A diode mixer is used in AM radio circuits to combine and modulate two signals together to create the desired frequency. It uses the nonlinear response properties of a single diode to create frequency permutations of the two input signals. The output can be composed of the sum or difference of the frequencies, but unwanted signals must be filtered. Diode mixers can be inefficient and cause unwanted signals, resulting in fuzzy signals and static in inexpensive AM radios.
A diode mixer is a special type of frequency mixer used in the manufacture of circuits for amplitude modulation (AM) radios. A diode is a nonlinear device in that an alteration in the current or voltage across the diode does not cause a corresponding change in the respective current or voltage, giving it a varying degree of resistance. A linear system is one whose output is directly proportional to its input. Consequently, diodes are used in the construction of frequency mixers to combine and modulate two signals together to create the desired frequency.
A diode mixer circuit uses the nonlinear response properties of a single diode to create frequency permutations of the two input signals. Designers, especially those of AM radios, use this type of circuit to convert received radio frequency (RF) signals to a lower intermediate frequency. This makes it much easier to design radio signal processing circuits. Alternatively, the diode can, if necessary, create a desired frequency from an input signal that contains more harmonics.
A diode mixer can be used to create frequencies of different basic types, depending on the designer’s needs. The first is the creation of an output composed of the sum of the frequencies of the two input signals; the signals are simply added together. The second is an output composed of the difference between the frequencies, effectively removing the value of one frequency from the other to create an output at a lower frequency. The third is to pass both signals. The first method creates a distinct and individual frequency by summing both inputs, but the third method is composed of both signals and unwanted aspects must be further filtered along the circuit.
The inherent non-linearity of diode mixers is why they are used in a variety of things, but their simplicity often results in a large degree of “output loss”, i.e. unwanted signals mixed in with the desired output frequency . For this reason, a diode mixer can be very inefficient and, in more sensitive systems, additional equipment must be used to filter unwanted signals. Inexpensive AM radios use diode mixers, which is often the cause of fuzzy signals and overly obtrusive static.
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