RF Electronics Chapter 5: Frequency Mixers Page 117
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Single Diode Mixer
Single diode mixers use a single diode to produce the required frequency components.
Single diode mixers are often used in cost critical applications, such as Radio or TV
receivers, where the low cost is more important than good performance. With the advent
of low cost active mixer IC's, single diode mixers are progressively being used less.
Single diode mixers are very suitable for microwave applications like speed guns and
shopping centre door openers, where the transmitted signal is used as the LO for the
received signal, and the receiver diode is simply mounted in the antenna horn. The
resulting IF signal is the difference frequency, which is due to the speed of the car being
detected or the speed of the person moving towards the door.
Figure 5.11. Low return loss diplexer circuit.
A single diode mixer requires a diplexer to separate the high frequency RF and LO signal
from the low frequency IF signal. Since the single diode mixer is normally a lower cost
consumer type application, the diplexer is normally kept simple with either a first or
second order high pass and low pass filters.
Figure 5.12. Frequency response of low return loss diplexer after optimisation.
Figure 5.11 shows a simple diplexer consisting of second order high pass and low pass
filters. The crossover frequency is a compromise with a lower corner frequency of the
low pass filter removing more LO and RF signals from the IF signal but causing an loss
RF Electronics: Design and Simulation
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