[Elecraft] Measuring power output of an SSB transmitter

[Andrew Faber]

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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