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Semiconductor technology has advanced to the point that the digital representation of a modulated RF signal can be applied directly to a DAC for transmission. Is there, then, any distinction between a "baseband" signal and the RF transmitted signal?

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Yes there is an important distinction which is independent of the semiconductor technology. In radio terminology, "baseband signals" are typically centered on "DC" and as such are required to be complex (such as I and Q) to represent a waveform that does not have a symmetrical spectrum about the carrier (basically nearly every modern waveform where modulation of amplitude and phase is involved) while IF signals and "RF Transmitted" signals can be real to represent these same waveforms.

In general "baseband" is the modulated waveform with DC (f=0) as the carrier while "IF signal" and "RF Transmitted signal" are representations with a non-DC carrier.

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Terms are going to differ. I think the best I can come up with is that if the signal occupies a relatively narrow slice of the spectrum then it's "RF". If it fully occupies the spectrum -- particularly if it has a DC component -- then it's baseband.

So a recording of me talking, from 300Hz to 3kHz, is baseband (that little slice missing around DC doesn't matter much). A 2.5kHz-wide, SSB-modulated version of me talking, centered around 7.25MHz, is RF, even if it's going out a DAC.

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  • $\begingroup$ What if the source of the "voice" was a synthesizer which only exists as a digital representation? $\endgroup$
    – Brian K1LI
    Mar 14, 2021 at 11:42
  • $\begingroup$ What do you mean by "synthesizer"? If you're back to your single tone at RF, with great swaths of spectrum at zero -- then it's not baseband. $\endgroup$
    – TimWescott
    Mar 14, 2021 at 16:39
  • $\begingroup$ "Synthesizer" as in no analog source; the source "only exists as a digital representation.":The source information has non-zero bandwidth. The RF signal that comprises the source information may be processed to recover the source information. $\endgroup$
    – Brian K1LI
    Mar 14, 2021 at 18:29
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Do you mean either that 1. the sampling bandwidth is large enough to cover the carrier frequency, or 2. demodulation occurs via aliasing when sampling at baseband a modulated signal? I think typically a signal is downconverted to baseband (or upconverted even to a certain frequency) by an analog mixer before sampling. If a signal is sampled at IF, you'll still need to demodulate it digitally. Otherwise it looks as a matter of fact as baseband signal

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  • $\begingroup$ A school of thought inclines to think that the "baseband" signal is fictitious since it only exists as a digital representation. Even though a human may type text into a digital system, it never exists as an analog signal that modulates a carrier for conversion to a higher frequency for transmission. $\endgroup$
    – Brian K1LI
    Mar 14, 2021 at 11:40
  • $\begingroup$ We definitely invented digital modulations but analog modulations have been invented by nature in order to convey information, think about bird songs $\endgroup$
    – ZiglioUK
    Mar 15, 2021 at 9:10

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