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I read that a negative frequency implies "forward" travelling wave and a positive frequency implies "backward" travelling wave, so.. does a suppressed carrier amplitude modulated wave not exist due to destructive interference? As it has 2 exact frequencies but one in negative and one in positive.

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    \$\begingroup\$ The frequency bands of the information are not the same after modulation (the 2 sidebands exist separately). en.wikipedia.org/wiki/…. Also, where did you read this information? \$\endgroup\$ Commented Dec 30, 2019 at 14:50
  • \$\begingroup\$ "Negative frequency" doesn't usually mean anything at all? Destructive interference can only happen at a single frequency - it sounds like this is a mangled explanation of how the carrier itself is suppressed? \$\endgroup\$
    – pjc50
    Commented Dec 30, 2019 at 14:51
  • \$\begingroup\$ No - negative frequencies do not exist in reality. They are (fictitous) parts of mathematical tools only... \$\endgroup\$
    – LvW
    Commented Dec 30, 2019 at 15:00
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    \$\begingroup\$ blogs.zhinst.com/sadik/2013/08/dsb-sc Negative is below fc, but above 0 \$\endgroup\$ Commented Dec 30, 2019 at 15:38

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Negative frequencies exist (outside of complex math) in strictly real signals only in relation to a higher frequency carrier frequency, and only over a long enough observation window to discriminate them orthogonally from the carrier and any other sidebands. (Over shorter time windows, spectral content is very likely ambiguous.)

Negative frequencies can also exist at baseband as complex IQ signals inside an SDR algorithm (FFTs, et.al.). Negative frequency signals have the opposite relationship between the cosine and sine basis vectors from positive frequency signals, and thus appear in the opposite half of a complex FFT result, which can allow an SDR radio to separate the upper and lower sidebands during USB or LSB SSB filtering or demodulation.

Essentially, quadrature or complex heterodyning allows moving a strictly real RF signal containing low, middle, and high frequency spectrum, down to baseband as mathematical complex or IQ representation containing negative, DC (0 Hz), and positive frequency spectrum, respectively. Why do this? At baseband an SDR radio can potentially use a much lower sample rate, and thus run on a less expensive CPU or DSP chip, and/or require less power to process (filter, demodulate, etc.)

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  • \$\begingroup\$ Also, the math becomes way way easier when you're in complex baseband, as you can stop lugging around a carrier wave and can simply concentrate on the aspect of the signal that actually carries your information. \$\endgroup\$ Commented Dec 30, 2019 at 17:25

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