RC filter or a ferrite bead at the connector); an unprotected connector (add a TVS diode right at the connector); a large loop (a continuous ground plane); insufficient decoupling (a capacitor close to the pin).
A module’s certification covers the module itself, under the conditions it was tested in. The main board, wiring, power supply, and enclosure can all emit and pick up interference on their own — the whole product still needs to pass EMC and safety testing per the requirements of whichever market it will be sold in. Radio requirements for the finished product must also be checked with that country’s own authority.

Check your understanding

Answer at least 4 of the 5 questions in quiz.yaml correctly.

Lab

Part A: read a board by eye. Using a magnifying glass or close-up photos of your board (and its schematic, if available), fill in the table.

What to find	Where / how many	What it does
Decoupling capacitors around the main chip
Vias arranged in a row (stitching vias)
A metal shield can
An antenna area with no copper
Protection parts or filters near the USB connector
Ferrite beads (marked FB on the schematic)

Part B: a loop is an antenna (oscilloscope, no need to connect to the board)

Set the probe to 10×. Clip the probe’s ground lead onto its own tip, forming the ground lead into a loop. Do not touch the board.
Set 20 mV/div, AC coupling, timebase about 1 µs/div, trigger mode Auto.
Hold the loop above a powered, running board (about 1 cm away), and slowly sweep it across the board — close to the main chip, close to the voltage regulator, and along the USB cable. Note where the signal is strongest.
Repeat with the loop squeezed smaller (coil the ground lead into a tighter loop), and compare the signal size at the same spot.
Conclude how loop size affects the picked-up signal, and connect this to why a ground spring gave a cleaner picture in the oscilloscope lesson.

Part C (if time allows): noise on the supply rail. Measure the 3V3 rail with AC coupling and a short ground lead, as in the lesson Oscilloscope basics, while the board is lightly loaded, then compare against while the screen is updating or wireless is active. Record the peak-to-peak ripple voltage in both conditions.

Going further

You have completed all six modules of this course. These skills carry straight over into courses that work with a real board, such as TESAIoT Firmware Stack, which writes C firmware for the TESAIoT Dev Kit. Every time a program says “sent” or “read successfully,” you now have the tools to prove for yourself whether that is really true.

Reflect

If you were designing your own first product, which point from this lesson would you check first, right from component placement, and why?

References

Review questions

Answer on your own first, then open the answer.

Why must a decoupling capacitor sit as close as possible to the chip's power pin? (Objective 1)
1. เพื่อให้ค่าความจุเพิ่มขึ้น
2. เพื่อให้ลูประหว่างขาไฟ ตัวเก็บประจุ และกราวด์เล็ก ความเหนี่ยวนำต่ำ จ่ายกระแสเร็วได้ทันที
3. เพื่อให้ประกอบง่าย
4. เพื่อป้องกันไฟฟ้าสถิตที่ขั้วต่อ
Show answer

Answer: B. เพื่อให้ลูประหว่างขาไฟ ตัวเก็บประจุ และกราวด์เล็ก ความเหนี่ยวนำต่ำ จ่ายกระแสเร็วได้ทันที

ลายวงจรทุกมิลลิเมตรเพิ่มความเหนี่ยวนำราว 1 nH เหนือความถี่เรโซแนนซ์ตัวเก็บประจุทำตัวเหมือนตัวเหนี่ยวนำ ตำแหน่งจึงสำคัญกว่าค่าความจุที่ความถี่สูง
What does a continuous ground plane under signal traces help most with? (Objective 1)
1. ทำให้กระแสไหลกลับวิ่งใต้ลายสัญญาณพอดี ลูปเล็ก แผ่คลื่นและรับสัญญาณรบกวนน้อย
2. ทำให้บอร์ดแข็งแรงขึ้น
3. ลดแรงดันไฟเลี้ยง
4. ทำให้สัญญาณวิ่งเร็วขึ้นกว่าความเร็วแสง
Show answer

Answer: A. ทำให้กระแสไหลกลับวิ่งใต้ลายสัญญาณพอดี ลูปเล็ก แผ่คลื่นและรับสัญญาณรบกวนน้อย

