Re: Summing Noise Sources

Phil Allison wrote:

"Nick Gorham"

Phil Allison wrote:


Following on from the " Familiar formula ? " thread:

If we have two or more random ( band limited) noise sources ( be they pink, white or whatever ) and we sum them, then the TOTAL noise is found by either summing the individual power levels OR by taking the RMS voltage of each noise source, squaring the values, summing the results and then taking the square root of that sum.

The latter gives a total RMS noise voltage while the former gives the total noise power.

OK ??

But what about the peak value ?????

Any steady noise source will have a "peak to average ratio" or Crest Factor ( CF) - which is the number ratio of the magnitude of the peak value to the steady RMS voltage level.

This is my point,



** Now, put back the line you just snipped.

Cos I wish to rub your smelly nose in it.





........ Phil



Sorry, I assumed you knew what the rest of your own post said. But here you go.

The CF for band limited pink noise is often quoted as being about 4 or 12 dB.

But if you sum two pink noise sources of the same average amplitude, the peak voltage value should double. I say this because there will be regular points in time when BOTH noise sources attain maximum ( or near maximum) values and have the same sign.

So, for the sum, the average power is double but the peak power is four times that of a single source.

Sounds like the CF of the sum has increased by a factor of sq rt 2 - ie from 4 to 5.65.

With more independent sources it gets even worse.

Is anomalous - no ??


Ok, what in there helps? Do you mean the "is quoted" bit? I don't see any difference, just because its "quoted" how does that make any difference. My question is still as valid. Just what does "peak value" mean in respect to a true noise source, not the amplitude limited one that you seem to require.

Of course I was ignoring the novel idea that uncorrelated noise sources will at "regular" points do anything. That a strange idea of random to my way of thinking.

--
Nick
.

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