A diode mixer is a particular kind of frequency mixer used in the production of circuits for amplitude modulation (AM) radios. A diode is a nonlinear device in that an alteration of the current or voltage across the diode does not cause a corresponding change in the in the respective current or voltage, giving it a degree of variable resistance. A linear system is one whose output is directly proportional to its input. As a result, diodes are used in the construction of frequency mixers in order to combine and modulate two signals with one another in order to create the desired frequency.
The circuit of a diode mixer utilizes the nonlinear response properties of a single diode to create permutations of frequencies of the two input signals. Designers, especially those of AM radios, use this type of circuit to downconvert received radio frequency (RF) signals to a lower intermediate frequency. This makes it a lot easier to design the radio's signal processing circuitry. Alternatively, the diode can, if needed, create a desired frequency from one input signal that contains multiple overtones.
A diode mixer can be used to create frequencies of several basic types, depending on the requirements of the designer. 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 push both signals through. The first method creates a distinct, individual frequency by adding both inputs together, but the third one is composed of both signals, and the unwanted aspects must be filtered out further along the circuit.
The inherent nonlinearity of diode mixers is why they are used in a variety of things, but their simplicity often results in a large degree of "output leakage" — that is, 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 out the unwanted signals. Inexpensive AM radios use diode mixers, which is often the cause of fuzzy signals and overly obtrusive static.