IP3 and SFDR Calculator

Current linearity window

{{ ip3Badge }} {{ noiseBadge }} {{ statusBadge }} Disabled

Noise IM3 Tone {{ visualMarker }}

Intercept reference:

Use IIP3 for receivers and cascades; use OIP3 when a power amplifier or mixer datasheet gives output intercept.

IP3 value:

The calculator derives the opposite reference plane using the small-signal gain.

dBm

Small-signal gain:

Gain shifts IIP3 to OIP3 and input noise to output noise without changing input-referred SFDR.

Noise basis:

SFDR uses the integrated noise or minimum detectable signal at the selected input bandwidth.

Input noise density:

For thermal noise plus receiver noise figure, switch the noise basis instead of doing the arithmetic manually.

dBm/Hz

Noise figure:

The derived density is -174 dBm/Hz plus this noise figure.

Measurement bandwidth:

Use the receiver, IF, FFT bin, or channel bandwidth used for the SFDR requirement.

Detection SNR margin: {{ detectionSnrDisplay }}

This converts integrated noise floor to the minimum detectable signal used by the SFDR formula.

Per-tone input power: {{ tonePowerDisplay }}

Use this to see whether a planned blocker or lab two-tone level keeps IM3 below the selected MDS.

dBm

{{ error }}

Tone 1 frequency:

The second tone is placed above this frequency by the spacing below.

MHz

Tone spacing:

Use a spacing that matches the lab two-tone setup or adjacent blocker spacing.

kHz

P1dB offset from IP3: {{ p1dbOffsetDisplay }}

This planning estimate affects only the compression headroom rows.

Intercept chart span: {{ chartSpanDisplay }}

This affects only the Intercept Map chart and its CSV export.

Metric	Value	Detail	Copy
{{ row.metric }}	{{ row.value }}	{{ row.detail }}

Product	Frequency	Input Power	Output Power	Detail	Copy
{{ row.product }}	{{ row.frequency }}	{{ row.inputPower }}	{{ row.outputPower }}	{{ row.detail }}

Check	Status	Action	Copy
{{ row.check }}	{{ row.status }}	{{ row.action }}

{{ jsonOutput }}

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Third-order intercept point, usually shortened to IP3, is an RF linearity figure used to estimate how strongly a receiver, amplifier, mixer, or converter will create third-order intermodulation products during a two-tone test. Those products are important because the products at 2f1 - f2 and 2f2 - f1 sit close to the original tones, so ordinary filtering may not remove them.

Spurious-free dynamic range, or SFDR, connects that linearity limit to the lowest signal level worth detecting. For an IM3-limited receiver estimate, the clean input span runs from the minimum detectable signal to the per-tone input level where predicted IM3 reaches that same threshold. A better IP3 raises the upper end of the span. A higher noise floor, wider measurement bandwidth, or extra detection SNR margin raises the lower end and shrinks the span.

Two equal RF tones, nearby third-order intermodulation products, an MDS noise boundary, and an extrapolated IP3 point.

IP3 is not a safe drive level and not a promise that every spur is below the noise floor. It is an extrapolated small-signal measure. Compression, blocker power, gain distribution, filters, impedance mismatch, ADC clipping, LO leakage, and second-order products can still limit the real system before the simple IM3 estimate does.

The useful comparison is made at one reference plane. Input-referred numbers are common for receivers and cascades. Output-referred numbers are common for amplifiers and mixers. Gain translates between the two, but it does not by itself improve the input-referred dynamic range because noise and signal levels move together.

How to Use This Tool:

Use the calculator as a planning check for an equal-power two-tone case. Keep the reference plane and bandwidth tied to the receiver, mixer, amplifier, or converter condition you want to compare.

