[Elecraft] Measuring power output of an SSB transmitter

[Alan Bloom]

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" <n1al at cds1.net>
> To: "Romanchik Dan" <kb6nu at w8pgw.org>
> Cc: "Mailing List Elecraft" <elecraft at 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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