Illustrated Dictionary of Electronics
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RF Mixer

In the general sense a mixer is basically any circuit that combines two or more circuits or signals into a common output. A more commonly used definition defines a mixer as a nonlinear circuit as one that accepts as its input two different frequencies [F1 & F2] and than outputs between two and four signals:
F1 & F2; the original input frequencies, if they are not filtered out.
F1 + F2; A signal equal in frequency to the sum of the frequencies of the input signals.
F1 - F2; A signal equal in frequency to the difference between the frequencies of the input signals.

Signals generated by a mixer block
Mixer Section

The term non-linear refers to a component which causes a current to with the application of a voltage, but the current does not change in a linear relationship with the voltage. See Heterodyning.

The standard symbol for a mixer, shown in the right side-bar is a circle with an 'X' in the center. The diagram also depicts one of the most common uses of a mixer, that is combing a local oscillator with an RF signal to produce an IF signal.

Mixer Variations

The term mixer describes a function, that of mixing two signals together. So there are a vast number of ways to implement a mixer. The mixers shown on this page include a mixer made from a diode, a transistor mixer, and a vacuum tube mixer. However those are just the component used to provide the nonlinear function, within those three groupings there are even more variations.

There isn't necessarily a need to design a mixer using discrete components. Mixers are also sold as complete self-contained components, refer here for a lost of Manufacturers of RF Mixers. The circuits shown here are just intended to show a few different circuit variations, and not intended to cover the design of a mixer.

Each example circuit also mixes an RF signal with a local oscillator to produce an IF [Intermediate Frequency] signal, again a common implementation.

Balanced vs Unbalanced

Unbalanced mixers is a style that allows some of the input signal power to pass through to the output. While a single-balanced mixer is configured so the local oscillator, or RF input, cancels and cannot pass through to the output. However a double balanced mixer has symmetrical paths for both the oscillator and RF inputs, and will have no output if either input signal is not present.

Diode Mixer

A diode mixer is the simplest form of mixer circuit, and uses a diode as the nonlinear device. An example of an unbalanced diode mixer is shown in the right side-bar. This example has tank circuits on each port.

Semiconductor diode mixer circuit diagram
Diode Mixer

For reference here is a list of Diode Manufacturers.

Transistor Mixer

Transistor mixer circuit diagram
Transistor Mixer

As with each of these examples a number of variations are possible. The oscillator signal could be injected into the Emitter lead as shown in this example, or the Base of the transistor. A capacitor is used as an isolator in this case, but a transformer could also be used.

As a side not the two variable capacitors [C5 & C6] should be shown ganged together, so they move as one. Although that line was removed for clarity.

For reference here is a list of Transistor and FET Manufacturers. A FET could replace a transistor in any of the applications using a transistor as the mixer.

Vacuum Tube Mixer

This example provides another example of a diode mixer, now using a vacuum tube diode instead of a semiconductor diode. The circuit configuration is also different and represents yet another method of injecting the two signals into the circuit. However the circuit could have been constructed in an identical manner as the one using the semiconductor diode.

Vacuum tube diode mixer circuit diagram
Vacuum Tube Diode Mixer

In addition a 3-terminal triode tube, or other types of vacuum tubes, could have been used to generate the mixer function. Also a number of different ways could have been used to couple the signals into the circuit, creating an endless number of different circuit configurations.

For reference here is a list of Vacuum Tube Types, although not all of them would be suitable for use in a mixer circuit.

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