Signal frequency spectrum down-conversion is achieved by multiplying the radio-frequency signal by a local oscillator signal in a circuit known as a mixer. This multiplication produces two signals whose frequency content lies about the sum and difference frequencies of the center frequency of the original signal and the oscillator frequency. Two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 - f2 of the original frequencies.
A simple technique which uses a diode can be is employed for the purpose of obtaining multiplication. A diode is a non-linear (or non-ohmic) device/element, which means its response (current) is not proportional to its input (voltage). The diode therefore does not reproduce the frequencies of its driving voltage in the current through it, which allows the desired frequency manipulation. The current 'I' through an ideal diode as a function of the voltage 'V' across it is given by
And can be approximated for small x (that is, small voltages) by the first few terms of that series. ...Show more