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Physics behind signal reflections and series termination

With electrical transmission lines, it all has to do with the speed of light being finite, thus so is the speed of EM propagation in a wire. You can think of a wire as a long series of infinitesimal ...
hotpaw2's user avatar
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22 votes

Physics behind signal reflections and series termination

The impedance of a transmission line, in ohms, is the ratio of voltage wave and current wave that travels down the line. For a 100 ohm line for instance, a 1 volt wave will always be accompanied by a ...
Neil_UK's user avatar
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16 votes
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Do USB Data Wires (D+/D-) have 90 ohm differential impedance and single ended 45 ohm impedance to ground and if so how is this made?

USB cables do require some precision engineering. There are stringent requirements on value of differential impedance, quality of interconnects, and amount of losses per cable. The high-speed part of ...
Ale..chenski's user avatar
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14 votes
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Impedance/Termination of Marvell PHY

I've been working on a Gigabit Ethernet project for months, and so far I checked datasheets from Realtek, TI, Microchip, and a few reference designs, here is what I've found out. 0. High-speed routing ...
比尔盖子's user avatar
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13 votes

Are I/O buffers of ICs design to have 50ohm impedance?

Very few do. If your chip's datasheet does not say it has 50-ohm termination, then it almost certainly does not. Traditional CMOS and TTL logic does not provide matching termination, though a few ...
The Photon's user avatar
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Characteristic impedance of a BNC jack to jack adapter

Why am I measuring such a low resistance? It's the resistance between centre pin and shield that is important to measure then: If you connected an infinite number of these together end-to-end and ...
Andy aka's user avatar
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12 votes

Are I/O buffers of ICs design to have 50ohm impedance?

No, the 50-Ohms is not a convention for PCB tracks to carry signals. The 50 Ohms is a standard for coaxial cables and corresponding interconnects - dozens and dozens various SMA/SMB, BNC, type-N, etc. ...
Ale..chenski's user avatar
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10 votes

Using a miniature BNC connector on a PCB to measure high-frequency content with an oscilloscope

You might consider adding an attenuator probe to your PCB consisting of a single series resistor. The 50-ohm coax cable to your oscilloscope can be any length - the oscilloscope input at the far end ...
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10 votes
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Unexpected low characteristic impedance using the JLCPCB impedance calculator

JLCPCB Let's first consider the case of JLCPCB. The last time I investigated the issue, JLCPCB's calculator was based on a lookup table using the model and results from Polar Instruments Si9000. ...
比尔盖子's user avatar
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10 votes

Characteristic impedance of a BNC jack to jack adapter

mΩ are a good reading for the resistance between the opposite pins! You are measuring a low resistance because the inner pin is a solid piece of metal. You can't measure the 50 Ω with a DMM, it ...
Neil_UK's user avatar
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Does coiling a transmission line affect its characteristic impedance?

That's not a transmission line, that's a trace in space. It would be a transmission line if the trace were over ground plane, and spaced adequately from nearby traces (microstrip geometry). Follow ...
Tim Williams's user avatar
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8 votes
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How to estimate characteristic impedance for a random cable and tolerance of the termination resistor?

It seems there's a fairly simple way to get a good approximation of the impedance of a random piece of cable. It requires a fast oscilloscope, a pulse generator with a fast rising edge, and a ...
JRE's user avatar
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What is the difference between characteristic impedance and input impedance in transmission lines?

What is the difference between characteristic impedance and input impedance in transmission lines? Characteristic impedance (\$Z_0\$) depends on the transmission line and its physical properties. ...
Andy aka's user avatar
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RF layout bend over Component

Yes, a smooth bend would be better for signal integrity than the square corner. All other things being equal, that corner sticking out represents an unnecessary impedance "bump" in the transmission ...
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7 votes

Transmission Line Transient Response To Step Voltage Prior To Any Reflections

I am specifically asking whether the driving circuit will see the transmission line as a pure resistance according The Telegrapher's Equations, or whether those equations predict the driving circuit ...
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Why are we able to ignore the henries, Hz, farads, siemens and ohms when calculating the characteristic impedance of a transmission line?

