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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Undersampling and its complement?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
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Date: Sat, 2 Nov 2002 21:36:23 -0000
NNTP-Posting-Date: Sat, 02 Nov 2002 21:36:29 GMT
"Jeroen" wrote in message
> "Kevin Aylward" wrote in message
> > "Jeroen" wrote in message
> > news:DtFw9.16424$I6.email@example.com...
> > >
> > > "James Meyer" wrote in message
> > > news:firstname.lastname@example.org...
> > > >
> > > > I have to extract information from an 8 MHz signal. Nyquist
> > that
> > > > 16 MHz should be my minimum sampling frequency. However... if
> > > bandwidth of
> > > > the signal I'm interested in is something like 10 KHz centered
> > around 8
> > > MHz,
> > > > then I really only need to sample at 2 times 10 KHz, let the
> > be
> > > "folded"
> > > > or aliased down to 0 to 10 KHz and I wil not have lost any
> > information.
> > > That's
> > > > undersampling.
> > > >
> > > > My question is.... is there anything similar regarding
> > 8
> > > > MHz signal? Do I really need a 16 MHz or better DAC in order to
> > generate
> > > 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
> > 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
> > is *not* the bandwidth of the signal. Depending on the modulation
> > method, it might be +/- fmod, or approximately, say, 5 to 10 times
> > modulation frequency. For AM the BW is indeed 10Khz in this example.
> But how do you create an 8MHz signal from an DAC if you feed only
> at 10KHz?
> I'm assuming the 8MHz a kind of a carrier signal and there are not
> filters after the DAC, just the standard LP (in this case 20KHz).
I was referring to the principle, not to methods of how to use that
principle. The most obvious technique is to downconvert/upconvert the
signal with a multiplier.
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