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Re: [Amps] Measuring RF Power

To: Michael Tope <W4EF@dellroy.com>
Subject: Re: [Amps] Measuring RF Power
From: David Kirkby <david.kirkby@onetel.net>
Date: Sun, 27 Mar 2005 02:31:58 +0100
List-post: <mailto:amps@contesting.com>
Michael Tope wrote:

>>The confusion reminds me of the Nyquist Sampling Theorem. 99% of 
>>professional engineers think you have to sample a signal at 2 or more 
>>times the highest frequency component to recover it without aliasing. 
>>That is not true, but many engineers think it is true, and many books 
>>say it is true.
>>
>>    
>>
>
>Okay, David, I'll bite (I am among the 99%, I guess). Are you referring
>to the highest frequency component of a modulated carrier or the 
>information bandwidth of a modulated signal? If the former, I agree (e.g. 
>undersampling of modulated RF carriers is done all the time). If the 
>latter, then please explain :)
>
>73 de Mike, W4EF.......................................
>  
>

Well you are not amongst the 99%, but the 1%, as most people are not 
aware that you only need to sample at twice the *bandwidth* of the 
signal, and not twice the highest frequency.

Ask many engineers what the Nyquist Sampling Theorem is and they will 
say something like "you need to sample a signal at twice the maximum 
frequency component".

For audio, (20Hz to 20kHz), it makes no difference, as twice 20kHz is 
not too different from twice 19.98kHz. However, if the same audio is 
modulated on a 100 MHz carrier, it makes a huge difference if you can 
(in theory at least) sample at close to 40kHz, rather than 200MHz.


I've been sampling a 70MHz signal at around 3.9MHz. I do intend sampling 
faster, as jitter is more of a problem the more you under sample. I'll 
probably sample at around 35.01 MHz, which will then convert the 70MHz 
signal down to 10 kHz, which is fine for me, as the information 
bandwidth is 1kHz or less.


-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/



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