Near Field Notes

Families10 notes

Signal integrity

A fast signal fails at a connector.

Traces, vias and returnsLinks and codingConnectors and cables

Find your problem

What is the board doing wrong?

  1. aImpedance dips at connector padsGround cut
  2. bAn eye looks closedEye diagram
  3. cImpedance dips at connector pinsTDR
  4. dA fast link fails past a viaVia stubs
  5. eA gigabit link comes up at 100 Mb/sPHY and link layer
  6. fA sensor board runs out of interrupt pinsI3C vs I2C
  7. gSATA at 6 Gb/s stops short of 750 MB/sLine codes
  8. hA switch counts stomped CRCs on a portCRCs on fast links
  9. iA dock’s USB ports drop to USB 2 speedUSB-C alt modes
  10. jA camera needs power over its video coaxCar camera coax

Traces, vias and returns

4 notes

What the board does to a fast edge, and how to see it.

Solid ground and a pad-sized hole
Fig. 1b2 mm1 · Solid ground−5.6% impedance2 · Pad-sized hole−0.5% impedanceFig. 1b2 mm1 · Solid ground−5.6% impedance2 · Pad-sized hole−0.5% impedanceFig. 1b2 mm1 · Solid ground−5.6% impedance2 · Pad-sized hole−0.5% impedance

Note 1Ground cut

Should you cut ground under connector pads?

−5.6% → −0.5%impedance error at the pads

USB-C · HDMI · 0402

The parts of an eye
Fig. 8c101234651 bit, 97 psFig. 8c101234651 bit, 97 psFig. 8c101234651 bit, 97 ps

Note 8Eye diagram

How do you read an eye diagram?

82% → 70%eye width after 10⁵ bits and after 10¹²

SFP+ · SFF-8431

Six things on a line
Fig. 16c0.35 pF to ground0902000.8 nH in series75 Ω from here090200Open endShort090200Matched end50 mm down a 90 Ω line, 25 ps edgeΩFig. 16c0.35 pF to ground0902000.8 nH in series75 Ω from hereOpen end090200ShortMatched end50 mm down a 90 Ω line, 25 ps edgeΩFig. 16c0.35 pF to ground0902000.8 nH in series75 Ω from hereOpen end090200ShortMatched end50 mm down a 90 Ω line, 25 ps edgeΩ

Note 16TDR

How do you read a TDR trace?

67 → 83 Ωthe USB-A socket pins at a 15 ps edge, then at USB 3.0’s 50 ps

USB-A · UBAF30-D2011 · CM5 IO board

The longest stub each link takes
Fig. 17d00.511.522.53Longest stub, mmThis board3.2 mm board10GBASE-KR10.3 GBd2.3PCIe 416 GBd1.5USB4, DP UHBR2020 GBd1.225G Ethernet25.8 GBd0.9PCIe 5, PCIe 632 GBd0.7112G PAM453.1 GBd0.4Fig. 17d00.511.522.53Longest stub, mmThis board3.2 mm board10GBASE-KR10.3 GBd2.3PCIe 416 GBd1.5USB4, DP UHBR2020 GBd1.225G Ethernet25.8 GBd0.9PCIe 5, PCIe 632 GBd0.7112G PAM453.1 GBd0.4Fig. 17d00.511.522.53Longest stub, mmThis board3.2 mm board10GBASE-KR10.3 GBd2.3PCIe 416 GBd1.5USB4, DP UHBR2020 GBd1.225G Ethernet25.8 GBd0.9PCIe 5, PCIe 632 GBd0.7112G PAM453.1 GBd0.4

Note 17Via stubs

How long a via stub can a fast link take?

0.94 mm → 25 Gb/sa via to layer 3 of a 1.6 mm board, under 0.5 dB

MNT Reform

Connectors and cables

2 notes

What one connector or one coax can carry, and how.

