Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Undersampling and its complement?
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Date: Sat, 2 Nov 2002 08:10:24 -0000
NNTP-Posting-Date: Sat, 02 Nov 2002 08:10:26 GMT
"Jeroen" wrote in message
> "James Meyer" wrote in message
> > I have to extract information from an 8 MHz signal. Nyquist says
> > 16 MHz should be my minimum sampling frequency. However... if the
> bandwidth of
> > the signal I'm interested in is something like 10 KHz centered
> > then I really only need to sample at 2 times 10 KHz, let the signal
> > or aliased down to 0 to 10 KHz and I wil not have lost any
> > undersampling.
> > My question is.... is there anything similar regarding generating an
> > MHz signal? Do I really need a 16 MHz or better DAC in order to
> 8 MHz
> > signals if the signal will not have a very wide bandwidth?
> Yes you do.
No you dont. The Nyquist Sampling theorem says that you have to sample
at twice the *bandwidth* of the signal, not twice its frequency.
>The bandwidth of the signal is 8Mhz, not the 10Khz you want
No its not.
> modulate it with. The 10kHz is modulation depth, not bandwidth.
By assumption the poster said the BW was not large. The centre frequency
is *not* the bandwidth of the signal. Depending on the modulation
method, it might be +/- fmod, or approximately, say, 5 to 10 times the
modulation frequency. For AM the BW is indeed 10Khz in this example.
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