At DC, the characteristic impedance equation becomes: - $$\sqrt{\dfrac{R}{G}}$$ Because G is the inverse of resistance we can say it is the inverse of ohms so, when you analyse the above formula it ...
Andy aka's user avatar
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How much is the propagation velocity affected by the characteristic impedance?

The velocity that electricity propagates at is related to the dielectric in the cable. In free-space, an EM wave travels at the speed of light but there is only the permittivity of free space (\$\...
Andy aka's user avatar
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6 votes

How can PCB trace have 50 ohm impedance regardless of length and signal frequency?

There is a simple hand-waving explanation why the effective impedance of a (ideal) transmission line is a constant. Other explanations leave some confusion on how do we "select" Li and Ci in the model ...
Ale..chenski's user avatar
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6 votes

Routing USB, DVI and Ethernet signals on a large PCB vs using long cables

I am by no means an expert in this field (there is a lot going to consider), but... Are USB/DVI/Ethernet cables a trivially easier solution for tens of cm? USB, DVI, and Ethernet are all very ...
uint128_t's user avatar
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6 votes

50 Ohm Single Ended Signal Impedances on PCBs

A rule of thumb is that characteristic impedance becomes seriously important when your trace length gets above a tenth of a wavelength. Below that you can mostly get away with treating your trace like ...
Peter Green's user avatar
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6 votes

Using a miniature BNC connector on a PCB to measure high-frequency content with an oscilloscope

I like to use this sort of BNC to scope probe adapter, which frequently is included in the accessory kit you get with the probe. You put it on the tip of your oscilloscope probe, then plug it into a ...
Hearth's user avatar
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6 votes

Transmission Line Transient Response To Step Voltage Prior To Any Reflections

I am specifically asking whether the driving circuit will see the transmission line as a pure resistance according The Telegrapher's Equations, or whether those equations predict the driving circuit ...
Andy aka's user avatar
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5 votes

What is the physical meaning of the characteristic impedance of a transmission line?

At a risk of collecting few negative points, I will try to answer this question as follows: The "characteristic impedance" has no direct physical meaning. It is just a constant in amplitude ...
Ale..chenski's user avatar
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Characteristic impedance when routing differential

No, it can't be ignored. The following discussion assumes traces on a PCB. The situation is different for interfaces like twisted pairs. For edge coupled (that is co-planar) traces, the differential ...
SteveSh's user avatar
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5 votes

Unexpected low characteristic impedance using the JLCPCB impedance calculator

Check the higher-up (calculators) page; Chemandy has two other microstrip formulas as well. The Wadell variant gives 75.25 ohms for the given dimensions, and IPC-2141, 83.93 ohms. It seems likely JLC ...
Tim Williams's user avatar
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4 votes

Characteristic impedance of a D-subminiature connector

D-sub connectors are not designed to be used at frequencies where transmission line effects in the connector are important, so their characteristic impedance is not well defined and it is not ...
The Photon's user avatar
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4 votes

Transmission line reflection. I would like a non-mathematical explanation

This fascinating Bell Labs video beautifully demonstrates wave motion, SWR, and impedance matching sections on a completely mechanical tabletop device without needing mathematics. It's presented in a ...
Mike Waters's user avatar
4 votes

What is the physical meaning of the characteristic impedance of a transmission line?

You seem to confuse two concepts: Attenuation is the amount of something (in this case electrical energy) that is absorbed by a medium. Attenuation is linear with the distance. Reflection is ...
Wouter van Ooijen's user avatar
4 votes

How is transfer impedance of a cable different from its characterstic impedance

Transfer impedance: Transfer impedance is used to determine shield effectiveness at lower frequencies (< 1 GHz) against both ingress and egress of interfering signals. This is an indication of ...
Bimpelrekkie's user avatar
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4 votes

Are I/O buffers of ICs design to have 50ohm impedance?

It's a convention, but we have lots of conventions. You mentioned DDR3, so here's Altera on the subject. The DDR3 SDRAM uses a programmable impedance output buffer. Currently, there are two ...
pjc50's user avatar
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