RF Electronics Chapter 5: Frequency Mixers Page 131
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
DBMs can be used as analogue multipliers in applications like a phase detector or a true
RMS power meter. For phase detection, the IF port of a DBM provides the phase output.
For multiplier operation, the DBM is operated in an unsaturated mode. For phase
detection, the mixer is operated in a saturated mode, where large signals are applied to
both the LO and RF ports. In addition the IF port must be DC coupled. Mini-Circuits
make Phase Detectors, which are optimised for phase detection.
The circuit diagram of a DBM is shown in figure 5.31. The two transformers provide
isolation for all ports. Four diodes are now required.
For the analysis of the mixer, consider a +ve input signal being applied to the IF port in
figure 5.31. Under those conditions, the diodes shown in figure 5.32 conduct and the
others are an open circuit. The resulting connection of the transformers cause a positive
signal at the LO port, to cause a negative signal at the RF port. Thus, for a +ve IF signal,
the mixer inverts the LO signal to the RF port.
Figure 5.31. Circuit diagram of a DBM.
Figure 5.32 Mixer with +ve voltage applied at the IF port.
Figure 5.33. Mixer with –ve voltage applied at the IF port.
If a -ve input signal is applied to the IF port, the diodes shown in figure 5.28 conduct and
the others are an open circuit. The resulting connection of the transformers cause a
RF Electronics: Design and Simulation
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