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RF Electronics Chapter 5: Frequency Mixers Page 159 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. same frequencies are used as for figures 5.50 and 5.57. Using a lower RF power level gives a better performance with a better LO and RF isolation at the IF output. The RF signal is at 1.7 GHz and the LO signal is at 1.585 GHz resulting in a difference signal at 135 MHz and a sum signal at 3.265 GHz. Figure 5.79 shows that these nearly the same power level and are the largest signals. The Frater-Gilbert cell acts thus as a near ideal mixer. Figure 5.81 shows a plot of the conversion gain for Frater-Gilbert Cell up-converter as both the LO and the IF level are varied. The IF level of -18.13dBm corresponds to a 1 dB reduction of the conversion gain of the mixer and thus corresponds to the 1 dB compression point. Figures 5.82 to 5.86 show the RF spectra of the Frater-Gilbert Cell mixer used as an up- converter, when the IF signal level is varied. The LO signal is -6 dBm and IF signal level is -35 dBm, -30 dBm, -25 dBm and -20 dBm and -15 dBm. For an IF signal of -35 dBm, apart from the LO feedthrough, the unwanted components are more than 58 dB below the wanted components, so that the Frater-Gilbert Cell is near ideal mixer. In precision applications, calibrated biasing can be used to minimise any LO feedthrough. Figure 5.81. Up-conversion gain of the Frater-Gilbert Cell mixer of figure 5.80. Changing the IF level by 10 dB, from -35 dBm to -25 dBm, causes a 10 dB change in level of the 1.45 GHz and 1.55 GHz components and a 30 dB change in the 3IM component at 1.35 GHz. The mixer is thus operating in a linear range and the expected third order output intercept point (OIIP3) from figures 5.5 and 5.77 is at (0.5*(-11.708 - (-83.162)) + (-11.708)) = 24.019 dBm. Figure 5.76 shows that the conversion gain at a LO of -6 dBm and an IF of -35 dBm is 23.28 dB, the IIP3 point occurs at an IF input level of 24.019 -23.28 = 0.789 dBm. Figure 5.76, shows that at a LO of -6 dBm, an IF signal of -13.5 dBm has 1 dB less conversion gain than an IF of -30 dBm. The 1 dB compression point is thus at an IF level of -13.5 dBm. The IIP3 point is thus 14.3 dB above the 1 dB compression point. That is similar to the corresponding 14 to 16 dB for a typical DBM. These computer simulations or similar measurements on the actual devices can thus easily determine the critical mixer parameters. Passive DBMs, can operate at higher input levels, but produce lower output levels than Frater-Gilbert Cell mixers, because Frater-Gilbert Cell mixers have a conversion gain. RF Electronics: Design and Simulation 159 www.cadence.com/go/awr

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