Envelope-to: dave@picks.force9.co.uk
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From: "Andy" <actalbot@southsurf.com>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 1 Feb 2006 14:55:01 -0000
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Subject: LF: Can't see the wood for the trees
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Can someone help with what should really 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/