Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [controlled-impedance]

41 questions
19
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1 answer

Reasoning behind Ethernet pair order

An Ethernet connector seems to use a surprising choice of signal placement in its RJ45 connector. Does anyone know the reasoning behind: The decision to make pair 2 straddle pair 1? Why Ethernet uses Pairs 2&3 as in the left hand image? Why…
Rocketmagnet
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Why doesn't controlled impedance depend on track length?

High speed design noob here. Resistance increases as wire gets longer, but I found in saturn pcb calculator the impedance depends only on track geometry and distance from plane. Say for DDR3 single ended signals with controlled impedance, does a…
Hemal Chevli
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Why Characteristic Impedance must be 50 ohms?

Where does this number come from? For single ended it must be 50 ohm and for differential pairs 100 ohm. Why? For PCB with controlled-impedance these are that common numbers. Outside of the PCB you can find others numbers for characteristic…
Jesus Castane
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9
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3 answers

DDR1 Layout Considerations - DOs and DONTs

I am novice to high speed design. Before getting in to DDR, I recently learned about impedance matching and how it is done, likewise I learned about length matching and how it is done.(Baby steps towards Signal Integrity) Now, I need to Place and…
V V Rao
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9
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2 answers

Length Matching Differential Pairs

I am routing a PCB with an Ethernet connection and I am having a bit of trouble deciding on how best to route the TX and RX differential pairs. I have done the impedance calculations to figure out the track geometry needed for 100 ohm differential…
IgorEE
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8
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2 answers

Testing of impedance test coupons

I have a board with a 50 Ω impedance controlled track. I have got the boards manufactured. The manufacturer has sent me 2 additional boards viz - test coupons. I understand that test coupons have the same impedance controlled design and are used by…
Board-Man
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6
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3 answers

Controlled impedance in presence of vias and through-hole components (PTHs)

We have some controlled impedance traces on layer 4 of a board. Layer 3 is a GND plane. Layer 5 is a 3.3V plane. Both planes are unbroken (they occupy the entire layer), with the exception of vias and holes. There are a lot of holes on this PCB,…
SomethingBetter
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Can I slow a CMOS output through an RC filter?

Is it good practice to slow down the slew rate of a CMOS output by putting an RC filter on it? What happens with impedance matching after I do this? Or can I just set the RC filter with such a low cut-off that transmission line effects don't need to…
cksa361
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When to Remove the Ground Plane under SMD components for High-Speed Signals?

I'm reading TI's recommendations on routing high-speed signals. Most of the guidelines are common sense, but there's one guideline that I haven't read before: It recommends completely removing the ground plane below the mounting pad of a SMD…
比尔盖子
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Is this layout good? (RF - 2.45 GHz)

I finished laying out HACK, my first RF (2.45 GHz) layout. The sensitive part is the connection between the IC (Atmel SAM R21G18A), the balun (Johanson 2450BM15A0015), and the PCB antenna (copied from a TI app note, thank you Texas Instruments!). I…
MickMad
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What are "slash sheets", and how do they relate to PCB substrates?

I am trying to specify the stackup for a PCB. Previously I have used whatever the board house considered to be their "standard" stackup. This time I am looking at using FR-408 (pdf) because of its (relatively) low cost and (relatively) favorable…
bitsmack
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Unattached tails (stubs) on SPI lines

I have a circuit design coming together that uses either of 2 sockets to plug in a controller. The idea is to allow a user to enable either of the controllers to control a radio using SPI. In the design, the user is only allowed to plug in one…
Peter Laidlaw
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Controlled Traces Length in SD card

I my new design I need to use SD card communication. I would like to match all the impedance traces and control the length of the traces. Regarding the controlled length, is there a rule of thumb to determine which can be the maximum difference…
MatD
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Implementing 1000BASE-T (Twisted pair) over PCB backplane

Problem: We are designing a small payload which will contain several ethernet capable devices connected via a backplane we must design. All guidelines concern the scenario where the PHY is connected to an RJ45 connector, but in my case I am…
Alexis
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Signal reflections at the driver?

This question is related to How do I use directional couplers on a differential signal? I understand that if I have an impedance mismatch at the end of a signal cable, E.G. from a terminator which doesn't match the cable impedance, then I can expect…
Rocketmagnet
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