IP3, intermodulation and dynamic range calculator

A number that does not exist

The third-order intercept is an extrapolation of two lines that never actually meet: the amplifier goes into compression first. But as a measure of linearity it is convenient, and the dynamic range is worked out from it — and that is what really decides whether a receiver survives next to a powerful station.

Third-order products fall three times as fast as the fundamental tones

What to enter	Result of a two-tone measurementKnown IIP3
Level of the wanted tone at the output (dBm)
Level of the third-order product (dBm)
Stage gain (dB)
Level of each input tone (dBm)
Receiver noise floor MDS (dBm)
Intercept points
IIP3 — referred to the input
OIP3 — referred to the output
Estimated P1dB compression point
What you get at the chosen level
Level of the third-order products
Offset from the carrier
Dynamic range DR3

📐 The formulas

From a two-tone measurement:

Δ = P_wanted − P_IM3          (at the output, dB)
OIP3 = P_out + Δ/2
IIP3 = OIP3 − G

Product level for a given input:

P_IM3 = 3·P_in − 2·IIP3

Dynamic range:

DR3 = (2/3)·(IIP3 − MDS)
P1dB ≈ IIP3 − 9.6 dB

Cascade (in milliwatts):

1/IIP3 = 1/IIP3₁ + G₁/IIP3₂ + G₁·G₂/IIP3₃ + …

Check points:

• Two tones at −20 dBm, IIP3 = +10 dBm → products at −80 dBm, Δ = 60 dB
• IIP3 = +10 dBm, MDS = −140 dBm → DR3 = 100 dB
• A cascade of IIP3 +20 dBm / G 15 dB → IIP3 +30 dBm gives +13.8 dBm overall

Where this calculator stops:

• IIP3 is an extrapolation: you cannot actually apply that level to the input, the amplifier compresses first
• The third-order model holds only while the products fall three times as fast as the signal; near compression it no longer does
• P1dB ≈ IIP3 − 9.6 dB is a statistical rule, not a law: for a particular device the gap runs from 8 to 12 dB
• Second-order products are not accounted for, and on a wideband input they can matter more

🎯 In practice

• Working up the result of a two-tone measurement on your own receiver.
• Judging whether a receiver will survive next to a broadcast station.
• Deciding where to put an attenuator — and understanding that it does not make the receiver better.

Practical advice:

1. Look at the attenuator column: DR3 does not change, only the operating point moves.
2. Third-order products rise three times as fast as the interference — which is why one strong station nearby kills the whole band at once.
3. A DR3 above 95 dB is a good receiver, above 100 dB a very good one.
4. The mixer is usually the weaker link, not the amplifier: it is what sets the IIP3 of the whole chain.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «IP3 Calculator: Intermodulation and Dynamic Range». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/ip3/ (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.