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RF Electronics Chapter 5: Frequency Mixers Page 164 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. A mixer operating as a down-converter will shift RF signals that are the IF frequency above and below the LO frequency to the IF frequency. The unwanted signal is called the "image signal" in figure 5.2. Normally a filter is used at the RF front-end to filter out any signals inside this image frequency band. Any thermal noise inside this image frequency band will contribute to the noise at the IF frequency band. An image reject mixer will only shift the required signals to the IF frequency and will not shift the signals or noise of the image band to the IF frequency. Image reject mixers can thus provide an improved noise performance. By simply using a non-zero IF frequency, IQ mixers can be used as image reject mixers, for both up-conversion, by applying an input signal at the IF port 1, as shown in figure 5.91, or for down-conversion, by applying an input signal to the RF port 3. For Up-conversion, a Single Sideband signal is generated. For details of the mathematics involved, see articles on using the Hilbert Transform for generating Single Sideband [16]. In this design, Branchline couplers are used to produce the required 90-degree phase shifted signals. Figure 4.39 shows that Branchline couplers maintain a 90 ±1.22º phase shift over a ±10% bandwidth, which is sufficient for many communication systems. To simplify the design, ideal transmission lines of 90º electrical length are used, without regard of the physical length of those lines. In figure 5.91, at port 1, a Branchline coupler (Branchline IF) is used at the IF frequency of FIF=50 MHz, clearly those lines would be too long to realise and normally LC networks or Digital Signal Processing (DSP) techniques are used to produce the required phase shifts or the required IQ signals at those frequencies. The block diagram is the same as that for the IQ mixer in figure 5.88, with the IF-Q signal being constrained to be the Hilbert transform (90-degree phase shift) of the IF-I signal. In figure 5.88, the in-phase combiner uses a transformer Wilkinson hybrid, however this can also be realised using a Wilkinson transmission line hybrid. Figure 5.92. Image reject mixer spectrum for Frater-Gilbert Cell upper sideband up-converter. For active IQ mixers, the Frater-Gilbert cell mixers in figure 5.91 are the same as the one shown in figure 5.76. For figure 5.83, a LO level of -6 dBm and an IF signal of -30 dBm were used. Since the LO signal is split to drive the 2 mixers, a LO of -3 dBm and an IF RF Electronics: Design and Simulation 164 www.cadence.com/go/awr

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