I've previously worked with super-heterodyne receivers where the IF spectral content has been above DC.

Reading the below tutorial; direct down conversion receivers are used to mix a signal of interest down to base-band. Figure 7 shows the output of the direct conversion process where the modulated signal has its frequency content either side of DC i.e. half as negative frequencies. https://www.microwavejournal.com/articles/3226-on-the-direct-conversion-receiver-a-tutorial

A similar question was asked in the link below which explains that there will be indeed overlap and corruption because of the negative frequencies. Downconversion / Demodulation of RF signal to DC

If that is the case, how is the signal recovered and what do the resultant I and Q samples look like in terms of magnitude and frequency content?

If specifics help; if we assume an already modulated carrier 1500MHz with +/-1MHz FSK signal is mixed with a 1500MHz LO. The output (and image) will be centered around DC but half the signal bandwidth will be negative, the missing half will be on-top. Its this with which I'm confused.

  • $\begingroup$ Hi! That answer applies 100% to your question, so I'm not sure in which way yours isn't answered by that – it 1. describes how the I and Q look in spectrum, 2. it explains how the signal is "recovered" (there's really no recovery – I and Q are the signal, without anything needing recovery). $\endgroup$ – Marcus Müller Nov 12 '19 at 17:53
  • $\begingroup$ This answer may help you see how quadrature demodulation can recover all information in the signal despite the "spectral overlap". $\endgroup$ – MBaz Nov 12 '19 at 18:18
  • $\begingroup$ OK, re-read the answer. So because there is a phase shift before both are shifted down, the lower half is lost from I and the upper half from Q, which is flipped due to being out of phase, both are recovered. $\endgroup$ – electronpygmy Nov 12 '19 at 18:29

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