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In https://ham.stackexchange.com/a/1068/9 it's shown that one can use

$$ r = \sqrt{I^2+Q^2} $$

to decode AM signals from a direct conversion receiver. It appears that, in this particular case, I and Q are interchangeable, thus the SDR doesn't care which is which.

Is this true, and for straight AM they can be switched without issue?

Are there other modes, such as SSB and CW, where switching I and Q wouldn't matter?

If I'm operating an SDR, what clues can I look for that would indicate a swapped I and Q?

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Swapping I and Q reverses all the frequencies. For example, a signal 5 kHz above the mixer's LO will appear at -5 kHz, instead of 5kHz. CW and AM are symmetrical in the frequency domain, so it doesn't matter for the purposes of demodulation, though your software is likely to display the wrong frequency. SSB is not: reversing I and Q will make USB look like LSB, and LSB look like USB.

If you have a waterfall with lower frequencies on the left, then tuning to a higher frequency should shift everything to the left. If it goes to the right instead, you know I and Q are reversed.

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    $\begingroup$ Might be useful to note that the software will display the wrong frequency only if a software frequency shift is being used, that is, if the demodulator is set to a carrier frequency different from the hardware LO frequency. The shift operation is what "does care" even though AM demodulation doesn't. $\endgroup$ – Kevin Reid AG6YO Jan 2 '14 at 21:58
  • $\begingroup$ Does your comment imply that I and Q can be reversed (from quadrature ADC) if the frequency shift is zero, and the waterfall won't be affected? $\endgroup$ – hotpaw2 Feb 4 '17 at 19:59
  • $\begingroup$ I'm not really sure what you mean by "frequency shift". $\endgroup$ – Phil Frost - W8II Feb 5 '17 at 2:00
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If you demodulate CW to audio by the same method (and frequency readout) as SSB, then swapping I & Q will often move the CW signal frequency by twice your chosen side tone (depending on how your receiver's CW frequency display algorithm works).

The CW audio side-tone, using direct conversion complex demodulation SDR methods, is produced by the rate of change in phase between the I near-sinusoid carrier and the Q near-sinusoid carrier. If the phase between Q and I is, say, increasing, then the opposite part of the phase circle, the phase between I and Q will be decreasing, thus representing a different frequency. This will commonly move the side tone outside a SSB+CW filter's passband unless you retune the complex heterodyning frequency (DSP/SDR frequency synthesizer) of the receiver.

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