Current linearity window
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{{ ip3Badge }} {{ noiseBadge }} {{ statusBadge }} Disabled
Use IIP3 for receivers and cascades; use OIP3 when a power amplifier or mixer datasheet gives output intercept.
The calculator derives the opposite reference plane using the small-signal gain.
dBm
Gain shifts IIP3 to OIP3 and input noise to output noise without changing input-referred SFDR.
dB
SFDR uses the integrated noise or minimum detectable signal at the selected input bandwidth.
For thermal noise plus receiver noise figure, switch the noise basis instead of doing the arithmetic manually.
dBm/Hz
The derived density is -174 dBm/Hz plus this noise figure.
dB
Use the receiver, IF, FFT bin, or channel bandwidth used for the SFDR requirement.
{{ detectionSnrDisplay }}
This converts integrated noise floor to the minimum detectable signal used by the SFDR formula.
dB
{{ tonePowerDisplay }}
Use this to see whether a planned blocker or lab two-tone level keeps IM3 below the selected MDS.
dBm
The second tone is placed above this frequency by the spacing below.
MHz
Use a spacing that matches the lab two-tone setup or adjacent blocker spacing.
kHz
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This planning estimate affects only the compression headroom rows.
dB
{{ chartSpanDisplay }}
This affects only the Intercept Map chart and its CSV export.
dB
Metric Value Detail Copy
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Product Frequency Input Power Output Power Detail Copy
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Check Status Action Copy
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Tags: Diagnostics, Electrical, Wireless
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A weak adjacent transmitter can be harmless in one receiver and disruptive in another even when both receivers have the same tuned frequency. The difference often comes from linearity. Real amplifiers, mixers, converters, and receiver front ends are never perfectly linear, so strong signals create extra tones that were not present at the input. The most troublesome products are often third-order intermodulation products because they land close to the original signals and can fall inside the wanted channel.

Two-tone testing is the common way to describe that behavior. Two equal RF tones at f1 and f2 pass through the device or chain, and the spectrum is checked for products near 2f1 - f2 and 2f2 - f1. Those IM3 products rise faster than the wanted tones as input power increases. A 1 dB increase in each input tone raises the wanted tones by 1 dB, while the predicted third-order products rise by 3 dB. That 3:1 slope is why a small power change can quickly turn a buried spur into a visible interferer.

IP3, the third-order intercept point, is an extrapolated crossing of the wanted-tone line and the IM3 line. It is not a safe drive level. Hardware usually compresses or reaches a rating limit well before the mathematical crossing. IP3 is still useful because it lets engineers compare low-level intermodulation behavior without forcing every test up to damaging power levels.

Term Meaning in RF planning
IIP3 Input-referred third-order intercept, useful for receiver and cascade comparisons.
OIP3 Output-referred third-order intercept, often quoted for amplifiers and mixers.
IM3 Third-order products near the desired tones, especially 2f1 - f2 and 2f2 - f1.
MDS Minimum detectable signal boundary used to decide when a spur becomes important.
SFDR Dynamic range before a predicted spur reaches the detection boundary.
Two RF tones, nearby third-order products, an MDS noise boundary, and an extrapolated IP3 point.

SFDR ties the linearity estimate to the receiver's lower limit. The lower limit may be a measured noise density integrated over a bandwidth, or it may start from the room-temperature thermal-noise reference plus noise figure. Wider bandwidth raises the integrated noise floor. A required detection SNR margin raises the boundary again, so the same IIP3 can produce a smaller useful dynamic range in a wider channel or a stricter detection requirement.

The reference plane must stay consistent. Input-referred values are usually the clearest choice for receiver sensitivity and blocker planning. Output-referred values are convenient when an amplifier or mixer datasheet quotes output intercept. Gain converts IIP3 to OIP3, but moving the reference plane does not make the input-referred SFDR larger because signal, noise, and intermodulation levels move together.

How to Use This Tool:

Use the calculator for equal-power two-tone planning. Keep the IP3 value, gain, noise assumption, and bandwidth tied to the same receiver, mixer, amplifier, or converter condition.

  1. Set Intercept reference to match the number you have. Choose Input IP3 (IIP3) for input-referred receiver or cascade data, or Output IP3 (OIP3) when a datasheet gives the output intercept.
  2. Enter IP3 value and Small-signal gain. Use positive gain for active stages, conversion gain for mixers, and negative gain when the path has passive loss.
  3. Choose the Noise basis. Measured input noise density uses a direct dBm/Hz value. Thermal noise plus noise figure starts from -174 dBm/Hz and adds the entered receiver noise figure.
  4. Set Measurement bandwidth and Detection SNR margin. The result summary should move as bandwidth or SNR margin changes because both affect MDS for SFDR.
  5. Enter Per-tone input power for the planned two-tone lab level or blocker case. Watch Current IM3 clearance and the status badge as the tone power approaches the Max clean input tone.
  6. Open Advanced when frequency placement or compression planning matters. Tone 1 frequency and Tone spacing place the products in Two-Tone Products; P1dB offset from IP3 changes the compression estimate; Intercept chart span changes the plotted input-power window.
  7. Read Linearity Ledger for the main numeric result, then check Operating Guidance for margin actions. Use Intercept Map to see where the fundamental, IM3 line, and MDS boundary cross, and use Bandwidth Sweep to compare SFDR across common bandwidths.

