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Theory of mixers

A mixer circuit normally has two inputs - from two separate signal sources. In the diagram below, the sources are two oscillators. Each oscillator is a generator producing a sinewave output, one at frequency f1 and the other at frequency f2.
The mixer multiplies the signals together,while the balanced modulator is a combination of 2 mixers.
Remember that the output comprises a complex mixture of separate sinewaves at many different frequencies beco'z of the square law equation of the current .


The main point to note is that the output comprises the two separate input frequencies f 1 and f 2 and their sum, (f 1 + f2), and their difference, (f 1 - f2). In practice, there are other component signals too - but we can ignore those.

A filter - which can be any one of various sorts - selects the required output from the mixer. In this diagram, a simple parallel tuned circuit is shown. The output will normally be tuned to the SUM, (f 1 + f2), or tuned to the DIFFERENCE, (f 1 - f2), signal as required.
The output from a mixer contains many more combinations of frequencies - generated from the harmonics of the input signals mixing with the component signals.
Almost any electronic device, diode, transistor, valve, can be used as a mixer. A "square-law" characteristic device is preferred - to minimise unwanted outputs.
The principle is: In a mixer stage, the output contains the SUM and the DIFFERENCE of the input signal frequencies.

let me correct a common mistake ...mixer is not a sum and difference frequency signal producer!

its any device having non-linear transfer characteristics .it is simply like our "mixer".it produces many components of frequencies sum ,difference ,secondorder sum second oerder difference .the no: of frequency components generated depends on the extent nonlinerarity of the device transfer characteristics .We filter out the desired frequency component using filters.Any way mixers contribute to most of the noise produced in the receiver or we can say that mixers are the sources of noice at the receiver end of a transmission system

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