What is the correct setup for measuring two-tone intermodulation distortion and IP3?
IP3 Measurement Techniques
IP3 is the most important linearity specification for components in multi-signal environments. Accurate IP3 measurement requires careful attention to test setup to avoid measurement artifacts that can make the DUT appear better or worse than it actually is.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
Frequently Asked Questions
What is the difference between IIP3 and OIP3?
IIP3 (input IP3): the intercept point referred to the input of the device. IIP3 = P_in + delta/2, where delta is the difference between the fundamental and IM3 at the output. OIP3 (output IP3): the intercept point referred to the output. OIP3 = P_out + delta/2. Relationship: OIP3 = IIP3 + gain (dB). For a 20 dB gain amplifier with IIP3 = +10 dBm: OIP3 = +10 + 20 = +30 dBm. Convention: for receivers and LNAs, IIP3 is preferred (it indicates how strong an input signal the device can handle). For transmitters and power amplifiers, OIP3 is preferred (it indicates the linearity of the output signal).
How do I calculate cascade IP3?
For cascaded stages: the overall IP3 is limited by the individual stage IP3 values: 1/IIP3_total = 1/IIP3_1 + G1/IIP3_2 + G1×G2/IIP3_3 + ... (all values in linear power, not dB). In dB: use: 10^(-IIP3_total/10) = 10^(-IIP3_1/10) + 10^((G1-IIP3_2)/10) + 10^((G1+G2-IIP3_3)/10). The stage with the worst IIP3 relative to the signal level reaching it dominates. Usually: the mixer or the IF amplifier (which sees the highest signal levels due to pre-amplification) limits the cascade IP3. Design rule: the mixer IIP3 should be at least 15 dB above the maximum signal level at the mixer input.
What about IP2 (second-order intercept)?
IP2 characterizes the second-order nonlinearity that produces output at f1±f2 (sum and difference frequencies). IP2 is particularly important for: (1) Direct-conversion (zero-IF) receivers: the difference product at f1-f2 falls at baseband (DC to a few MHz), directly interfering with the desired signal. (2) Wideband receivers: second-order products from two strong signals can fall on a weak desired signal. IP2 measurement: similar to IP3 but measure the product at f1+f2 or f2-f1 instead of 2f1-f2. IIP2 = 2×P_in - P_IM2 + gain (note: the slope for IM2 is 2:1, not 3:1). Typical values: IIP2 = +40 to +65 dBm for receiver front-ends (much higher than IIP3 because even-order distortion is largely canceled in balanced/differential circuits).