If results disappear, fix the validation message first. Non-numeric IP3, gain, or tone power; zero or negative bandwidth, tone frequency, or tone spacing; negative noise figure; negative detection margin; and chart spans outside 40 to 180 dB are rejected before results are shown.

Interpreting Results:

The main SFDR value is the input-referred clean span from MDS for SFDR up to the point where predicted IM3 reaches that boundary. Max clean input tone is the equal per-tone input level at that crossing. A planned tone level below that value leaves positive IM3 clearance; a planned tone level above it predicts an intermodulation product above the selected detection boundary.

Output Useful reading Check before acting
IIP3 and OIP3 The same intercept point expressed at the input or output reference plane. Verify the datasheet reference plane and gain sign before comparing devices.
Integrated input noise Input noise density integrated across the selected bandwidth. Keep bandwidth and noise basis unchanged when comparing several runs.
MDS for SFDR The noise boundary after adding the selected detection SNR margin. Do not compare a 0 dB margin estimate with a requirement that needs extra SNR.
Current IM3 clearance The distance between predicted IM3 and MDS for the entered per-tone power. Positive clearance is useful only while the device remains out of compression.
Two-Tone Products The desired tone frequencies and the nearby lower and upper IM3 products. Check whether those products fall inside the receiver channel, IF passband, FFT bin, or filter window that matters.
Estimated input P1dB A planning compression point from the selected offset below IIP3. Replace the estimate with measured P1dB when a datasheet or lab result provides it.

The status badge compresses the current margin into one warning. No SFDR window means MDS is above IIP3, so the formula yields a negative clean span. IM3 above MDS means the entered two-tone level already predicts a visible third-order product. Tight IM3 margin is below 6 dB, Usable IM3 margin is below 20 dB, and Clean IM3 margin is 20 dB or more.

A strong SFDR number does not prove that the hardware can accept the entered input power. Check Compression estimate, absolute maximum ratings, attenuator losses, analyzer overload limits, and measured two-tone data before using the estimate for receiver protection, compliance work, or expensive hardware tests.

Technical Details:

Third-order intermodulation is modeled from the small-signal slopes of the desired tone and the third-order product. In logarithmic power units, the desired tone changes 1 dB for each 1 dB input change, while the IM3 product changes 3 dB. The separation between the desired tone and IM3 therefore changes by 2 dB for every 1 dB input change.

SFDR for this two-tone estimate uses the input level where the IM3 line reaches the minimum detectable signal boundary. The lower boundary is not fixed by IP3. It comes from input noise density, bandwidth, and any required detection SNR margin. That is why the same IIP3 can look generous in a narrow FFT bin and much smaller in a wide receiver channel.

Formula Core:

All powers below are handled in dBm or dB at the input reference plane unless the equation explicitly translates to output.

OIP3 = IIP3+G Ndensity = -174+NF when noise figure is used Nint = Ndensity+10log10(B) MDS = Nint+SNRdet PIM3 = 3Ptone-2IIP3 SFDR = 23(IIP3-MDS) Pclean = MDS+SFDR

G is small-signal gain in dB, NF is noise figure in dB, B is bandwidth in hertz, Nint is integrated input noise, SNRdet is the detection SNR margin, Ptone is the equal per-tone input power, and Pclean is the max clean input tone. Output MDS and output IM3 are found by adding gain to the input-referred values.

Input or status Rule Effect
OIP3 entry IIP3 equals OIP3 minus gain. The calculation returns to the input reference plane before SFDR is computed.
Measured noise density The entered dBm/Hz value is integrated over bandwidth. It represents the input noise basis directly.
Thermal noise plus noise figure Noise density equals -174 dBm/Hz plus nonnegative noise figure. It approximates room-temperature input noise for receiver planning.
Bandwidth Must be greater than zero after unit conversion. A 10x increase raises integrated noise by 10 dB and lowers SFDR by 6.67 dB.
Detection margin Must be zero or greater. Every added 3 dB of required SNR lowers SFDR by 2 dB.
Margin badge Clearance below 0 dB is failed, below 6 dB is tight, below 20 dB is usable, and 20 dB or more is clean. The badge reflects the entered tone level, not the device's absolute safe input rating.

