41 votes
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Why doesn't controlled impedance depend on track length?

The characteristic impedance of a transmission line is not the same thing as a lumped resistance, it just happens to have same units. Similarly, certain amplifiers are designed to have a current as ...
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25 votes

How is 10G Ethernet physically possible?

Chasing this answer down took a few different links, but it appears to boil down to this: 1. 4 differential pairs (8 wires total, but only 4 lanes). 2. 800 Mega Symbols a second. 3. Using PAM16, 16 ...
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24 votes
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PCIe, diagnosing and improving an eye diagram

There are quite a number of things that will do this to you. You have not stated the length of the interface. I do direct chip to chip PCIe frequently and you really need to take this into account as ...
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23 votes
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More Vdd than Vss Pins

As a chip maker, it's easy for me to explain the cause of the imbalance. It's that there are several different rings of VDD in the IC for different purposes, but only a single ground. The different ...
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  • 7,217
22 votes
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Understanding the Maximum Speed that can be Transmitted over a Cable

The maximum frequency is mostly related to the frequency-dependent loss characteristics of the cable. Eventually you get to a frequency where you simply don't get enough signal at the other end to use....
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21 votes

Trace Length Tolerance Calculation - High Speed PCB Design

Length matching is about timing so if you want to know how tightly you have to length match you have to understand the timing budget for your interface. Signals will leave your source, and arrive at ...
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20 votes
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Why is the AC coupling capacitor always 0.1uF or 0.01uF in high speed transceivers?

It's a HIGH-pass filter, not a low-pass filter. That cutoff frequency you calculated is the LOWEST that can get through, not the highest. Frequencies lower than that are blocked, and frequencies ...
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18 votes

Under what conditions does an optocoupler work fastest?

I actually have some relevant experience on this very subject. Many, many years ago I grabbed a bunch of PS2505 optoisolators which turned out to be PS2506s. No big deal right? It turns out the ...
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16 votes
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Why would you stack a resistor and capacitor on top of each other?

Forget the 40kHz- this kind circuit really likes to oscillate at very high frequency- the feedback resistor is almost open (1M) at high frequencies in comparison to a few pF and the amplifier has a ...
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15 votes

How is 10G Ethernet physically possible?

10G ethernet (as described by other answers) does not do signal transitions at 10 GHz, it uses multiple level encoding spread across 4 pairs to achieve 10 Gb/s. However, 10+ gigabit serial ...
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15 votes
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Effect of Ethernet PHY interface on the speed of Ethernet

The Ethernet controller datasheet you link claims 10/100 Mbit/s speeds, not Gbit/s speeds. The controller is capable of negotiating with other 10BASE-T or 100BASE-TX interfaces on the network, and of ...
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14 votes
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Quad SPI PCB layout

For FR4, using effective epsilon of 3.25 we get the wavelength of a 80 MHz signal in the PCB at 80 by calculating wavelength = (c/f) * (1/sqrt(epsilon)) = (300000000 m/s / 80000000 1/s) * (1/sqrt(3....
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14 votes

Why is the AC coupling capacitor always 0.1uF or 0.01uF in high speed transceivers?

A binary data signal isn't a single frequency. If it's truly random, it will have frequency content from near DC all the way up to about half the data rate (i.e. a 1 Gbps signal has content from DC to ...
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12 votes

What are these small stubs on the USB data pins of this device?

This question is already answered, but my guess is that they're for testing the PCB while it is still panelized. Check out this episode of EEV Blog where Dave talks about mass production testing ...
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12 votes

How this DIY High-Speed Probe doesn't reflect signals?

The point of this probe is that the scope has its 50Ω termination enabled. Since the scope input impedance is matched to the cable, there is no reflection from the scope. Once the load side is matched,...
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11 votes
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CAN bus layout design

You are getting confused about the impedance. The type of CAN you are apparently using is implemented as twisted pair with roughly 120 Ω impedance. That is why there is a 120 Ω resistor ...
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11 votes
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Why application notes advise series resistance to be added on clock lines for high-speed interfaces, like SDIO, and not on data lines?

The receiving circuit only cares about voltage levels on data lines during a time interval centered on the clock edge, with setup/hold times on both sides. So... even if the data lines are ...
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10 votes
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Why don't I need to consider trace width for a high speed oscillator crystals?

A 100MHz oscillation has a wavelength of 3 metres and, a general rule of thumb is that you don't need to consider track length impedance matching if the trace length is less that one-tenth of the ...
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9 votes
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high speed microcontroller - pc communication

A bigger issue may be the operating system on the PC itself. Unless you have significant buffering on your sampling device, you will run into delays in the PC that will cause you to lose data. I got ...
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9 votes

RF PCB Design Basics

I believe the first image you posted is meant to accommodate a machined EMI shield that is screwed down. You can see that it would provide EMI shielding as well as isolating the circuits from each ...
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9 votes

Effect of Ethernet PHY interface on the speed of Ethernet

The chip can connect at 10 Mbps and 100 Mbps Ethernet link speeds, and it has a maximum SPI bus speed of 40 MHz. So yes, even if it can communicate with 100 Mbps link speed, it won't achieve 100 Mbps ...
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8 votes

More Vdd than Vss Pins

Just connect the remaining VDD pins to the ground plane via decoupling capacitors. It is not always necessary that power and ground pins be equal. If you have a solid ground reference throughout the ...
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8 votes

How can a SerDes based link be used to replace the age old UART?

You need to choose one of the high-speed interfaces that are available on the PC, and implement that in your FPGA (or by a companion chip of some sort) including any upper protocol layers that are ...
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8 votes
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Routing USB 3.x Type-C Signals

I have several suggestions. First up, I would not swap the P and N over in the way you have shown. All via crossings must be symmetric, so dropping one half of the pair to the other side is a bad ...
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8 votes

At what data rate should we consider PCB via back drilling? How about flex PCB?

For a 4-layer with signals on the outer layers, never. 3-layer flex… also never. The whole point of back drilling is to remove stubs on the outer parts of vias, that if left undrilled, would cause ...
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8 votes

Effect of Ethernet PHY interface on the speed of Ethernet

Ethernet link speeds range from 10 Mbit/s to 400 Gbit/s currently. Generally, each Ethernet frame must be transmitted atomically, without any pauses. For low-speed devices, that mandates decoupling ...
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  • 218
7 votes

Making prototypes with high speed SMD components

There are techniques that can be used for prototyping. If you're prototyping a whole system you're probably not doing it right- but to test smallish bits of analogish circuitry, it's practical. It's ...
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7 votes
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What are these small stubs on the USB data pins of this device?

I believe they are just connections that are removed by the milling tool that defines the PCB outline- probably for hard gold electroplating. Notice that all four gold contacts have the connections ...
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7 votes

CAN bus layout design

You are knocking yourself out for no good reason. CANBus, with a 1 MHz maximum bit rate, is largely impervious to PCB termination issues. A few inches of mismatch on a PC board simply doesn't count in ...
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7 votes
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Why place inline capacitors on PCIe traces?

PCI Express Electrical Interconnect Design from Intel Press (2004), p19 The PCI Express Base Specification (PCI-SIG 2004a) requires each lane of a link to be AC-coupled between its corresponding ...
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