Choose Intercept reference. Select Input IP3 (IIP3) when the datasheet or cascade budget is input-referred, or Output IP3 (OIP3) when the available number is output-referred.
Enter IP3 value and Small-signal gain. Gain converts IIP3 to OIP3 and also moves input noise to output noise, so use conversion gain for mixers, receiver gain for front ends, or a negative value for passive loss.
Set Noise basis. Use Measured input noise density when you already know the input spectral density in dBm/Hz. Use Thermal noise plus noise figure when the estimate should start from -174 dBm/Hz and add receiver noise figure.
Enter Measurement bandwidth and Detection SNR margin. The bandwidth integrates the noise density; the SNR margin raises the minimum detectable signal used by the SFDR estimate.
Enter Per-tone input power for the planned two-tone test or blocker condition. The summary and Operating Guidance show whether the predicted IM3 product is below the selected MDS.
Open Advanced only when the lab setup needs it. Tone 1 frequency and Tone spacing place the two desired tones and their IM3 products; P1dB offset from IP3 controls the compression estimate; Intercept chart span changes the plotted input-power window.
Read Linearity Ledger first for IIP3, OIP3, integrated noise, MDS, input SFDR, max clean input tone, and estimated P1dB. Use Two-Tone Products for product frequencies and powers, then check Operating Guidance before relying on the number.

If validation appears, fix the named field before interpreting results. Common causes are nonnumeric IP3 or gain, a bandwidth at zero or below, a negative detection margin, a negative noise figure, tone frequency or spacing at zero or below, or a chart span outside 40 to 180 dB.

Interpreting Results:

The headline SFDR is the input-referred clean span for the selected MDS threshold. The Max clean input tone is the per-tone input level where the predicted third-order product reaches that MDS threshold. The Current IM3 clearance says whether the entered two-tone power is below that limit.

How to read IP3 and SFDR outputs
Output	What it means	What to verify
IIP3 and OIP3	Same intercept point expressed at the input or output reference plane.	Confirm the datasheet reference plane and gain sign before comparing parts.
Integrated input noise	Noise density integrated across the selected measurement bandwidth.	Use the same bandwidth and noise basis when comparing runs.
MDS for SFDR	Integrated noise plus the selected detection SNR margin.	Do not compare a 0 dB margin estimate with a requirement that needs extra SNR.
Input SFDR	Clean input span from MDS to the IM3 crossing level.	Check compression and other spur mechanisms before treating it as the whole dynamic range.
Max clean input tone	Highest equal per-tone input level where predicted IM3 reaches MDS.	Keep blocker or lab tones at or below this level unless measured data says otherwise.
Two-Tone Products	Desired tone frequencies and the lower and upper IM3 product frequencies.	Check whether those products land in the channel, IF, FFT bin, or filter passband that matters.
Estimated input P1dB	A planning compression point derived from the selected offset below IIP3.	Use measured P1dB and maximum input ratings for real hardware decisions.

The status badge is a fast margin check. No SFDR window means the chosen MDS is above IIP3, so the simple formula gives a negative span. IM3 above MDS means the entered tone power already predicts a visible third-order product. Tight IM3 margin means clearance is below 6 dB, Usable IM3 margin is below 20 dB, and Clean IM3 margin is 20 dB or more.

A high SFDR estimate does not prove the hardware can tolerate the entered tone power. Check Compression estimate, measured P1dB, absolute maximum ratings, and a real two-tone measurement when the result will guide RF power levels, receiver protection, or compliance-sensitive work.

Technical Details:

Third-order intermodulation follows a 3:1 slope in dB while the desired fundamental follows a 1:1 slope. If each input tone rises by 1 dB, the predicted third-order product rises by 3 dB. The distance between a tone and its IM3 product therefore changes by 2 dB for every 1 dB of input-tone change.

SFDR uses that slope difference to find the input level where IM3 just reaches the minimum detectable signal. The lower boundary is not only thermal noise. It is the integrated noise floor after bandwidth, plus any SNR margin required before a signal should count as detectable.

Formula Core:

The core calculation keeps all powers in dBm or dB at the same reference plane.

\begin{array}{rcl} OIP3 & = & IIP3 + G \\ N_{int} & = & N_{density} + 10 \log_{10} (B) \\ MDS & = & N_{int} + {SNR}_{det} \\ P_{IM3} & = & 3 P_{tone} - 2 IIP3 \\ SFDR & = & \frac{2}{3} (IIP3 - MDS) \\ P_{clean} & = & MDS + SFDR \end{array}

Here, G is small-signal gain in dB, B is bandwidth in hertz, Ndensity is input noise density in dBm/Hz, SNRdet is the detection SNR margin in dB, Ptone is the equal per-tone input power, and Pclean is the max clean input tone. When the thermal-plus-noise-figure path is selected, Ndensity is -174 dBm/Hz plus noise figure.

