[time-nuts] Help w/integration problem

John Miles jmiles at pop.net
Mon Jan 2 03:49:19 EST 2006

Never mind, I think I see what's wrong... you can't integrate the dBc/Hz
values directly.  You have to turn them back into linear ratios, do the
interval sum, and then, if you want dBc coming out, take 10*log10(sum).

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of John Miles
> Sent: Monday, January 02, 2006 12:00 AM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] Help w/integration problem
> Does anyone have a piece of C (BASIC, whatever) code that turns
> an array of
> dBc/Hz values into integrated RMS noise?
> I'm trying to use a simple rectangular integrator to divide a log-log plot
> into "bins":
>   for (i=L_column; i < U_column-1; i++)
>      {
>      sum += ((value[i] - ((value[i] - value[i+1]) / 2.0)) *
> (frequency[i+1] - frequency[i]));
>      }
> This just takes the midpoint dBc/Hz value between successive columns of a
> phase-noise plot, multiplies it by the frequency step between the
> columns in
> question, and sums the result for all columns in the range of interest.
> The output of this process, when I feed a typical noise graph with values
> around -110 dBc/Hz to it, with frequency values at the lower and upper
> limits of 1000 and 10000 Hz, is around -1E+6.  What I'd *like* is a value
> corresponding to the "-63 dBc" value cited on pages 7 and 8 in
> this Zarlink
> app note:
> http://assets.zarlink.com/CA/Phase_Noise_and_Jitter_Article.pdf
> In this note, the author shows a noise curve similar to the ones
> I'm working
> with, and magically pulls -63 dBc out of the ether with no explanation of
> the integration process that obtained it.  (What does it mean, in the
> author's words, to take the area "under" a phase-noise curve, anyway?
> What's the bottom dBc/Hz value?)
> Being from the instant-gratification generation, I really don't want (and
> won't understand) a calculus lecture.  I want the 5 lines of code that do
> the integration. :-)  This is for the next release of my freeware GPIB
> noise-measurement app, so your karma will be integrated along
> with the noise
> if you're able to help!
> -- john, KE5FX
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