Documentation sections

PCB Crosstalk Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug pcb-crosstalk. Calculate PCB trace crosstalk NEXT, FEXT, and coupling coefficient for signal integrity analysis. Determine critical length and guard trace spacing. Free, instant results.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of pcb-crosstalk
Name	Unit	Range	Default	Description
traceWidthnumber	mm	at least 0.01	0.2	Trace Width
traceSpacingnumber	mm	0.01 … 10	0.2	Edge-to-edge spacing between traces
dielectricHeightnumber	mm	at least 0.01	0.1	Height of dielectric layer between trace and reference plane
frequencynumber	MHz	at least 0.1	100	Highest significant frequency component of the signal
traceLengthnumber	mm	0.1 … 10000	50	Length over which the traces run parallel
dielectricConstnumber	none	1 … 15	4.5	Relative permittivity of the substrate under the traces; 4.5 is typical FR-4.

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of pcb-crosstalk
Field	Unit	Description
couplingCoeff	%	(Z_even − Z_odd)/(Z_even + Z_odd), from the coupled-microstrip mode impedances
next_dBheadline	dB	Near-end crosstalk with both ends matched: K_b = (√Z_even − √Z_odd)/(√Z_even + √Z_odd) on a long run, K_b·2T_d/T_r on a run shorter than half the rise time
fext_dB	dB	Far-end crosstalk from the delay difference between the even and odd modes over the rise time, bounded at (1 − K_b²)/2 of the aggressor at any length
criticalLength	mm	Quarter-wavelength at the signal frequency in PCB medium

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "pcb-crosstalk",
    "inputs": {
      "traceWidth": 0.2,
      "traceSpacing": 0.2,
      "dielectricHeight": 0.1,
      "frequency": 100,
      "traceLength": 50,
      "dielectricConst": 4.5
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("pcb-crosstalk", {
    "traceWidth": 0.2,
    "traceSpacing": 0.2,
    "dielectricHeight": 0.1,
    "frequency": 100,
    "traceLength": 50,
    "dielectricConst": 4.5,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "pcb-crosstalk",
    "inputs": {
      "traceWidth": 0.2,
      "traceSpacing": 0.2,
      "dielectricHeight": 0.1,
      "frequency": 100,
      "traceLength": 50,
      "dielectricConst": 4.5
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "pcb-crosstalk",
	  "inputs": {
	    "traceWidth": 0.2,
	    "traceSpacing": 0.2,
	    "dielectricHeight": 0.1,
	    "frequency": 100,
	    "traceLength": 50,
	    "dielectricConst": 4.5
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "pcb-crosstalk",
  "values": {
    "couplingCoeff": 5.722274079743187,
    "next_dB": -45.96243171195444,
    "fext_dB": -51.709356640198116,
    "criticalLength": 406.33867957581043
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:pcb-crosstalk",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "S. H. Hall, G. W. Hall & J. A. McCall, High-Speed Digital System Design, Wiley 2000, ch. 3 (crosstalk on coupled lines); the coupled-microstrip mode model of lib/pcb/impedance.ts, fitted to the site's 2-D method-of-moments solver",
    "assumptions": [
      {
        "code": "matched-both-ends",
        "text": "Both traces are matched at both ends to √(Z_even·Z_odd), and the aggressor edge is a linear ramp of rise time 0.35/f."
      },
      {
        "code": "zero-thickness-microstrip",
        "text": "The pair is edge-coupled microstrip of zero thickness on one substrate, with no solder mask; its mode impedances and permittivities come from the coupled-microstrip fit, whose worst-case error on a mode impedance is 2 % inside its range."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.01,
          "max": null,
          "unit": "mm"
        },
        "traceSpacing": {
          "min": 0.01,
          "max": 10,
          "unit": "mm"
        },
        "dielectricHeight": {
          "min": 0.01,
          "max": null,
          "unit": "mm"
        },
        "frequency": {
          "min": 0.1,
          "max": null,
          "unit": "MHz"
        },
        "traceLength": {
          "min": 0.1,
          "max": 10000,
          "unit": "mm"
        },
        "dielectricConst": {
          "min": 1,
          "max": 15,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 0.2,
      "traceSpacing": 0.2,
      "dielectricHeight": 0.1,
      "frequency": 100,
      "traceLength": 50,
      "dielectricConst": 4.5
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "traceWidth"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pagePCB Crosstalk Calculator, at these values