About Digital mixer and DDC

[rezvania]

Hi, what is difference between digital signal in base band and digital signal in RF band? I want to know when we shift base band signal to Rf band in analogue domain, in digital it change? and When we use digital instead of analogue signal, for mixer block we need to multiplier? we multiply LO in analogue signal to central frequency shift to zero but in digital signal what we do?

Thanks for your responses ...

 

[Dan Mills]

You can do the mixing in the digital domain at least if the carrier frequency is not too high, but while this is entirely practical at HF, it becomes a much harder problem at UHF as the required sample rates become insane.

Now some of the more interesting parts from folks like analogue, TI & linear tech support an interesting mode where the DAC chip can both upsample and mix with (typically Fs/4 or a multiple thereof) which allows a digital IF to be moved up to a UHF output in the digital domain.
You can of course subsample if the sample and hold bandwidth is sufficient and the bandwidth limits meets the ampling theory limit.

Note that a mixer block in the digital domain is simply a half complex multiplier, and so has none of the non linearities inherent to a diode ring or gilbert cell mixer done in the analogue domain, however there is a limit to the maximum speed that a digital multiplier can operate which is lower then the limit for a analogue mixer.

 

[rezvania]

Thank you Dan,
when you use digital mixer you just pass it from CIC filter or must multiply it in Sin and Cos and next pass it from CIC filter?