A default receiver case shows the units. With IIP3 at +10 dBm, input noise density at -150 dBm/Hz, 100 kHz bandwidth, and 0 dB detection margin, integrated input noise is -100 dBm. MDS is also -100 dBm. The SFDR is two thirds of the 110 dB gap from IIP3 to MDS, or 73.33 dB. The max clean input tone is -26.67 dBm. If each input tone is -35 dBm, predicted IM3 is -125 dBm, leaving 25 dB of clearance below MDS.

Two-tone product frequencies follow the same spacing rule. 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 small-signal behavior, so unequal tones, compression, filtering, impedance mismatch, ADC clipping, and analyzer overload can make measured results depart from the simple model.

Accuracy and Safety Notes:

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

  • The IM3 estimate assumes equal tone powers, small-signal operation, and the 3:1 third-order slope.
  • The thermal-noise path assumes the standard room-temperature -174 dBm/Hz reference plus receiver noise figure. Antenna temperature, external interference, cryogenic operation, and measurement setup noise can shift the real floor.
  • The P1dB value is estimated from an offset below IP3. Use a measured compression point whenever one is available.
  • SFDR based on IM3 does not cover second-order products, harmonics, LO leakage, image responses, ADC clipping, blocker desensitization, or filter leakage.
  • Do not treat Max clean input tone as permission to apply RF power. Check attenuators, power ratings, thermal limits, and lab safety practice first.

Advanced Tips:

  • Keep Noise basis, Measurement bandwidth, and Detection SNR margin unchanged when comparing two devices; otherwise the SFDR change may come from the noise boundary rather than the intercept point.
  • Use Input IP3 (IIP3) for receiver blocker planning unless the whole requirement is written at the output reference plane. Output IP3 (OIP3) is convenient for amplifier or mixer datasheets, but the calculator still derives input-referred SFDR from IIP3 and MDS.
  • Check Two-Tone Products before trusting a clean margin. A spur outside the wanted channel may be less important than the same predicted power inside an IF filter, FFT bin, or demodulator passband.
  • Use Intercept Map to confirm that the entered per-tone level is not near the extrapolated IP3 crossing or the compression estimate. The map is a small-signal planning view, not a safe-power envelope.
  • Use Bandwidth Sweep when the requirement can be expressed in several channel widths. The same IIP3 and noise density can produce very different clean-tone limits as bandwidth grows.
  • Export the JSON or chart data when sharing a lab note so the reference plane, noise basis, bandwidth, detection margin, and tone level travel with the reported SFDR value.

Worked Examples:

A VHF receiver budget uses 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. With Per-tone input power at -35 dBm, Current IM3 clearance is 25.00 dB, so the status reads Clean IM3 margin.

An amplifier datasheet example starts from OIP3 at +30 dBm and gain at +15 dB, so the input-referred intercept is +15 dBm. With the noise basis set to Thermal noise plus noise figure, an 8 dB noise figure, 2 MHz bandwidth, and 10 dB detection margin, MDS for SFDR is about -92.99 dBm and Input SFDR is about 71.99 dB. That run is useful for reference-plane translation, but measured P1dB still matters before choosing a real input drive level.

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

A validation failure is easier to diagnose. If Measurement bandwidth is 0 Hz or Tone spacing is 0 kHz, no result table is shown because noise integration and product placement both need positive values. Restore positive values before using Two-Tone Products or the chart tabs.

FAQ:

Is IP3 a real power level I can drive to?

No. IP3 is an extrapolated intercept point. Use Estimated input P1dB, measured P1dB, absolute maximum input ratings, and attenuator settings for real drive-level decisions.

Should I enter IIP3 or OIP3?

Enter the reference plane used by your source data. Choose Input IP3 (IIP3) for input-referred receiver or cascade numbers, and choose Output IP3 (OIP3) when the datasheet gives output intercept with a known gain.

Why does wider bandwidth reduce SFDR?

Wider bandwidth integrates more noise. A 10x bandwidth increase raises integrated noise by 10 dB, so the IM3-based SFDR estimate drops by 6.67 dB when the other inputs stay fixed.

What does IM3 above MDS mean?

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

Why are there two IM3 frequencies?

Equal two-tone testing creates nearby products on both sides of the desired tones. Two-Tone Products lists the lower product at 2f1 - f2 and the upper product at 2f2 - f1.

Are RF values sent for remote processing?

The arithmetic runs in the browser from the values you enter. If you share a filled-in page URL, the selected values may be included in 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, usually near 2f1 - f2 or 2f2 - f1 in a two-tone test.
MDS
Minimum detectable signal boundary used for the SFDR estimate.
SFDR
Dynamic range before predicted IM3 reaches the selected MDS boundary.
Noise figure
Receiver noise degradation expressed as dB above the standard thermal-noise reference.
P1dB
One-dB compression point, where gain has dropped 1 dB from its small-signal value.
dBm
Power in decibels relative to one milliwatt.

References: