Downconverter

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In digital communications signals can be transmitted in baseband or passband. On receiver side usually a downconverter is used to transform the signal from the passband back to the baseband for further processing.

Image:downconverter.png

The local oscillator frequency is \sqrt{2} e^{j2 \Pi f_c t}, with fc being the carrier frequency. This results in a scaling of the received signal by \sqrt{2} and a phase shifting by fc to the left, so that the resulting signal is located in the baseband.

Image:downconverter_process.png

[edit] Further reading

Barry, Lee, Messerschmitt; Digital Communication 3rd edition, Springer: p.17f

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