24 pins, 12 ways in
Plain USBThe defaultDisplayPort, 2 lanesVESA, 2014 · in useDisplayPort, 4 lanesVESA, 2014 · in useThunderbolt 3Intel, 2015 · in useUSB4USB-IF, 2019 · in useMHLMHL, 2014 · no phone foundHDMIHDMI, 2016 · no adapters by 2023VirtualLink2018 · abandoned 2020Analog audioDeprecated in 2023Chromebook debugGoogle’s debug cableAppleApple’s debug messagesGoogleGoogle’s firmware updatesNVIDIANVIDIA’s test modeSwitch 2Nintendo, 2025A faceUSB 3USB 2CCDPAUXDPTB3TB3SidebandUSB4USB4MHLHDMIHDMIHEACUSB 3RLMicTo groundDebug USBPulled upSerial05AC18D1095530+ messagesB faceUSB 3USB 2DPAUXDPTB3TB3SidebandUSB4USB4MHLHDMIHDMIHEACUSB 3RLAGNDTo groundDebug USBPulled upSerialPlain USBThe defaultDisplayPort, 2 lanesVESA, 2014 · in useDisplayPort, 4 lanesVESA, 2014 · in useThunderbolt 3Intel, 2015 · in useUSB4USB-IF, 2019 · in useMHLMHL, 2014 · no phone foundHDMIHDMI, 2016 · no adapters by 2023VirtualLink2018 · abandoned 2020Analog audioDeprecated in 2023Chromebook debugGoogle’s debug cableAppleApple’s debug messagesGoogleGoogle’s firmware updatesNVIDIANVIDIA’s test modeSwitch 2Nintendo, 2025A faceUSB 3USB 2CCDPAUXDPTB3TB3SidebandUSB4USB4MHLHDMIHDMIHEACUSB 3RLMicTo groundDebug USBPulled upSerial05AC18D1095530+ messagesB faceUSB 3USB 2DPAUXDPTB3TB3SidebandUSB4USB4MHLHDMIHDMIHEACUSB 3RLAGNDTo groundDebug USBPulled upSerialPlain USBThe defaultDisplayPort, 2 lanesVESA, 2014 · in useDisplayPort, 4 lanesVESA, 2014 · in useThunderbolt 3Intel, 2015 · in useUSB4USB-IF, 2019 · in useMHLMHL, 2014 · no phone foundHDMIHDMI, 2016 · no adapters by 2023VirtualLink2018 · abandoned 2020Analog audioDeprecated in 2023Chromebook debugGoogle’s debug cableAppleApple’s debug messagesGoogleGoogle’s firmware updatesNVIDIANVIDIA’s test modeSwitch 2Nintendo, 2025A faceUSB 3USB 2CCDPAUXDPTB3TB3SidebandUSB4USB4MHLHDMIHDMIHEACUSB 3RLMicTo groundDebug USBPulled upSerial05AC18D1095530+ messagesB faceUSB 3USB 2DPAUXDPTB3TB3SidebandUSB4USB4MHLHDMIHDMIHEACUSB 3RLAGNDTo groundDebug USBPulled upSerial

Note 23USB-C alt modes

What can a USB-C port carry besides USB?

24 pins, 12 waysto take over a USB-C port

USB-C receptacle

The board’s filter
Fig. 27eImpedance, Ω101001k10k100k1 MHz100 MHz10 GHz365 ΩFour coils22 µH aloneFig. 27eImpedance, Ω101001k10k100k1 MHz10 MHz100 MHz1 GHz10 GHz365 ΩFour coils22 µH aloneFig. 27eImpedance, Ω101001k10k100k1 MHz10 MHz100 MHz1 GHz10 GHz365 ΩFour coils22 µH alone
  • Four coils
  • Coilcraft
  • 22 µH alone

Note 27Car camera coax

How does one coax carry a car camera’s video, control and power?

86% vs 36%of ADI’s band at 1 kΩ, four coils vs one

MAX96717 · MAX96724 · PFL1609-471 · 1210POC-682 · MSS6132T-223 · FAKRA · Antmicro GMSL Serializer · Antmicro GMSL Deserializer