2.3.3 Second-Order Intercept Point

Chapter 2.3.3 Second-Order Intercept Point

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

  • 24 Issues in RFIC Design: Noise, Linearity, and SignalsOf course, the third-order intercept point cannot actually be measured directly, since by the time the amplifier reached this point, it would be heavily overloaded. Therefore, it is useful to describe a quick way to extrapolate it at a given power level. Assume that a device with power gain G has been measured to have an output power of P1 at the fundamental frequency and a power of P3 at the IM3 frequency for a given input power of Pi, as illustrated in Figure 2.10. Now, on a log plot (for example when power is in dBm) of P3 and P1 versus Pi, the IM3 terms have a slope of 3 and the fundamental terms have a slope of 1. Therefore: 1OIP31IIP3iPP-=- (2.56) 3OIP33IIP3iPP-=- (2.57)since subtraction on a log scale amounts to division of power. Also noting that: 1OIP3 IIP3iGPP=-=- (2.58)These equations can be solved to give: 1131311IIP3[][]22iPPPG PPP=+-- =+- (2.59)2.3.3  Second-Order Intercept PointA second-order intercept point (IP2) can be defined similarly to the third-order in-tercept point. Which one is used depends largely on which is more important in the system of interest; for example, second-order distortion is particularly important in direct down-conversion receivers.If two tones are present at the input, then the second-order output is given by 2IM22 ivk v= (2.60)Note that the IM2 terms rise at 40 dB/dec rather than at 60 dB/dec, as in the case of the IM3 terms as shown in Figure 2.10.The theoretical voltage at which the IM2 term will be equal to the fundamental term given in (2.52) can be defined: 22 IP21 IP21k vk v= (2.61)This can be solved for vIP2: 1IP22kvk= (2.62)