What Is a Double-Balanced Mixer?
Double-balanced mixers are a type of passive mixer circuit.
A mixer circuit is generally a circuit that outputs the multiplied value of two different frequency signals, with the sum and difference frequencies of the two signals as its output signal components. Double-balanced mixers are also called DBMs, from the English acronym, or double-balanced modulators. Double-balanced mixers have a two-input, one-output configuration for the mixer circuit and consist of two transformers and four diodes.
Applications of Double-Balanced Mixers
Double-balanced mixers are often used in modulation circuits for wireless communications as mixer circuits. In recent years, wireless communications, including smartphones, have become increasingly popular, and modulation by mixer circuits plays an important role in wireless communications.
The mixer circuit consists of only a diode and a transformer and can be operated with a relatively simple mechanism. By changing the direction of inputs and outputs, it can be used as either the receiving side or the transmitting side of a modulation/demodulation circuit.
Principle of Double-Balanced Mixers
Double-balanced mixers are mixer circuits, generally used as multipliers, that consist of two inputs and one output, and output the sum and difference of the frequencies of the two input signals.
If the frequencies of the two input signals are f1 and f2, respectively, the output signal frequencies are f1+f2 and f1-f2 (when f1>f2). When using double-balanced mixers as a modulation circuit, if f1 is the carrier wave and f2 is the low-frequency signal, an amplitude modulation signal in which f2 is superimposed on f1 can be output.
The mechanism by which the two sum and difference frequencies are output is as follows: When two frequencies f1 and f2 are used and α = 2πf1 and β = 2πf2, multiplication of the two single frequencies yields the following equation.
sinα×sinβ=1/2{cos(α-β)-cos(α+β)}
Here, α-β=2π(f1-f2) and α+β=2π(f1+f2), indicating that the frequency components are divided into a sum and a difference. Therefore, when the two AC signals are input and multiplied by the mixer circuit, the sum and difference of the input frequencies are output.