Output-referred SFDR has the same numeric span after gain translation because OIP3 and output MDS both move by G. Gain can still matter in a real chain because later stages, ADC full scale, filters, and compression limits may be referenced to output power rather than input power.

IP3 and SFDR variable meanings
Quantity	Unit	Role in the estimate
IIP3	dBm	Input-referred third-order intercept. Higher values reduce predicted IM3 for the same tone power.
OIP3	dBm	Output-referred third-order intercept. It equals IIP3 plus small-signal gain.
Integrated input noise	dBm	Noise density integrated across the selected bandwidth with 10 log10 of bandwidth.
MDS	dBm	Noise boundary used for SFDR after adding the selected detection margin.
IM3 input power	dBm	Predicted third-order product for the entered per-tone input level.
IM3 clearance	dB	MDS minus predicted IM3. Positive clearance means the product is below the selected detection threshold.

A default-style receiver example makes the arithmetic concrete. With IIP3 at +10 dBm, measured input noise density at -150 dBm/Hz, 100 kHz bandwidth, and 0 dB detection margin, the integrated input noise is -100 dBm and MDS is also -100 dBm. The SFDR is two thirds of 110 dB, or 73.33 dB. The max clean input tone is therefore -26.67 dBm. If the planned two-tone input is -35 dBm per tone, IM3 is -125 dBm, which leaves 25 dB of clearance below MDS.

Validation boundaries for IP3 and SFDR calculations
Input	Accepted values	Why it matters
Intercept reference	IIP3 or OIP3	The selected reference decides whether gain is added or subtracted to derive the other intercept value.
IP3 value and gain	Finite numeric dBm and dB values	The intercept and gain define the input and output reference planes.
Noise basis	Measured density, or thermal noise plus nonnegative noise figure	MDS depends on the noise density before bandwidth integration.
Measurement bandwidth	Greater than zero after unit conversion	Noise integration needs a positive bandwidth in hertz.
Detection SNR margin	Zero or greater	A higher required SNR raises MDS and lowers SFDR.
Tone frequency and spacing	Greater than zero	The product table needs real tone locations for 2f1 - f2 and 2f2 - f1.
P1dB offset from IP3	Zero or greater	The compression estimate subtracts this offset from IIP3.
Intercept chart span	40 to 180 dB	The chart window is bounded so the intercept map remains readable.

The frequency products use ordinary two-tone third-order relationships. If tone 2 is tone 1 plus the selected spacing, the lower IM3 product lands one spacing below tone 1 and the upper IM3 product lands one spacing above tone 2. The power estimate assumes equal-amplitude tones and the small-signal IP3 slope, so it should be checked against measured two-tone data when compression or wideband spur behavior matters.

Accuracy and Safety Notes:

Use the result as an RF planning estimate. It is not a substitute for a calibrated two-tone measurement, a device maximum-rating check, or a compliance test.

The IM3 estimate assumes equal tone powers, small-signal behavior, and the third-order 3:1 slope.
The thermal-noise option uses the standard -174 dBm/Hz room-temperature reference plus noise figure; unusual antenna temperature, cryogenic systems, or external interference can shift the real noise floor.
P1dB is estimated from an offset below IP3. Use the measured compression point when the hardware datasheet provides one.
SFDR based on IM3 does not cover second-order products, harmonics, LO leakage, image responses, ADC clipping, filter leakage, or blocker desensitization.
Do not use a calculated clean-tone level as permission to apply RF power to a receiver input. Check attenuators, maximum input ratings, thermal limits, and lab safety practice first.

