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RF Electronics Chapter 5: Frequency Mixers Page 113 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The second order intermodulation produces the required mixing action in a mixer and is thus of utmost importance. Figure 5.4. IM distortion of an amplifier or frequency mixer. The third order IM (3IM) and fifth order IM performance is very important in linear amplifiers since when two tones are used as an input to the amplifier, the 3IM and 5IM distortion results in additional frequency components, which again cannot be filtered out, as can be seen in figure 5.4. The 5IM components are often too small to be observed in a spectrum like figure 5.4. For mobile phone base-stations, these IM signals are likely to create interference in adjacent mobile phone channels, as a result the IM performance of amplifiers and mixers are a critical part of their specification. Third Order Intercept Point In practice a 1 >> a 3 >> a 5 , so that the fundamental frequency components are proportional to a 1 , the 3IM components are proportional to a 3 and the 5IM components are proportional to a 5 . If the input signals are increased by 1 dB, then the 3IM components will increase by 3 dB since they are caused by 3 2 1 3 ) ( ) ( t X t X a in equation 5.2 and the 5IM components will increase by 5 dB since they are caused by 5 2 1 5 ) ( ) ( t X t X a in equation 5.2. A popular method of determining the "linearity" of a frequency mixer is the "third-order intercept" approach [1]. The Third-Order Intercept Point is a theoretical point on the RF input versus IF output curve where the desired output signal and third order products become equal in amplitude as RF input is raised. For a down-converter, the RF power is the input and as the RF signal increases by 1 dB, the 3IM distortion component increases 3 dB. The Third Order Intercept Point is determined by increasing the RF level and noting both the desired and the 3IM levels. The third order intercept point is the point where the extension of the plotted desired output and 3IM output level versus RF input meet, as shown in figure 5.5. The corresponding level on the horizontal, input, axis (RF) is the third order input intercept point (IIP3) and the corresponding level on the vertical, output, axis (IF) is the third order output intercept point (OIP3). Sometimes the input, output or both is implied and just IP3 is used. The difference is the conversion loss. It is not possible to drive the mixer to those RF levels. As a rule of thumb the third order intercept point is about 8 to 10 dB above the LO level for a typical diode based double balanced mixer and up to 15 dB above the LO RF Electronics: Design and Simulation 113 www.cadence.com/go/awr

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