Calculate the third-order intercept point (IP3), the figure of merit for the linearity of amplifiers and mixers. Adjust the input-referred IP3, gain and two-tone input level to see the output IP3, third-order intermodulation (IM3) product level, IM3 suppression and spurious-free dynamic range update in real time.
Parameters
Input-referred IP3 (IIP3)
dBm
Third-order intercept point referenced to the input port
Gain G
dB
Power gain of the amplifier or stage
Two-tone input level (per tone)
dBm
Input power of each tone in the two-tone test
Noise floor
dBm
Input-referred noise power level
Results
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Output IP3 (OIP3) (dBm)
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Fundamental output level (dBm)
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IM3 product level (dBm)
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IM3 suppression (dBc)
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SFDR (dB)
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Linearity verdict
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Output spectrum — two tones and IM3 products
The two tall bars are the fundamental tones (f1, f2); the shorter bars outside them are the third-order intermodulation (IM3) products (2f1-f2, 2f2-f1). The baseline is the noise floor, and the height difference is the IM3 suppression.
The output IP3 (OIP3) is the input-referred IP3 plus the gain G. The IM3 product level P_IM3 follows from the fundamental output P_out and OIP3. All quantities are in dBm.
Spurious-free dynamic range. The fundamental rises 1 dB/dB while the IM3 product rises 3 dB/dB of input level, which gives the factor 2/3.
What is the Third-Order Intercept Point (IP3)?
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"IP3" is a number I keep seeing on RF component datasheets. What does it actually represent?
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In short, it is a linearity figure of merit: it tells you how large a signal an amplifier or mixer can handle without distorting it. An ideal amplifier scales perfectly — input it 10× and the output is 10×. But real circuits are slightly non-linear, and when you feed in a strong signal they create extra frequency components — distortion. IP3 bundles that "tendency to distort" into a single number.
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There seem to be many kinds of distortion — why is "third-order" treated as special?
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Good question. Picture a "two-tone test" where you apply two strong signals, f1 and f2, at once. A non-linear circuit generates frequencies at 2f1-f2 and 2f2-f1. That is third-order intermodulation, IM3. With f1=100.0 MHz and f2=100.1 MHz, the IM3 products fall at 99.9 MHz and 100.2 MHz — right next to the wanted signals. Second-order distortion flies out of band and can be filtered away, but IM3 lands inside the passband, so a filter cannot remove it. That is why RF designers care about IM3, and therefore IP3, the most.
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I see. When I raise the "two-tone input level" slider on the left, the IM3 product level climbs really fast — faster than the fundamental, it looks like.
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That is the heart of IP3. Raise the input 1 dB and the fundamental output rises 1 dB — a slope of 1. But the IM3 product rises 3 dB for that same 1 dB — a slope of 3. So as you keep raising the input, the two lines with different slopes eventually cross. That imaginary crossing is the "intercept point", IP3. In practice the amplifier saturates well before reaching it, so IP3 is an extrapolated, fictitious point — but it is a handy way to compare linearity with a single number.
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On the "IM3 suppression vs input level" chart below, the suppression gets 2 dB worse for every 1 dB I add to the input. Does that also come from the slope difference?
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Exactly. If the fundamental rises 1 dB and the IM3 rises 3 dB, the gap between them — the IM3 suppression — shrinks 2 dB per 1 dB of input. Conversely, drop the input 3 dB and the suppression improves 6 dB. That is the design technique called "back-off". When a receiver struggles with interference, you first reduce the input to suppress distortion. But back off too far and the signal sinks into the noise. So you set the usable window by watching both the noise floor and the distortion — and that window is the SFDR, the spurious-free dynamic range.
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SFDR is computed as SFDR = (2/3)(IIP3 - noise floor). Where does the 2/3 come from?
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It is that slope-1 versus slope-3 story again. SFDR is the span from a signal just at the noise floor up to the signal where the IM3 product pokes up to the noise floor. As you raise the input, the fundamental's margin above the noise grows — but the IM3 chases after it three times as fast. Subtract the two effects and the usable range works out to 2/3 of the gap between IIP3 and the noise floor — that is where the factor comes from. In practice, when you sum up a receiver in one breath, you look at this SFDR together with the noise figure (NF).
Frequently Asked Questions
IP3 is the standard figure of merit for the linearity of non-linear circuits such as amplifiers and mixers. When two strong signals (a two-tone test) are applied, the non-linearity creates third-order intermodulation (IM3) products at 2f1-f2 and 2f2-f1. The fundamental rises 1 dB per dB of input while the IM3 product rises 3 dB per dB, so as you raise the input the two lines eventually cross at a single point. The output (or input) level of that imaginary intercept is IP3. A higher IP3 means the circuit stays linear up to higher signal levels.
IM3 products appear at the frequencies 2f1-f2 and 2f2-f1. For example, with f1=100.0 MHz and f2=100.1 MHz the IM3 products land at 99.9 MHz and 100.2 MHz - right next to the wanted signals. They fall inside the receiver passband, so a filter cannot separate them. Second-order distortion tends to fall out of band and can be filtered away, but IM3 cannot - which is exactly why IP3 matters. The only remedies are a more linear circuit (higher IP3) or backing off the input level.
IIP3 (input-referred IP3) is referenced to the input port and OIP3 (output-referred IP3) to the output port; the relation is OIP3 = IIP3 + gain. IIP3 is usually used for receiver front-end sensitivity and distortion design, while OIP3 is used for transmit chains and inter-stage linearity. To combine the IP3 of a multi-stage system, refer each stage's IIP3 back to the input before accumulating: later stages contribute more strongly because the gain of preceding stages is applied to them.
