One might, at the risk of some oversimplification, think of the sampling
process as demodulation. To recover the envelope, one does not care
about the absolute frequency. What you are concerned with is the
variation in amplitude.
As I said, over-simplified but nonetheless perhaps useful as a starting
point.
Jack K8ZOA
A recovering lawyer as well as engineer.
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!
>
>
>
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