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RF Electronics Chapter 5: Frequency Mixers Page 139 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 5.45 shows the down conversion spectra for DBM1, DBM3, DBM4 and DBM5. The conversion loss and IF spectra produced by DBM 2 and DBM3 are near identical. That is expected, since the LO and RF ports are terminated in 50 Ω, which is a reflectionless termination and for a down converter we do not consider the spectra at the RF and LO ports. As a result, DBM2 is not plotted in figure 5.45. The relationship between the spectral components and LO and RF frequencies are shown in table 5.1. DBM1 and DBM3, have similar performance below 100 MHz. At 200 MHz, the low pass section of the IF filter reduces those signal amplitudes by 40 dB. The 3 rd harmonic component at 15 MHz for DBM 1 and DBM3 is much lower than those of DBM4 and DBM5. At the sum frequency of 205 MHz, DBM3 gives the best performance. However if low cost and low conversion loss are critical, then DBM 5, which provides a low impedance at the RF and LO frequencies is the best option. Figure 5.46. Down-conversion loss DBM2 with different reflectionless filter terminations. Since the DBM2 circuit has each port connected to a reflectionless filter, it is worthwhile to see how terminating the out of band ports changes the performance, by leaving R1 to R3 as 50 Ω, or changing them to an open or short circuit. Figure 5.46 shows that the conversion loss is not affected by changing the resistors R2 and R3 at the RF and LO ports. The conversion loss is reduced by replacing the IF termination, R1, by a short circuit and it is increased by replacing R1 by an open circuit. The effect is similar to what is shown in figure 5.44 for DBM3 and DBM4. For the corresponding spectra, the lowest level of unwanted signals are obtained when the IF filter is terminated into 50 Ω, just like figure 5.45. For Up-conversion, the conversion loss is similar to figure 5.46, with the RF and LO terminations not affecting the conversion loss, but an open circuit R1 also resulting in a higher conversion loss and a short circuited R1 resulting in a lower conversion loss than R1= 50 Ω. The corresponding spectrum is shown in figure 5.47. Similar to figure 5.45, the lowest level of unwanted signals are obtained by having a 50 Ω termination at the IF port. There is little change in the IF spectrum when the out of band termination of the RF and LO ports are changed by changing resistors R2 and R3. In summary, the lowest level of unwanted spectral components are obtained when the IF port of a mixer is terminated into 50 Ω. If a low conversion loss is more critical than low unwanted signals, then providing a low impedance for RF/LO frequencies at the IF port can reduce the conversion loss by 1 dB. RF Electronics: Design and Simulation 139 www.cadence.com/go/awr

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