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- an overview of the basics of RF mixers and the process of RF mixing used in radios and RF circuits.

One of the most useful RF or radio frequency processes is that of mixing. Unlike an audio mixer where signals are simply added together, when a radio or RF engineer talks about mixing, he means a whole different process. Here signals are multiplied together and signals an new frequencies are generated.

The process of RF or non-linear mixing or multiplication is used in virtually every radio set these days and also in many other circuits beside. It enables signals to be changed from one frequency to another so that signal processing for example can be undertaken on a low frequency where it is easier to perform, but the signal can be changed to a from a higher frequency where the signal is to be transmitted or received.

What happens when signals are mixed

It is found that if two signals are passed through a non-linear circuit, then additional signals on new frequencies are formed. These appear at frequencies equal to the sum and difference frequencies of the original signals. In other words if signals at frequencies of f1 and f2 enter the mixer, then additional signals at frequencies of (f1+f2) and (f1-f2) will also be seen at the output.

To give an example if the two original signals are at frequencies of 1 MHz and 0.75 MHz, then the two resultant signals will appear at 1.75 MHz and 0.25 MHz.

Mixing two RF signals

Why RF mixing or multiplication works

To understand a little more about the RF mixing or multiplication process it is necessary to look at exactly how the mixing process occurs. As mentioned before the two signals are actually multiplied together, and this occurs as a result of a non-linear element in the circuit. This may be a diode, or active devices such as transistors or FETs that are suitably biased.

The two signals can be considered as sine waves. The instantaneous output level is dependent upon the instantaneous level of signal A multiplied by the instantaneous level of signal B. If points along the curve are multiplied, then the output waveform is more complex as shown below.

Mixing or multiplying two signals together

The frequencies used to generate the example below fir the frequencies mentioned above, i.e. 0.75 MHz and 1.0 MHz. It can be seen that in the output there is a low frequency component (the difference frequency at 0.25 MHz) and high frequency component (the sum frequency at 1.75 MHz).