the general rule for aliasing. If my sampling frequency is Fs=800Mhz signal frequency =120Mhz.

for the 4th harmonics 480MHz its 80MHz above nyquist frequency(400Mhz) thus its mirrored 80MhZ back to 320MHz.

What about 1120MHz the closest nyquist frequnecy to it is 2*400 so its supposed to be mirrored 800-320=480 why we do double mirroing for it? why it also mirroed to 320Mhz?

  • 1
    $\begingroup$ How is 1120MHz harmonically related to anything? $\endgroup$ – TimWescott Oct 2 '20 at 18:32
  • $\begingroup$ i ask purely theoreticaly $\endgroup$ – rocko445 Oct 2 '20 at 19:10
  • $\begingroup$ That's fine. The question would make sense if 1120MHz / 120MHz were an integer -- it's not, but you're calling it a "harmonic" -- that does not compute. Was it a typo, or do you misunderstand what a harmonic is, or what? $\endgroup$ – TimWescott Oct 2 '20 at 20:08

Aliasing is more like circular wrapping around rather than just mirroring. Frequencies wrap circularly between -Fs/2 to Fs/2, an 800 Hz range from -400 to 400 at a sample rate of 800 sps.

120 is inside -Fs/2 to Fs/2, so doesn't need to wrap around at all.

But 480 wraps 80 past 400, and shows up at -320, which looks like 320 if you don't care about phase.

And 1120 wraps all the way around the circles range of 800 Hz, plus another 320, so it shows up at 320.

12000 Hz (12 kHz) would wrap around 15 times and show up identical to DC or 0 Hz.

  • $\begingroup$ for 480 we have its pass by 80 the 400 line. So by wrapping we do 400-80=320 . with 1120 we have 800 line which we pass it by 320 its not the same thing they pass th nyquist line by diiferent amount. $\endgroup$ – rocko445 Oct 2 '20 at 22:38
  • $\begingroup$ -400+80 = -320, 1120-800 = 320, -320 is different from 320 (if you don't ignore phase info) $\endgroup$ – hotpaw2 Oct 3 '20 at 4:09

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