Microstrip Crosstalk Calculator: Does Length of Parallel Routes Mean Net Length or Track Width?

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#1 21683184 13 Sep 2019 22:49

Didan Ribeiro Didan Ribeiro

Anonymous
Post #1
21683184 13 Sep 2019 22:49

Dear forum, I have a question about microstrip crosstalk calculator from eeweb (https://www.eeweb.com/tools/microstrip-crosstalk) I attached figure with input parameters I desire to know. Im designing a 2 layer pcb and Im going to use ethermet module, which use W5200 chip. As far as I know, the speed of my SPI SCLK pin on master configuration reaches 80Mhz and for slave 20Mhz. From W5200 datasheet, it says for stable operation at least 33Mhz. anyway I would like to know if the parameter "Length of Parallel Routes" is about my net length (size of my track from SCLK pin from ethernet module to my microcontroler) or it is its width (like 10 mils)

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#2 21683185 16 Sep 2019 14:48

Anglina Miller Anglina Miller

Anonymous

Post #2
21683185 16 Sep 2019 14:48

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#3 21683186 18 Sep 2019 20:40

Mike P OKeeffe Mike P OKeeffe

Anonymous

Post #3
21683186 18 Sep 2019 20:40

Hi Didan,
The length in the case of calculating crosstalk is the length of the track that it is in parallel with another track. The calculator looks at the crosstalk or noise that could potentially be coupled into your track, which in this case is your SPICLK, which we want clean.In your capture of your layout, this would relate to the two tracks you have going to GPIO13 and GPIO14.
-Mike
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#4 21683187 19 Sep 2019 00:56

Didan Ribeiro Didan Ribeiro

Anonymous

Post #4
21683187 19 Sep 2019 00:56

Hi, thanks for reply. What you are talking about is parameter "s" ? I would like to know "L" parameter, length of parallel route. About "s" its 0.5mm as I measured it.
#5 21683188 19 Sep 2019 01:16

Mike P OKeeffe Mike P OKeeffe

Anonymous

Post #5
21683188 19 Sep 2019 01:16

s is the distance between the two tracks.
L is what I am talking about. It is the length of the track where the two signals are parallel to each other. This is different to the length of the net.
Why are you worried about cross talk? Are there other signals (clocks or orherwise) that you are worried about interfering with? The length of the net is important for impedance matching and minimising propagation delay. Ideally clk, mosi and miso tracks should be the same length, kept away from other noisy signals and not too long. Even better if you can route them all beside each other, coupling them together.
#6 21683189 19 Sep 2019 01:26

Didan Ribeiro Didan Ribeiro

Anonymous

Post #6
21683189 19 Sep 2019 01:26

Looking for screenshots can you help me to determine lenth of the track? I didnt understand how to get it, yet.
#7 21683190 19 Sep 2019 15:48

Mike P OKeeffe Mike P OKeeffe

Anonymous

Post #7
21683190 19 Sep 2019 15:48

Check out the this article on EDN: https://www.edn.com/electronics-blogs/bakers-best/4426162/PCB-signal-coupling-can-be-a-problemCypress have a decent PDF on SPI Layout guidelines here with some nice diagrams: https://www.cypress.com/file/196526/download
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#8 21683191 19 Sep 2019 17:00

Didan Ribeiro Didan Ribeiro

Anonymous

Post #8
21683191 19 Sep 2019 17:00

Hello Mike. I knew about EDN article but not Cypress. Anyway, thanks for links. By the way about my question for parameter"length" what is this number? What screen do you need to help me with this? Its not total net length.
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#9 21683192 19 Sep 2019 17:20

Mike P OKeeffe Mike P OKeeffe

Anonymous

Post #9
21683192 19 Sep 2019 17:20

Hi Didan,Taking your screenshot you submitted previously, I've highlighted two different examples of parallel length tracks. The distances between each track are different, so the crosstalk will be worse as this distance between the parallel tracks decreases. Taking the red highlighted area, you need to measure the length of the tracks when they are this far apart. Looking at the blue highlighted area, you measure the length of the nets when they have this amount of seperation between them.Then take your worst case value and that's your crosstalk value.
#10 21683193 19 Sep 2019 17:49

