Say you have a narrow-band signal at 1350 kHz, the frequency of our local AM broadcast station KSRO. You could use a sampling frequency of, say, 1000 kHz. The samples from the 1350 kHz signal would be identical to the samples of a signal at 350 kHz or 2350 kHz or 3350 kHz, etc. But if you know in advance that the signal is band-limited, then you can accurately re-create the 1350 kHz signal eliminating the "images" at the other frequencies.
If you measure the 1350 kHz AM signal with a digital oscilloscope that has a 1 MHz sample rate, it will appear that the signal is at 350 kHz. (That is, the samples are identical to those from a 350 kHz signal.) But that's OK if you only care about the modulation. Al N1AL On Tue, 2008-04-29 at 16:43, Andrew Faber wrote: > Al, > I'm confused. I always thought (at least before I became a lawyer and > stopped thinking about such things) that to reconstruct a waveform you > needed to sample at twice the highest frequency of the Fourier components > contained in the signal waveform. For a sine wave, that would be twice the > frequency. For a square wave, that would be virtually impossible to do > accurately, since it is composed of an infinite series of sine wave > harmonics, though you could approximate an accurate waveform to any degree > desired by sampling ever higher in frequency. Are you saying something > different, or do you mean that for a narrow-band signal, you have a > practical high-frequency cut-off, and thus a practical limit on how fast you > need to sample? > 73, Andy, AE6Y > > ----- Original Message ----- > From: "Alan Bloom" <[EMAIL PROTECTED]> > To: "Romanchik Dan" <[EMAIL PROTECTED]> > Cc: "Mailing List Elecraft" <elecraft@mailman.qth.net> > Sent: Tuesday, April 29, 2008 1:14 PM > Subject: Re: [Elecraft] Measuring power output of an SSB transmitter > > > > Actually, for a narrow-band signal, you only have to sample at twice the > > BANDWIDTH, not twice the frequency. So long as there are no interfering > > signals around (reasonably accurate when measuring a transmitter), > > so-called "undersampling" works fine so long as: > > > > 1. The sample rate is more than twice the bandwidth. > > 2. None of the harmonics (sample_rate/2) fall close to the signal. > > > > Al N1AL > > > > > > On Tue, 2008-04-29 at 11:05, Romanchik Dan wrote: > >> Sampling at twice the bandwidth will allow you to accurately extract > >> frequency information about a signal, or if all you were viewing were > >> sine waves. To view the actual waveform, however, you really want to > >> sample at at least 4x or 5x the bandwidth, or, even better, 10x the > >> bandwidth. > >> > >> 73! > >> > >> Dan KB6NU > >> ---------------------------------------------------------- > >> CW Geek and MI Affiliated Club Coordinator > >> Read my ham radio blog at http://www.kb6nu.com > >> LET'S GET MORE KIDS INTO HAM RADIO! > >> > >> > >> > >> On Apr 29, 2008, at Apr 29, 1:32 PM, Stephen Prior wrote: > >> > >> > On a related topic, does anyone have any experience of using the > >> > usb types > >> > of 'oscilloscopes' which appear on eBay- dual channel 60MHz > >> > bandwidth and > >> > 150Mb/s sampling rate? It's a neat and cost-effective way of > >> > displaying the > >> > transmitted waveform for those of us without desk/bench room for a > >> > 'proper' > >> > scope. > >> > > >> > I've always been led to believe that sampling at twice the > >> > bandwidth is a > >> > decent decent rule of thumb and this one is better than that. > >> > > >> > I'd be grateful for any advice from someone who has one. > >> > > >> > Thanks > >> > > >> > Stephen G4SJP > >> > > >> > KX-1, K2, K3 almost! > >> > > >> > > >> > > >> > _______________________________________________ > >> > Elecraft mailing list > >> > Post to: Elecraft@mailman.qth.net > >> > You must be a subscriber to post to the list. > >> > Subscriber Info (Addr. Change, sub, unsub etc.): > >> > http://mailman.qth.net/mailman/listinfo/elecraft > >> > > >> > Help: http://mailman.qth.net/subscribers.htm > >> > Elecraft web page: http://www.elecraft.com > >> > > >> > >> _______________________________________________ > >> Elecraft mailing list > >> Post to: Elecraft@mailman.qth.net > >> You must be a subscriber to post to the list. > >> Subscriber Info (Addr. Change, sub, unsub etc.): > >> http://mailman.qth.net/mailman/listinfo/elecraft > >> > >> Help: http://mailman.qth.net/subscribers.htm > >> Elecraft web page: http://www.elecraft.com > > > > _______________________________________________ > > Elecraft mailing list > > Post to: Elecraft@mailman.qth.net > > You must be a subscriber to post to the list. > > Subscriber Info (Addr. Change, sub, unsub etc.): > > http://mailman.qth.net/mailman/listinfo/elecraft > > > > Help: http://mailman.qth.net/subscribers.htm > > Elecraft web page: http://www.elecraft.com > > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
