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RF Electronics Chapter 5: Frequency Mixers Page 161 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 5.87. RF Waveform for the mixer of figure 5.80. Quadrature (IQ) Mixers Most modern modulation methods, such as QAM, CDMA, OFDM and LTE, use vector modulation and require IQ modulators. The vector modulator typically uses DSP techniques to calculate a complex baseband signal with Real I and Imaginary Q components corresponding to the desired signal. Those digital signals are then converted to analogue signals using two Digital to Analogue converters, one for the baseband I (BB-I) signal and one for the baseband Q (BB-Q) signal. Since for quadrature mixers, a zero frequency IF is normally used, the term Baseband (BB) is a more accurate description and as a result is used here. In quadrature mixers a quadrature or 90 degree hybrid, like a Branchline coupler is used to produce two LO signals, corresponding to Cosine (LO-I) and Sine (LO-Q) of the LO frequency. In an up-converter, BB-I signal is multiplied with the LO-I signal using a DBM and the BB-Q signal is multiplied with the LO-Q signal using a DBM, as shown in figure 5.88. The resulting signals are then added, using a combiner like a Wilkinson hybrid to produce the RF signal. The result is the baseband spectrum shifted to the LO frequency. This can be demonstrated mathematically by considering the baseband signal to be a vector of magnitude A b and a phase angle b , then: �� �� � � ∅ � � ��� � � ∅ � � ����������� Eqn. 5.13 ���� � � � �� Eqn. 5.14 ������ � � �� Eqn. 5.15 ���� ���������� � � � � � � � ��∅ � � � � � ��∅ � � � Eqn. 5.16 ���� ���������� � � � � � � � ��∅ � � � � � ��∅ � � � Eqn. 5.17 �� ���������� �� � � � ��∅ � � Eqn. 5.18 The RF signal is thus the baseband vector with an amplitude and phase of the baseband signal. If needed, the phase angle + b in equation 5.18 can be changed to - b by either generating a – (BB-Q) signal, or swapping LO-I and LO-Q. When b = b then a single sideband RF signal results. RF Electronics: Design and Simulation 161 www.cadence.com/go/awr

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