Didan Ribeiro Didan Ribeiro

Anonymous

Post #10
21683193 19 Sep 2019 17:49

From blue mark distance, I got 0.5mm, my trace spacing "S" parameter. I wanna to know what value to input from eeweb crosstalk calculator (https://www.eeweb.com/tools/microstrip-crosstalk) for "L" parameter, so if I understood you, S is 0.5mm (blue marks) as I measured before and L is the fartherst distance between tracks? Dont make sense it to me but its right?
#11 21683194 20 Sep 2019 16:16

Mike P OKeeffe Mike P OKeeffe

Anonymous

Post #11
21683194 20 Sep 2019 16:16

Didan,"L" is not the distance between the tracks. "S" is the distance between the tracks.I've drawn the diagram more explicitly here. To measure the Crosstalk, you have to split up the track into multiple pieces depending on the distance between the tracks. In the diagram, we have the red and blue area. Red S and Blue S are different distances, so we will have different crosstalk values, hence the split areas. The closer each track is, the worse the crosstalk will be.Notice where I've marked Red L. This is the length of the track, where Red S is the same distance between the tracks. At a rough guess, Red S is about 0.7mm apart, red L looks about 25mm in length. L is the length of the net, where the distance between the tracks in question are equal. Sometimes this is the total length of the net if the distance between the two tracks is the same for entire length of the track.Does this make sense now?
#12 21683195 20 Sep 2019 16:18

Mike P OKeeffe Mike P OKeeffe

Anonymous

Post #12
21683195 20 Sep 2019 16:18

Just to clarify, you will have to calculate your Crosstalk values separately.First for the area in red, second for the area in blue.
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Topic summary

✨ The discussion clarifies the meaning of the "Length of Parallel Routes" parameter in the eeweb microstrip crosstalk calculator. It refers to the length of the PCB track segment where two signals run parallel at a consistent spacing, not the total net length or the track width. The spacing between the tracks is denoted by "S," while "L" is the length over which this spacing is maintained. For accurate crosstalk calculation, the track should be divided into segments with uniform spacing, and crosstalk values calculated separately for each segment. This distinction is important for minimizing noise coupling on high-speed SPI signals, such as the SCLK line operating at up to 80 MHz in a design using the W5200 Ethernet module. Additional references include SPI layout guidelines from Cypress and an EDN article on PCB signal coupling.

FAQ

TL;DR: In microstrip crosstalk tools, L is the length where two traces run parallel, not net length or width; S is the spacing. “L is the length of the track where the two signals are parallel.” Example stat: SPI SCLK discussed at 80 MHz. [Elektroda, Anonymous, post #21683194]

Why it matters: Entering the correct L and S gives realistic near/far‑end crosstalk so you can keep SPI clocks clean on 2‑layer boards.

Quick-Facts

L = parallel run length; S = edge‑to‑edge spacing. Use the longest parallel segment for worst‑case. [Elektroda, Anonymous, post #21683194]
If spacing changes, split into segments and compute crosstalk for each, then use the worst. [Elektroda, Anonymous, post #21683195]
Example from thread: S measured ≈ 0.5 mm; another segment S ≈ 0.7 mm with L ≈ 25 mm. [Elektroda, Anonymous, post #21683194]
SPI layout tips: match CLK, MOSI, MISO lengths; route together; keep away from noisy signals. [Elektroda, Anonymous, post #21683188]
SPI speeds noted: master SCLK up to 80 MHz; slave 20 MHz; stable operation ≥ 33 MHz mentioned for W5200. [Elektroda, Anonymous, post #21683184]

Quick Facts

L = parallel run length; S = edge‑to‑edge spacing. Use the longest parallel segment for worst‑case. [Elektroda, Anonymous, post #21683194]
If spacing changes, split into segments and compute crosstalk for each, then use the worst. [Elektroda, Anonymous, post #21683195]
Example from thread: S measured ≈ 0.5 mm; another segment S ≈ 0.7 mm with L ≈ 25 mm. [Elektroda, Anonymous, post #21683194]
SPI layout tips: match CLK, MOSI, MISO lengths; route together; keep away from noisy signals. [Elektroda, Anonymous, post #21683188]
SPI speeds noted: master SCLK up to 80 MHz; slave 20 MHz; stable operation ≥ 33 MHz mentioned for W5200. [Elektroda, Anonymous, post #21683184]

What does “Length of Parallel Routes (L)” mean in crosstalk calculators?