กระแสความถี่สูงเลือกทางกลับที่มีความเหนี่ยวนำน้อยที่สุด ซึ่งคือใต้ลายสัญญาณ ถ้าระนาบมีร่องใต้ลาย กระแสต้องอ้อมและลูปใหญ่ขึ้น
Which of these make a board emit or pick up more interference? (choose all that apply) (Objective 2)
1. ลายสัญญาณนาฬิกาวิ่งข้ามร่องของระนาบกราวด์
2. ขอบสัญญาณเร็วกว่าที่งานต้องการมาก
3. สายเคเบิลยาวที่ไม่มีตัวกรองที่ขั้วต่อ
4. ระนาบกราวด์เต็มแผ่นไม่มีร่อง
5. ขาเข้าที่ปล่อยลอย
Show answer

Answer: A. ลายสัญญาณนาฬิกาวิ่งข้ามร่องของระนาบกราวด์ · B. ขอบสัญญาณเร็วกว่าที่งานต้องการมาก · C. สายเคเบิลยาวที่ไม่มีตัวกรองที่ขั้วต่อ · E. ขาเข้าที่ปล่อยลอย

ลูปใหญ่ ขอบเร็ว สายเคเบิลที่เป็นเสาอากาศ และจุดความต้านทานสูงหรือขาลอย ล้วนเพิ่มการแผ่หรือการรับคลื่น ระนาบกราวด์เต็มแผ่นคือทางแก้ ไม่ใช่ต้นเหตุ
A 1 MHz digital signal with 1 ns edges has spectral content up to roughly what frequency? (Objective 2)
1. 1 MHz
2. 2 MHz
3. ราว 350 MHz
4. 1 GHz ขึ้นไปเสมอ
Show answer

Answer: C. ราว 350 MHz

ใช้กฎหยาบ 0.35 / t_r = 0.35 / 1 ns ≈ 350 MHz ความเร็วของขอบ ไม่ใช่ความถี่ของสัญญาณ เป็นตัวกำหนดว่าคลื่นรบกวนไปถึงความถี่สูงแค่ไหน
A product uses a certified WiFi module. Which is correct about testing before sale? (Objective 3)
1. ไม่ต้องทดสอบอะไรอีก เพราะโมดูลผ่านแล้ว
2. ผลิตภัณฑ์ทั้งชิ้นยังต้องผ่านการทดสอบ EMC และความปลอดภัย และตรวจข้อกำหนดด้านวิทยุของประเทศที่จะขาย เพราะบอร์ด สาย และกล่องเปลี่ยนการแผ่คลื่นได้
3. ทดสอบเฉพาะความปลอดภัยทางไฟฟ้าก็พอ
4. ทดสอบเฉพาะในประเทศที่ผลิต ส่วนประเทศที่ขายไม่เกี่ยว
Show answer

Answer: B. ผลิตภัณฑ์ทั้งชิ้นยังต้องผ่านการทดสอบ EMC และความปลอดภัย และตรวจข้อกำหนดด้านวิทยุของประเทศที่จะขาย เพราะบอร์ด สาย และกล่องเปลี่ยนการแผ่คลื่นได้

คลื่นวิทยุเป็นทรัพยากรที่ใช้ร่วมกัน และอุปกรณ์ต้องไม่รบกวนผู้อื่นหรือทำงานผิดเมื่อถูกรบกวน การรับรองโมดูลครอบคลุมแค่ตัวโมดูล ผลิตภัณฑ์ต้องผ่านข้อกำหนดของตลาดที่ขาย เช่น กสทช. ในประเทศไทย

Cite this lesson

If you teach from this lesson or reuse it in slides or documents, credit it with the text below. If you changed it, add (adapted) after the title.

"PCB and EMC basics" from TESA Open Knowledge by the Thai Embedded Systems Association (TESA), https://github.com/tesaiot/tesa-qualification-program, licensed under CC BY-NC 4.0

Thai attribution: "พื้นฐาน PCB และ EMC" จาก TESA Open Knowledge โดยสมาคมสมองกลฝังตัวไทย (Thai Embedded Systems Association: TESA) https://github.com/tesaiot/tesa-qualification-program สัญญาอนุญาต CC BY-NC 4.0

Lesson link: https://tesaiot.github.io/tesa-qualification-program/en/courses/electronics-and-instruments/m06-build-and-read/l04-pcb-and-emc-basics/

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