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RF Electronics Chapter 5: Frequency Mixers Page 166 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 5.93 shows the up-conversion gains for this mixer. Figure 5.81 shows the conversion gain of the same Frater-Gilbert Cell mixer when used as an up-converter operating at a similar LO frequency. The LO power of -3 dBm and IF power of -27 dBm in figure 5.87 correspond to a LO power of -6 dBm and IF power of -30 dBm in figure 5.78. The wanted sideband conversion gain in figure 5.93 of 23.241 dB is very similar to the 23.293 dB in figure 5.78. Figures 5.92 and 5.93 show that the image rejection, the difference between the wanted and unwanted sideband, is 23.241 -116.86 dB = 140.1 dB, so that this mixer works perfectly as an image reject mixer. In practice a perfect match between the conversion gain and phase shifts produced by 2 separate mixers is unlikely to be achieved, in addition, the hybrids used will not produce perfect 90 degree phase shifts, so that a 25 dB image rejection is more common [17-18]. Figure 5.89 shows the output current at the RF port of the mixer. For a -10 dBm LO, the output waveform is nearly a pure sine wave at 1.55 GHz. As the LO level is increased, the increase in the amplitude variation of the RF output due to the LO can be seen. Passive IQ Mixers The Frater-Gilbert Cell in figure 5.86 can be replaced by a DBM and the LO level and IF levels changed to match. Figure 5.90 shows the RF spectrum when the LO is +10 dBm and the IF level is -8 dBm. The conversion loss is -4.9 dB and the image rejection is more than 70 dB. In practice, the image /sideband rejection will be more like 22 dB [19] due to differences in diodes and the hybrids not producing perfect 90-degree phase shifts. Figure 5.95. Spectrum for DBM upper sideband up-converter. For an image reject down-conversion mixer, the same block diagram of figure 5.91 is used. At port1, the IF input, the IF frequency and the IF Branchline coupler of the up- conversion circuit, is changed to the RF input, the RF frequency and the RF Branchline coupler, for down-conversion. Also, the RF output port and the RF in-phase combiner of the up-conversion circuit is changed to the IF frequency and an IF in-phase combiner to achieve down-conversion. If a DBM is used, then the mixers low frequency ports should be used for the IF, as an input for up-conversion and as an output for the down-conversion. RF Electronics: Design and Simulation 166 www.cadence.com/go/awr

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