L is the trace length over which two signals run side‑by‑side with the same spacing. It is not total net length. You measure only the portions that are parallel, because coupling occurs there. “L is the length of the track where the two signals are parallel.” [Elektroda, Anonymous, post #21683194]

Does L mean my track width or my entire net length?

Neither. Track width is unrelated here, and entire net length can overstate coupling. Use only the segment where your victim and aggressor run parallel with the same spacing. [Elektroda, Anonymous, post #21683194]

What does the spacing “S” parameter represent?

S is the edge‑to‑edge distance between the two traces in the parallel section. Measure S for each distinct parallel region you plan to analyze. [Elektroda, Anonymous, post #21683188]

How do I measure L and S on my layout? (3‑step)

Highlight the two nets and mark where they run side‑by‑side.
Measure edge‑to‑edge spacing S within each parallel region.
Measure the length L of each region and analyze separately; take the worst case. [Elektroda, Anonymous, post #21683195]

If spacing changes along the route, how should I enter values?

Split the route into sections. For each section, use its own L and S to compute crosstalk. Compare results and design to the worst‑case section. [Elektroda, Anonymous, post #21683195]

Which nearby signals should I worry about for SPI SCLK?

Any trace that runs in parallel with SCLK is a potential aggressor or victim. The thread example highlights runs near GPIO13 and GPIO14 as relevant parallel sections to evaluate. [Elektroda, Anonymous, post #21683186]

What SPI speeds were discussed, and why do they matter for crosstalk?

The thread notes 80 MHz SCLK in master mode, 20 MHz in slave, and a 33 MHz stable‑operation note for W5200. Higher edge rates increase coupling risk, so layout discipline becomes critical. [Elektroda, Anonymous, post #21683184]

How should I route SPI to minimize crosstalk on a 2‑layer board?

Match CLK, MOSI, and MISO lengths, keep them short, and route them together to improve coupling symmetry. Keep them away from noisy clocks or switching nodes. [Elektroda, Anonymous, post #21683188]

Does crosstalk get worse with smaller spacing or longer parallel length?

Both increase coupling. The closer the traces and the longer they stay parallel, the higher the crosstalk. Prioritize increasing S and reducing L in critical areas. [Elektroda, Anonymous, post #21683194]

What number should I enter for L in the eeweb microstrip crosstalk tool?

Enter the length of the specific parallel section you are analyzing, not the whole net. If multiple sections exist, analyze each and use the worst result. [Elektroda, Anonymous, post #21683195]

Can L ever equal the entire net length?

Yes. If two traces maintain the same spacing for their whole run, then L equals the full net length for the calculation. This is an edge case that maximizes coupling. [Elektroda, Anonymous, post #21683194]

What is microstrip crosstalk in simple terms?

It is unwanted signal coupling between nearby traces that run in parallel on the same layer. The coupling mainly depends on spacing and the parallel length between aggressor and victim. [Elektroda, Anonymous, post #21683194]

How do I pick the worst‑case segment when several exist?

Choose the segment with the smallest spacing S and significant L. Compute each segment separately and design to the largest predicted crosstalk. [Elektroda, Anonymous, post #21683192]

Any quick numeric examples from the thread to guide inputs?

One section used S ≈ 0.5 mm; another had S ≈ 0.7 mm with L ≈ 25 mm. Start with the 0.5 mm segment as likely worst‑case, then compare. [Elektroda, Anonymous, post #21683194]

Why match SPI trace lengths at all?

Matched lengths reduce skew and jitter, which keeps sampling margins robust as frequency rises. “Ideally clk, mosi and miso tracks should be the same length.” [Elektroda, Anonymous, post #21683188]