Basic sampling frequency question

GNU Radio
1

I have a really simple question that I can’t seem to find the answer to
online (and pardon my limited signal processing background). I’m trying
to help a fellow colleague figure out a processing problem.

An external code modulates a real-valued signal that is eventually fed
into GRC. For now the signal is generated with some arbitrary positive
center frequency and then a GRC script converts it to complex and then
downshifts and centers it at 0Hz. This isn’t efficient because the
modulation in the external code takes a lot longer with higher sampling
frequency.

If the external code was modified to put the real-valued signal in the
-5 to 5kHz band (centered at 0Hz and assuming a real-valued signal can
represent negative frequencies), is the minimum sampling frequency to
avoid aliasing 20kHz or is it 10kHz? This is before the GRC script
converts it to complex.

Thank you in advance… Tom

2

On Mon, Jun 27, 2011 at 11:02 AM, Tom H. [email protected]
wrote:

If the external code was modified to put the real-valued signal in the -5
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A few things:

  • A real valued signal has a frequency spectrum that is symmetric around
    0Hz. So the negative frequency components are identical to the positive
    frequency components.

  • For your case, the minimum sampling frequency would be 10KHz. The
    signal
    being real or complex does not make a difference. If a signal occupies
    frequency -F to F at the baseband (centered), your Nyquist sampling rate
    is
    2F.

Hope this helps,

Colby