Documentation sections

Cable Capacitance High-Frequency Rolloff

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

Slug cable-capacitance-rolloff. Calculate the high-frequency rolloff (-3 dB point) caused by cable capacitance interacting with source impedance.

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 cable-capacitance-rolloff
Name	Unit	Range	Default	Description
sourceImpedancenumber	Ω	at least 1	10000	Source Impedance
cableCapacitancePfnumber	pF/m	at least 1	100	Per metre, from cable datasheet (typical: 80–150 pF/m)
cableLengthMnumber	m	at least 0.1	5	Cable Length
targetCutoffnumber	Hz	1000 … 10000000	100000	The −3 dB frequency the cable must stay above. Five times 20 kHz keeps the audio band within 0.2 dB. The status fails below 20 kHz.

Outputs

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

Outputs of cable-capacitance-rolloff
Field	Unit	Description
totalCapacitancePf	pF	Total Cable Capacitance
cutoffFrequencyheadline	Hz	-3 dB Frequency
rolloffAt20k	dB	Rolloff at 20 kHz

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": "cable-capacitance-rolloff",
    "inputs": {
      "sourceImpedance": 10000,
      "cableCapacitancePf": 100,
      "cableLengthM": 5,
      "targetCutoff": 100000
    }
  }'

import rftools

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

result = client.calculate("cable-capacitance-rolloff", {
    "sourceImpedance": 10000,
    "cableCapacitancePf": 100,
    "cableLengthM": 5,
    "targetCutoff": 100000,
})
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": "cable-capacitance-rolloff",
    "inputs": {
      "sourceImpedance": 10000,
      "cableCapacitancePf": 100,
      "cableLengthM": 5,
      "targetCutoff": 100000
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "cable-capacitance-rolloff",
	  "inputs": {
	    "sourceImpedance": 10000,
	    "cableCapacitancePf": 100,
	    "cableLengthM": 5,
	    "targetCutoff": 100000
	  }
	}`)
	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": "cable-capacitance-rolloff",
  "values": {
    "totalCapacitancePf": 500,
    "cutoffFrequency": 31830.988618379066,
    "rolloffAt20k": -1.4450701162052872
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:cable-capacitance-rolloff",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "sourceImpedance": {
          "min": 1,
          "max": null,
          "unit": "Ω"
        },
        "cableCapacitancePf": {
          "min": 1,
          "max": null,
          "unit": "pF/m"
        },
        "cableLengthM": {
          "min": 0.1,
          "max": null,
          "unit": "m"
        },
        "targetCutoff": {
          "min": 1000,
          "max": 10000000,
          "unit": "Hz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "sourceImpedance": 10000,
      "cableCapacitancePf": 100,
      "cableLengthM": 5,
      "targetCutoff": 100000
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "sourceImpedance"
      ],
      "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 pageCable Capacitance High-Frequency Rolloff, at these values