Two-Tone Vs. Single-Tone Measurement Of Second-Order Non-Linearity And IP2 Performance Of Direct Conversion Receivers

This article describes how to find the second-order intercept point (IP2) from one-tone and two-tone tests of Direct Conversion Receivers (DCRs). It also presents measurement results for the GSM900 receive path, as used in the AMPS band, for Skyworks first-generation of DCRs.

Relation Between 2-Tone and 1-Tone Tests for Second-Order Non-linearity

Second order non-linearity is an important phenomenon in DCR-type receivers. Here we set out to show the test result of this kind of non-linearity, which can be used to predict the DC offset, both with single tone or two tones. A two-tone condition is illustrated in Figure 1 and Figure 2.

The relationship between f₁, f₂ and f₃, f₄ is:

f₃ = f₂ - f₁

Likewise for f₄:

f₄ = f₂ + f₁

This shows that the unwanted output frequency components are mathematically related to the input tones. To have a better insight into the relationship between undesired components and the input terms, a more rigorous derivation is needed.

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•Application Note: Two-Tone Vs. Single-Tone Measurement Of Second-Order Non-Linearity And IP2 Performance Of Direct Conversion Receivers