2.3.2 Third-Order Intercept Point

Chapter 2.3.2 Third-Order Intercept Point

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 22 Issues in RFIC Design: Noise, Linearity, and SignalsExample 2.4: Determination of Frequency Components Generated in a Nonlinear SystemConsider a nonlinear circuit with 7-MHz and 8-MHz tones applied at the input. Determine all output frequency components, assuming distortion components up to the third order.Solution: Table 2.2 and Figure 2.9 show the outputs.It is apparent that harmonics can be filtered out easily, while the third-order intermodulation terms, being close to the desired tones, may be difficult to filter.2.3.2  Third-Order Intercept PointOne of the most common ways to test the linearity of a circuit is to apply two signals at the input, having equal amplitude and offset by some frequency, and plot fundamental output and intermodulation output power as a function of in-put power as shown in Figure 2.10. From the plot, the third-order intercept point (IP3) is determined. The third-order intercept point is a theoretical point where the amplitudes of the intermodulation tones at 2w1 - w2 and 2w2 - w1 are equal to the amplitudes of the fundamental tones at w1 and w2.From Table 2.1, if v1 = v2 = vi, then the fundamental is given by 3139fund4iik vk v=+ (2.51)The linear component of (2.51) given by: 1fundik v= (2.52)Table 2.2  Outputs from Nonlinear Circuits with Inputs at f1 = , f2 = MHzSymbolic FrequencyExample FrequencyNameCommentFirst orderf1, f27, 8FundamentalDesired outputSecond order2f1, 2f214, 16HD2 (Harmonics)Can filterf2 – f1, f2 + f11, 15IM2 (Mixing)Can filterThird order3f1, 3f221, 24HD3 (Harmonic)Can filter harmonics2f1 – f2,2f2 – f169IM3 (Intermod)IM3 (Intermod)Close to fundamental, difficult to filterFigure 2.9  The output spectrum with inputs at and MHz.