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RF Electronics Chapter 5: Frequency Mixers Page 125 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Advantages of single diode mixers: 1. They can be used at very high (microwave) frequencies. 2. They have a low cost, using only one diode. Disadvantages of single diode mixers: 3. They have a high conversion loss. 4. A high level of unwanted components are generated. These must be filtered out using additional circuitry. 5. There is no RF to LO isolation, IF to LO and IF to RF isolation only due to diplexer. Balanced Mixer Adding a second diode to the circuit shown in figure 5.25 results in a balanced mixer. The first diode has V a +V b across it and the second diode has V a -V b where voltage V a is the LO and V b is the RF voltage. The currents through the diodes are thus: 5 5 4 4 3 3 2 2 1 0 1 ) ( ) ( ) ( ) ( ) ( b a b a b a b a b a D V V a V V a V V a V V a V V a a I Eqn. 5.9 5 5 4 4 3 3 2 2 1 0 2 ) ( ) ( ) ( ) ( ) ( b a b a b a b a b a D V V a V V a V V a V V a V V a a I Eqn. 5.10 The difference between the diode currents flows into the IF port and RF port, so that the current flowing through the IF and RF port is: 5 5 3 2 5 4 5 3 3 4 3 2 3 2 1 2 1 2 20 10 ) ( 8 2 6 4 2 b b a b a b a b a b b a b a b D D RF IF V a V V a V V a V V V V a V V V a V V a V a I I I Eqn. 5.11 For a single diode mixer the current through the IF and RF port is the same as is shown in equation 5.9. Comparing equation 5.11 with equation 5.9, shows that most of the unwanted components cancel. The only components that are in the IF band are: ) ( 8 4 3 3 4 2 b a b a b a IF V V V V a V V a I Eqn. 5.12 Which is close to ideal multiplication as the a 4 term is normally very small. Since the LO voltage, V a >> the RF voltage, V b , the V a V b and the V a 3 V b terms dominate. The V b and the V a 2 V b terms do not produce any frequency components in the region of interest. For an ideal frequency mixer one wants the a, c, d and e components to be as small as possible, the manufacturers of mixers ensure their diodes satisfy this as much as possible. The balanced mixer will thus have a much better performance than the single diode mixer. The circuit diagram of the balanced mixer is shown in figure 5.25. The only difference between that and the circuit for the single diode mixer of figure 5.13 is the use of the second diode, having the second diode results in a significant performance improvement. The conversion loss of the frequency mixer is shown in figure 5.26. This shows that the reflectionless configuration of the RF/IF diplexer improves the conversion loss. The conversion loss is far less than that of a single diode mixer. For LO levels of around 7 dBm, any variation in LO power does not cause any change in conversion loss, so that RF Electronics: Design and Simulation 125 www.cadence.com/go/awr

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