The principle of signal mixing
A mixer is a circuit which combines 2 signals having different frequencies, generating a signal whose frequency is the sum or the difference of the input frequencies. One of the signals is generally an unmodulated carrier, so that mixer has the effect of converting a modulated signal at 1 frequency to a modulated signal at some other frequency. A mixer has 2 inputs and one output.
A mixer is a circuit which combines 2 signals having different frequencies, generating a signal whose frequency is the sum or the difference of the input frequencies. One of the signals is generally an unmodulated carrier, so that mixer has the effect of converting a modulated signal at 1 frequency to a modulated signal at some other frequency. A mixer has 2 inputs and one output.
Up conversion versus down conversion
Assume that the signals to a mixer are at frequencies f1 and f2, with f2 being the higher frequency. The mixer has a nonlinear element which is active, such as a diode or a class-B-biased transistor. The circuit generates new signals, in addition to passing original 2 signals. Outputs will appear at f1, f2, f2 + f1, and f2 - f1.
A characteristic mixer circuit is diagrammed in the figure given below. The output has a tuned circuit which is set to either f2 + f1 or at f2 - f1. If the output is tuned to sum of the 2 input frequencies, the output frequency, fout, is higher than the input frequencies. If output is tuned to the difference frequency, the new signal is in between the 2 inputs, or else lower than both.
Let f1 denotes the frequency of a signal which you want to convert via mixing. Let f2 be signal from the local oscillator (LO). If the sum output frequency is used, you have up conversion. If difference frequency is used, and if f2 is selected such that fout is less than f1, you have down conversion.