2.3.4 The 1-dB Compression Point

Chapter 2.3.4 The 1-dB Compression Point

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

  • 2.3 Linearity and Distortion in RF Circuits 25Assume 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 P2 at the IM2 frequency for a given input power of Pi, as illustrated in Figure 2.10. Now, on a log plot (for example when the power is in dBm) of P2 and P1 versus Pi, the IM2 terms have a slope of 2 and the fundamental terms have a slope of 1. Therefore: 1OIP21IIP2iPP-=- (2.63) 2OIP22IIP2iPP-=- (2.64)since subtraction on a log scale amounts to division of power. Note that 1OIP2 IIP2iGPP=-=- (2.65)These equations can be solved to give: 11212IIP2[][]iPPPG PPP=+-- =+- (2.66)2.3.4  The 1-dB Compression PointIn addition to measuring the IP3 or IP2 of a circuit, the 1-dB compression point is another common way to measure linearity. This point is more directly measurable than IP3 and requires only one tone rather than two (although any number of tones can be used). The 1-dB compression point is simply the power level, specified at either the input or the output, where the output power is 1 dB less than it would have been in an ideally linear device. It is also marked in Figure 2.10.We first note that at 1-dB compression, the ratio of the actual output voltage vo to the ideal output voltage voi is æö = -ç÷èø1020log1 dBooivv (2.67)or = 0.89125ooivv (2.68)Now referring again to Table 2.1, we note that the actual output voltage for a single tone is 31334oiivk vk v=+ (2.69)for an input voltage vi. The ideal output voltage is given by oi1 ivk v= (2.70)