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RF Electronics Chapter 5: Frequency Mixers Page 157 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The long tail multiplier, developed by Frater [13] and now more commonly known as the Gilbert Cell [14] provides close to ideal multiplier performance. As a result, it is used as an active mixer in many IC's. There is a Gilbert-Cell mixer in the examples provided by MWO. That Frater-Gilbert cell example has been simplified to illustrate its principle of operation as shown in figure 5.76. Commercial Frater- Gilbert Cell mixers with very good IM performances and a frequency range from DC to 5 GHz are available. The circuit consists of 3 parts, the left part of figure 5.76 consisting of transistors TR2 to TR7 is the Frater-Gilbert Cell, long tail multiplier. Transistor TR1 is a constant current source for this multiplier. Transistors TR8 and 9 are voltage followers for the output of the long tail multiplier. Transistors TR10 and TR11 are constant current sources for those voltage followers. Resistors R1 to R4 form a biasing chain to provide the correct biasing for the transistors. All the capacitors are for AC bypassing. The output is a differential signal and for convenience, an ideal transformer is used to convert this to a single ended output. For best performance, the inputs are differential, however to minimise the circuit complexity, Input A- and Input B- are connected to an AC ground. Either input can be used for the LO input, however for the circuit in figure 5.76, using input B for the LO results in a lower level of unwanted sidebands at the expense of a slightly lower conversion gain and a higher level of LO feed-through to the output. For the best input-match, resistor values R1 and R2 are tuned, to result in values slightly above 50 Ω. Normally all these components in figure 5.76 would be included in an IC. Figure 5.77. Frater-Gilbert Cell down-conversion mixer Figure 5.78. Conversion gain of the Frater-Gilbert Cell mixer of figure 5.76. RF Electronics: Design and Simulation 157 www.cadence.com/go/awr