SFDR is the usable input window between the noise floor and the level at which the IM3 product reaches the noise floor. Input-referred, it is SFDR = (2/3)*(IIP3 - noise floor) [dB]; the factor 2/3 comes from the IM3 product rising 3 dB for every 3 dB of input (a 2 dB relative degradation). A wider SFDR means a larger range of signal levels can be handled free of distortion, so SFDR is a key single-number measure of overall receiver performance.
Real-World Applications
Receiver front-end design: In receivers for mobile phones, Wi-Fi and GPS, the IP3 of the low-noise amplifier (LNA) and mixer governs sensitivity and interference immunity. The antenna brings in not only the wanted signal but also strong interferers, so a low IIP3 lets IM3 land in the wanted channel and degrade sensitivity. Designers trade off noise figure (NF) against IIP3 and set the LNA gain and bias so the SFDR meets the requirement.
Multi-stage RF chain budgeting: The RF chain from antenna to ADC is a cascade of stages — LNA, filters, mixer, IF amplifier and more. Each stage's IIP3 is referred to the input and combined to give the system IIP3. Later stages generally affect linearity more because the gain of preceding stages is applied to them, so the gain plan is optimized while watching both noise and IP3. This tool helps build intuition for the IP3 behaviour of a single stage.
Test instruments and spectrum analyzers: A spectrum analyzer itself has an input mixer with a finite IP3. If the signal under test is too strong, IM3 is generated inside the analyzer and becomes indistinguishable from the distortion of the device under test. Switching the input attenuator and checking whether the internally generated IM3 changes is a basic measurement technique. Understanding the two-tone test setup and SFDR pays off directly in the lab.
Transmitter linearity management: In power amplifiers (PAs), IM3 shows up as adjacent-channel leakage (ACLR/ACPR) and can cause a specification violation. OIP3 is a first-order indicator of PA linearity and gives a guide to how many dB of back-off is needed to meet the required ACLR. Modern designs combine it with digital pre-distortion (DPD) to achieve both efficiency and linearity.
Common Misconceptions and Pitfalls
The biggest misconception is thinking IP3 is an output level you can actually reach. IP3 is the imaginary crossing of the extrapolated fundamental (slope 1) and IM3 (slope 3) lines, and a real amplifier saturates with gain compression (the 1 dB compression point, P1dB) long before that. OIP3 is typically 10 to 15 dB above P1dB. IP3 is purely a "yardstick" for comparing small-signal linearity — it does not mean the device can amplify linearly up to that level.
Next, assuming "a high IP3 alone makes a good receiver". IP3 is only a linearity figure; sensitivity is set by the noise figure (NF). Increasing bias current to raise IIP3 may worsen the NF or raise power consumption — a trade-off is unavoidable. The true capability of a receiver should be measured by SFDR, the span between the noise floor set by NF and the distortion ceiling set by IP3 — do not be swayed by the IP3 number alone. Move the noise floor in this tool and you will see the SFDR change for the same IIP3.
Finally, believing "the two-tone spacing does not affect the result". This tool uses an ideal memoryless non-linear model, so the IM3 level does not depend on tone spacing. Real amplifiers, however, have "memory effects" from bias-circuit and thermal time constants, and changing the tone spacing alters the IM3 amplitude and phase. With wideband signals it is not unusual for the upper and lower IM3 products to become asymmetric. This tool is useful for early-stage estimates and conceptual understanding, but confirm the final result with a real-signal two-tone measurement or a system simulation.
How to Use
Enter Input IP3 (IIP3) in dBm—typical values range from +5 to +35 dBm depending on amplifier topology (BJT, FET, or GaAs). Use the slider or numerical input.
Set the amplifier Gain in dB (typically 15–40 dB for RF stages). Higher gain increases OIP3 but may reduce linearity margin.
Specify Input Power (Pin) in dBm relative to 1 mW reference. For two-tone testing, enter the power per tone; typical range is −30 to +10 dBm.
Input Noise Figure (NF) in dB if available from the device datasheet (0.5–6 dB for LNAs, 5–15 dB for mixers).
Click Calculate or drag sliders to update OIP3, IM3 product level, and Spurious-Free Dynamic Range (SFDR) in real time.
Worked Example
A 20 dB gain X-band amplifier with IIP3 = +18 dBm, Pin = −5 dBm per tone, and NF = 3.2 dB: OIP3 = 18 + 20 = +38 dBm; fundamental output = −5 + 20 = +15 dBm; IM3 product = 3(−5) − 2(18) + 20 = −49 dBm; IM3 suppression = 15 − (−49) = 64 dBc; SFDR (referenced to 1 dB compression) ≈ 92 dB·Hz^(2/3). This configuration meets typical satcom and radar receiver specifications.
Practical Notes
OIP3 = IIP3 + Gain. For cascaded stages, use Friis cascade formula: total OIP3 ≈ OIP3_stage1 + (OIP3_stage2 − IIP3_stage1)/Gain_stage1. Dominant stage (lowest IIP3) sets system linearity.
IM3 products fall at f1 + (f1 − f2) and f2 + (f2 − f1); suppress them via filtering or tuned circuit design. Two-tone spacing under 100 MHz risks broadband IMD in wideband systems.
SFDR = (2/3) × [OIP3 − Noise Floor] for tone-limited case. Trade gain against stability; excessive gain destabilizes feedback networks and degrades IP3 by 0.5–2 dB per 10 dB gain above 25 dB.
Temperature and bias current shifts alter IP3 by ±1–3 dB; verify across −40 to +85°C for military/aerospace applications.