Worked Examples:

A VHF receiver budget starts with IIP3 at +10 dBm, small-signal gain at +20 dB, measured input noise density at -150 dBm/Hz, 100 kHz bandwidth, and 0 dB detection margin. Linearity Ledger reports MDS for SFDR at -100.00 dBm, Input SFDR at 73.33 dB, and Max clean input tone at -26.67 dBm. A -35 dBm per-tone test predicts IM3 at -125.00 dBm, so Operating Guidance shows 25.00 dB of current IM3 clearance.

A mixer estimate uses IIP3 at -5 dBm, gain at +15 dB, noise figure at 8 dB, 2 MHz bandwidth, and 10 dB detection SNR margin. The thermal-noise path gives an input noise density of -166 dBm/Hz and MDS near -92.99 dBm. Input SFDR is 58.66 dB, with Max clean input tone near -34.33 dBm. A -55 dBm per-tone blocker leaves about 62.01 dB of IM3 clearance, but the estimated P1dB is still only -15 dBm input when the offset is 10 dB.

A troubleshooting run enters IIP3 at 0 dBm, measured noise density at -145 dBm/Hz, 1 MHz bandwidth, 6 dB detection margin, and -20 dBm per-tone input. The calculator reports MDS for SFDR at -79.00 dBm and Input SFDR at 52.67 dB, but the predicted IM3 is -60.00 dBm. The status becomes IM3 above MDS because the current product is 19 dB above the detection threshold. Lowering the tone level toward the Max clean input tone of -26.33 dBm is the direct fix for that setup.

A validation case is simpler: if Measurement bandwidth is set to 0 Hz or Tone spacing is set to 0 kHz, the result panel disappears and the error list names the field to fix. Restore a positive bandwidth and positive spacing before trusting the product frequencies or SFDR value.

FAQ:

Is IP3 a real power level I can drive to?

No. IP3 is an extrapolated intercept point from the small-signal fundamental and third-order slopes. Use Estimated input P1dB, measured P1dB, and maximum input ratings for real drive-level decisions.

Should I enter IIP3 or OIP3?

Use the reference plane from the number you have. If the datasheet gives OIP3, choose Output IP3 (OIP3) and enter gain so the calculator can derive IIP3. If a receiver cascade gives input-referred IP3, choose Input IP3 (IIP3).

Why does wider bandwidth reduce SFDR?

Wider bandwidth integrates more noise. A 10x bandwidth increase raises integrated noise by 10 dB, and the IM3-based SFDR estimate drops by 6.67 dB because SFDR is two thirds of the gap between IIP3 and MDS.

What does IM3 above MDS mean?

It means the entered per-tone input power predicts a third-order product above the selected minimum detectable signal. Reduce Per-tone input power, narrow the bandwidth, lower the required SNR margin, or improve the IP3 and noise terms.

Why did the result panel disappear?

The calculator hides results when required values fail validation. Check the error list for nonnumeric IP3, gain, or tone power; nonpositive bandwidth, tone frequency, or tone spacing; negative noise figure; negative detection margin; or a chart span outside 40 to 180 dB.

Are entered RF values sent for remote processing?

The arithmetic runs in the browser from the values you enter; there is no uploaded file or remote measurement step. If you share a filled-in page URL, the selected values may travel with that shared link.

Glossary:

IIP3: Input-referred third-order intercept point, expressed in dBm.
OIP3: Output-referred third-order intercept point, equal to IIP3 plus small-signal gain.
IM3: Third-order intermodulation product created by two input tones, commonly near 2f1 - f2 and 2f2 - f1.
MDS: Minimum detectable signal used as the noise boundary for this SFDR estimate.
SFDR: Spurious-free dynamic range, the clean input span before predicted IM3 reaches MDS.
Noise figure: Added receiver noise expressed as dB above the standard thermal-noise reference.
P1dB: One-dB compression point, where gain has dropped by 1 dB from its small-signal value.
dBm: Power level in decibels relative to one milliwatt.

References:

Use Selectivity to Improve Receiver Intercept Point, Analog Devices.
High Performance Active Mixer Overcomes RF Transmitter Design Challenges, Linear Technology and High Frequency Electronics, May 2006.
Measurement Methods, Mini-Circuits, October 31, 2015.
Noise Figure, Keysight Technologies.
SFDR, Analog Devices.