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RF Electronics Chapter 5: Frequency Mixers Page 132 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. positive signal at the LO port, to cause a positive signal at the RF port. Thus, for a -ve IF signal, the mixer does not inverts the LO signal to the RF port. If a zero voltage signal is applied at the IF port all the diodes are equal resistances and the LO signal is cancelled at the RF port. Having a smaller IF voltage, results in a smaller RF voltage. The voltage at the RF port is thus a multiplication of the LO signal and the IF signal. Similarly the signal at the IF port is a multiplication of the LO signal and the RF signal. Comparing figures 5.34 with figures 5.10, 5.24 and 5.30, shows that the DBM is a near ideal multiplier. At a high LO power some clipping of the RF signal occurs, but that does not affect the desired signals at frequencies of LO ± IF (100 MHz and 110 MHz in figure 5.38). Figures 5.35 and 5.38 show that the DBM is a near ideal device for producing sum and difference frequency signals. Unwanted components that cannot easily be filtered out are more than 50 dB below the desired signals. A practical DBM is operated at a LO power such that the diodes are turned on hard and the resistive losses are minimised. Figure 5.34. Down-conversion loss of a DBM compared with other mixers. Figure 5.34 shows that the conversion loss of a DBM and compares it with the conversion loss of the single diode and balanced mixers from figures 5.17 and 5.26. The conversion loss is approximately 0.9 dB less than that of a balanced mixer and more than 6.9 dB less than that of a single diode mixer. At an LO power of 7 dBm, the slope of the conversion gain is 0.06 dB/dB for the DBM. The DBM will this have an excellent LO AM noise immunity. Figure 5.35 shows the IF spectrum of a down-converting DBM. The relationship between these components and LO and RF frequencies are shown in table 5.1. Comparing this with figures 5.8(Left), 5.19 and 5.27 for the ideal multiplier and single and balanced diode mixers, shows that this mixer performs nearly as an ideal own converter. There are no unwanted signals in the 20 to 180 MHz frequency range. The difference signal at 5 MHz and the sum signal at 205 MHz are the same amplitude. The signal at 215 MHz is due to the third harmonic of the input signal mixing with the local oscillator. In a receiver, these harmonic signals must be evaluated, to ensure that they do not cause signals in the IF frequency band. RF Electronics: Design and Simulation 132 www.cadence.com/go/awr

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