Documentation sections

Ferrite Bead Filter Calculator

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

Slug ferrite-bead. Calculate ferrite bead impedance, insertion loss, and DC voltage drop at any frequency. Select EMI filter beads for power supply decoupling.

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 ferrite-bead
Name	Unit	Range	Default	Description
impedance100MHznumber	Ω	at least 1	600	Ferrite bead impedance at 100 MHz from datasheet (e.g. 600Ω for Murata BLM18PG601SN1)
dcResistancenumber	Ω	at least 0.001	0.3	DC resistance from datasheet
loadImpedancenumber	Ω	at least 1	50	Load Impedance
frequencynumber	MHz	at least 0.1	100	Frequency
beadInductancenumber	µH	at least 0.001	1.208	L of the bead's equivalent circuit, fitted where the impedance curve rises (AN-1368 eq. 1). The default is the note's fit of a Tyco BMB2A1000LN2
parasiticCapacitancenumber	pF	at least 0	1.678	C of the equivalent circuit, fitted where the impedance curve falls above its peak (AN-1368 eq. 2)
sourceImpedancenumber	Ω	at least 0	0	Resistance of the source that drives the bead; 0 Ω is an ideal supply rail

Outputs

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

Outputs of ferrite-bead
Field	Unit	Description
beadImpedance	Ω	Bead Impedance at f
insertionLossheadline	dB	Insertion Loss
voltageDivider	%	Voltage Ratio
dcVoltageDrop	mV/A	DC Voltage Drop

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": "ferrite-bead",
    "inputs": {
      "impedance100MHz": 600,
      "dcResistance": 0.3,
      "loadImpedance": 50,
      "frequency": 100,
      "beadInductance": 1.208,
      "parasiticCapacitance": 1.678,
      "sourceImpedance": 0
    }
  }'

import rftools

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

result = client.calculate("ferrite-bead", {
    "impedance100MHz": 600,
    "dcResistance": 0.3,
    "loadImpedance": 50,
    "frequency": 100,
    "beadInductance": 1.208,
    "parasiticCapacitance": 1.678,
    "sourceImpedance": 0,
})
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": "ferrite-bead",
    "inputs": {
      "impedance100MHz": 600,
      "dcResistance": 0.3,
      "loadImpedance": 50,
      "frequency": 100,
      "beadInductance": 1.208,
      "parasiticCapacitance": 1.678,
      "sourceImpedance": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "ferrite-bead",
	  "inputs": {
	    "impedance100MHz": 600,
	    "dcResistance": 0.3,
	    "loadImpedance": 50,
	    "frequency": 100,
	    "beadInductance": 1.208,
	    "parasiticCapacitance": 1.678,
	    "sourceImpedance": 0
	  }
	}`)
	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": "ferrite-bead",
  "values": {
    "beadImpedance": 599.9999999999999,
    "insertionLoss": 22.271137142684843,
    "voltageDivider": 7.6991564164251445,
    "dcVoltageDrop": 300
  },
  "warnings": [
    "High DC resistance — check voltage budget for load"
  ],
  "errors": [],
  "provenance": {
    "method": "calculator:ferrite-bead",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Eco & Limjoco, Ferrite Bead Demystified, Analog Devices AN-1368 (Rev. 0, 2015), Figure 3 and eqs. 1-2; Paul, Introduction to Electromagnetic Compatibility, 2nd ed. (2006), insertion loss of a series element",
    "assumptions": [
      {
        "code": "an-1368-equivalent-circuit",
        "text": "The bead is AN-1368's R_DC + (R_AC ∥ L ∥ C) circuit, with R_AC set so that |Z| at 100 MHz is the entered datasheet value; the default L and C are the note's fit of a Tyco BMB2A1000LN2."
      },
      {
        "code": "no-dc-bias",
        "text": "DC bias is not modelled. It lowers a bead's impedance: AN-1368 shows a TDK MPZ1608S101A falling from 100 Ω to 10 Ω at 100 MHz at half its rated current."
      },
      {
        "code": "resistive-source-and-load",
        "text": "The source and the load are resistive."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "impedance100MHz": {
          "min": 1,
          "max": null,
          "unit": "Ω"
        },
        "dcResistance": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "loadImpedance": {
          "min": 1,
          "max": null,
          "unit": "Ω"
        },
        "frequency": {
          "min": 0.1,
          "max": null,
          "unit": "MHz"
        },
        "beadInductance": {
          "min": 0.001,
          "max": null,
          "unit": "µH"
        },
        "parasiticCapacitance": {
          "min": 0,
          "max": null,
          "unit": "pF"
        },
        "sourceImpedance": {
          "min": 0,
          "max": null,
          "unit": "Ω"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "impedance100MHz": 600,
      "dcResistance": 0.3,
      "loadImpedance": 50,
      "frequency": 100,
      "beadInductance": 1.208,
      "parasiticCapacitance": 1.678,
      "sourceImpedance": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "impedance100MHz"
      ],
      "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 pageFerrite Bead Filter Calculator, at these values