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RF Electronics Chapter 5: Frequency Mixers Page 112 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Dynamic Range Dynamic range is the range over which a frequency mixer provides useful operation. The 1 dB compression point determines the upper limit of the dynamic range. The noise figure of the mixer determines the lower limit of the dynamic range. Since the mixer noise figure is only about 0.5 dB higher than its conversion loss, the lowest conversion loss is desirable to obtain the largest dynamic range. High and Extra High level mixers have a higher 1 dB compression point and thus a bigger dynamic range. Higher-level mixers are significantly more expensive and require more LO power, so that a compromise between cost, power consumption and dynamic range exists. Two-tone Third Order Intermodulation Distortion In this section one considers the frequency mixer as a "linear" device, since for a down- converter, the IF mixer output amplitude is directly related to the RF input amplitude. The output Y(t) of a frequency mixer or amplifier will depend on the input X(t). The gain of the device, relating the output to the input is a 1 . In addition, a DC component and harmonics of the input may be created due to the distortion of the device. The output is thus: .... ) ( ) ( ) ( ) ( ) ( ) ( 5 5 4 4 3 3 2 2 1 0 t X a t X a t X a t X a t X a a t Y Eqn. 5.1 For the devices we are considering, the terms above the 5 th harmonic are so small they can be ignored. When two signals, X 1 and X 2 are used as input. The output will then be: .... ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( 5 2 1 5 4 2 1 4 3 2 1 3 2 2 1 2 2 1 1 0 t X t X a t X t X a t X t X a t X t X a t X t X a a t Y Eqn. 5.2 When X 1 (t) is a sine wave of frequency F 1 and X 2 (t) is a sine wave of frequency F 2 , (F 2 > F 1 ) the frequency components at the fundamental and different intermodulation frequencies can be collected as follows: The fundamental frequency component, i.e. at F 1 due to X 1 (t) and at F 2 due to X 2 (t) is: 5 3 1 5 3 1 4 25 4 9 ) 8 3 5 8 3 10 8 5 ( ) 2 1 3 4 3 ( a a a a a a Y F Eqn. 5.3 The Third Order Intermodulation (3IM) frequencies are 3IM (upper) due to F 1 2F 2 and 3IM(lower) due to 2F 1 F 2 , and are given by: 5 3 5 3 3 8 25 4 3 ) 8 3 10 2 1 5 ( 2 1 4 3 a a a a Y IM Eqn. 5.4 The Fifth Order Intermodulation (5 IM) frequencies are 5IM (upper) due to 2F 1 3F 2 and 5IM(lower) due to 3F 1 2F 2 , and are given by: 5 5 5 8 5 8 1 10 2 1 a a Y IM Eqn. 5.5 The frequencies of these spectral-components are shown in figure 5.4. The second order intermodulation (2IM) and the fourth order intermodulation (4IM) distortion produced by the amplifier or mixer do not create any components near the desired frequency components. As a result, the 2IM and 4IM performance is less important for an amplifier. RF Electronics: Design and Simulation 112 www.cadence.com/go/awr

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