Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 01 Feb 2006 15:54:48 +0000
Message-ID: <002901c62746$fc8ed190$1a8cf8d4@standalone>
From: "Andy" <actalbot@southsurf.com>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 1 Feb 2006 15:48:43 -0000
MIME-Version: 1.0
Subject: Re: LF: Can't see the wood for the trees
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

Yes - that's the obvious bit I was missing - go back to first principles and
do the actual Fourier transform on the waveform.

There ought to be a short cut, though, knowing the integral is finite and
contains total power in the signal.   I had assumed the Dirichlet integral
converged, but didn't know the result, so maybe that is the way to go.

Tnx
Andy  G4JNT
www.scrbg.org/g4jnt/


-----Original Message-----
From: Warren K2ORS/WD2XGJ <k2ors@comcast.net>
To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>;
rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Cc: Andy <actalbot@southsurf.com>
Date: 2006/02/01 15:34
Subject: Re: LF: Can't see the wood for the trees


>Andy,
>
>      Unless I'm missing the meaning of your question, I believe you want
the fourier coefficients to determine the amplitudes of the components
making up the pulse train. Also, should you feel the need to integrate
sin(x)/x, the so-called Dirichlet integral, it does converge and the
definite integral from 0 to infinity of sin(x)/x = pi/2.  For reference see:
>http://www.pupress.princeton.edu/books/maor/chapter_10.pdf
>
> I'm in the middle of a project at the moment but let me know if I'm on the
right track - I could help with the fourier coefficients perhaps this
evening.
>
>--
>73 Warren K2ORS/WD2XGJ
>FN42hi
>http://www.w4dex.com/wd2xgj.htm
>
> -------------- Original message ----------------------
>From: "Andy" <actalbot@southsurf.com>
>> Can someone help with what should be obvious.
>>
>> I have a train of constant width pulses at a fixed repetition rate.   In
the
>> frequency domain these appear as a spectral comb with spacing at the
>> repetition rate, whose amplitude  follows a SIN(X) / X shape depending on
>> the pulse width, ie. the first null occuring at at 1/width and so on.
>>
>> What I'm getting tied up in knots trying to calculate is :
>>
>> What is the absolute amplitude (power) of just one individual tooth of
the
>> comb at any particular spacing.
>>
>> Assume the pulse waveform has, say,  1mW or 0dBm mean amplitude, and
>> consists of 500us pulses at 40Hz PRI.   The duty cycle is 0.02, so the
>> individual pulse power would have to be 50mW or 17dBm to get this mean.
>> But what is the amplitude of the component at, say, 1kHz, or 1040Hz, or
>> 10kHz ??
>>
>> It must be obvious, but I keep feeling the urge to integrate SIN(X) / X
>> which is not funny and way beyond my maths capabilities!!
>>
>> The figures given above are those for the 5MHz beacon sounder sequence.
>> The amplitude trace on the monitoring software during the sounder
sequence
>> is measuring just one line of the comb ( F = 0,  the carrier) , and
appears
>> to suggest this is about 30 - 35dB down on the CW part.  That is -17dB
from
>> the peak/mean  as above, but where does the other 13 - 18dB come from?
>>
>> Tearing hair out
>>
>> Andy  G4JNT
>> www.scrbg.org/g4jnt/
>>
>>
>>
>
>
>