Why are third-order products the most dangerous?

Second-order products (f1+f2 and f1-f2) fall far from the original signals and are easily filtered. Third-order products (2f1-f2 and 2f2-f1) fall close to the original signals, typically within the passband of the system. If f1 = 1000 MHz and f2 = 1001 MHz, the IMD3 products appear at 999 MHz and 1002 MHz, right next to the desired signals and inside any practical filter bandwidth. They masquerade as real signals and degrade the receiver's ability to detect weak signals in the presence of strong ones.

What is IP3 and how is it measured?

The third-order intercept point (IP3) is a theoretical power level where the extrapolated fundamental output power equals the extrapolated third-order product power. It never actually occurs because the device compresses first, but it serves as a figure of merit for linearity. To measure it: apply two equal-power tones (f1 and f2) at a power level well below compression (typically 10 to 15 dB below P1dB). Measure the output power of the fundamental tones and the IMD3 products. OIP3 = P_out + delta/2, where delta is the difference in dB between the fundamental and IMD3 output levels.

What is the relationship between P1dB and IP3?

For most devices, OIP3 is approximately 10 to 12 dB above P1dB. This rule of thumb holds for amplifiers, mixers, and ADC front-ends with weakly nonlinear behavior. GaN PAs tend toward the higher end (12 dB), while GaAs devices are often closer to 10 dB. Devices with strong odd-order nonlinearity (like class-AB amplifiers near cutoff) can deviate from this, with OIP3 only 7 to 8 dB above P1dB.

Signal Processing

Intermodulation Distortion

IMD

Feed two signals at 1000 and 1001 MHz into any amplifier, mixer, or even a corroded connector, and new signals appear at 999 and 1002 MHz. These phantom signals are not noise; they are mathematically precise products of the nonlinearity, falling exactly where you can not filter them out because they sit right next to the signals you want. Their power grows three times faster than the fundamentals (3 dB per 1 dB of input), which is why even a small amount of nonlinearity at high signal levels creates large intermodulation products that blind the receiver to weak signals.

Category: Signal Processing

Worst Products: 2f₁−f₂, 2f₂−f₁

Figure of Merit: IP3 (dBm)

The Products You Cannot Filter Away

Third-order intercept point:
OIP3 = P_out + Δ/2
IIP3 = P_in + Δ/2
where Δ = P_fundamental − P_IMD3 (in dB at the output)

IMD3 product level vs. input power:
P_IMD3 = 3·P_in − 2·IIP3 (dBm, each tone)

Worked example: LNA with OIP3 = +35 dBm, Gain = 20 dB:
IIP3 = 35 − 20 = +15 dBm
At P_in = −20 dBm per tone:
P_IMD3 = 3×(−20) − 2×15 = −60 − 30 = −90 dBm
Δ = (−20 + 20) − (−90) = 90 dB between fundamental and IMD3 at output

IP3 by Component Type

Component	OIP3 (typical)	P1dB	OIP3 − P1dB	Notes
GaAs LNA	+30 to +38 dBm	+18 to +25 dBm	10 to 12 dB	Standard front-end
GaN LNA	+40 to +48 dBm	+28 to +35 dBm	10 to 13 dB	High-dynamic-range
Passive mixer (DBM)	+15 to +25 dBm	+1 to +12 dBm	12 to 15 dB	IIP3 ≈ LO power + 10
Active mixer (Gilbert)	+10 to +20 dBm	−5 to +5 dBm	12 to 15 dB	Has conversion gain
ADC front-end	+25 to +40 dBm	Full-scale −1 dB	Varies	Defines SFDR

Common Questions

Frequently Asked Questions

Why are third-order products the worst?

IMD2 products (f₁+f₂, f₁−f₂) fall far away and are easily filtered. IMD3 (2f₁−f₂, 2f₂−f₁) fall right next to the desired signals, inside any practical filter bandwidth. They masquerade as real signals.

What is IP3?

The theoretical power where fundamental and IMD3 extrapolations intersect. Never actually reached (device compresses first). Measured with two tones 10 to 15 dB below P1dB: OIP3 = P_out + Δ/2, where Δ is the dB gap between fundamental and IMD3.

P1dB to IP3 relationship?

Typically OIP3 = P1dB + 10 to 12 dB. GaN devices: 12 to 13 dB. GaAs: 10 to 11 dB. Class AB near cutoff: may be only 7 to 8 dB. Passive mixers: 12 to 15 dB (IIP3 ≈ LO power + 10 dB).

Related Terms

← Insertion Loss IP3 →

Linearity Analysis

Two-Tone IMD Calculator

Enter IP3, input power per tone, and gain to compute IMD3 product levels, spurious-free dynamic range, and the maximum input level for a target IMD suppression.

Calculate IMD