RF Electronics Chapter 5: Frequency Mixers Page 162
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 5.88. Block diagram of a quadrature mixer in up-conversion.
From figure 5.88, for a down-converter, the directions of the RF and BB signals are
reversed, the Digital to Analogue converters are replaced by Analogue to Digital
converters and the vector modulator is replaced with a vector demodulator. To prevent
LO signals from overloading the ADC, low pass anti-aliasing are normally used before
the ADC. The resulting block diagram is shown in figure 5.89. When the LO frequency
and the RF frequency are not the same, then the IQ down-converter is called an Image
reject mixer. These are described in more detail later on.
Figure 5.89. Block diagram of a quadrature mixer in down-conversion.
The 90-degree hybrids required for quadrature mixers can also be produced using LC
networks, so that quadrature hybrids at frequencies below 500 MHz are possible. As an
example, Mini-Circuits make quadrature mixers, for IQ modulators and IQ demodulators,
up to 176 MHz as shown in figure 5.90. Analogue devices make IQ and Image-Reject
Mixers for up and down conversion with RF/LO frequencies from 2.5 GHz to 64 GHz.
Because of limitations in the bandwidth over which an accurate 90 phase shift can be
produced, the bandwidth of these mixers is typically limited to a 2:1 frequency range.
Local oscillators produced using phase locked loops or using Direct Digital Synthesis,
can have two outputs, which have an exact 90 phase difference over a wide range of
frequencies, as is required for the quadrature mixers. A simple twisted ring divide by 2
flip-flop circuit can produce two signals, which are exactly 90 apart. Some commercial
IQ mixers include such divide by 2 circuits.
RF Electronics: Design and Simulation
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