I'm not an RF engineer so any "direction" or "stay inside these boundaries,
Dood" assistance is appreciated! However, as a Systems Engineer with a
background in statistics, I missed the "what?" factor when he mentioned Central
Limit Theorem....I shoulda put on my "Huh?" hat so I'm almost
embarrassed....hihi
I re-read the "explanation," Rick and Tom, and walked away with a clearer -
"clearer" = more questions - view of its content.
The "Jupiter effect" is another one I shoulda looked for that hat as well anf
headed to Google to see how I could link that statement/phrase to ADC overload!
Thanks to you and Tom for your comments that caused me to re-read it and end up
at a point I shoulda arrived at earlier ---- Sherwood Engineering's rcvr
measurements are not there to debunk myths.....they support real
comparisons/analysis.
72 de Jim R. K9JWV
> Subject: Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
> To: rodenkirch_llc@msn.com; topband@contesting.com
> From: richard@karlquist.com
> Date: Sun, 11 Oct 2015 00:10:56 -0700
>
> > Steve Hicks, N5AC and the VP of Engineering at FlexRadio
> > has posted an excellent explanation and bust of the
> > ADC overload myth on the FlexRadio community. You don’t
> > need to be registered on the community to read this excellent write up:
> >
> >
> > https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link
> >
>
> I have no experience with Flex Radio equipment,
> (it might be great stuff for all I know),
> so I will confine my comments to the theory
> discussed in the "ADC overload myths debunked"
> paper. A lot of what I read didn't make a
> lot of sense to me, or seemed irrelevant.
>
> To begin with, I'm not sure as to the exact
> nature of the "myth". Initally, the myth is
> supposed to be that hams think average power
> of an ensemble of uncorrelated signals is
> the sum of the power of the components. This
> is not a myth, it is true. Then it is suggested
> that hams believe peak voltages add up, as
> in a 6 dB increase for two signals. Supposedly,
> hams don't realize that the high peaks only
> occur rarely. I'm not aware of any ham lore
> exhibiting this misunderstanding.
>
> The discussion of crest factor obscures the
> fact that average power still adds. 100 signals
> at S9 still has a power of 20 dB over S9, on
> the average. Once in a while it looks like 40
> dB over S9. The rest of the time, the combined
> power of all the signals still tests the
> dynamic range of the receiver. It's not like
> a bunch of S9 signals is no worse than a single
> S9 signal.
>
> Then there is this statement:
>
> "The individual data points that make up a signal
> you are listening to are almost never going
> to fall in the same time as the overload, statistically."
>
> I have no idea what this means in terms of
> Nyquist sampling theory. The paper goes on to
> say:
>
> "With a noise blanker, we remove thousands of samples
> with no negative effects to the signal being
> monitored and a momentary overload from the
> addition of many signals summing up will have a
> much lower effect"
>
> I don't know whether this means Flex (IE "we") has invented
> some sort of magic digital noise blanker that removes
> samples corrupted by overload (I'm skeptical) or
> whether it means that a noise blanking effect
> just happens as part of the sampling process
> (in which case, I'm still skeptical).
>
> Then the subject shifts to decimation and "processing
> gain", which are simply references to digital filters.
> These techniques are all based on linearity. Adding
> digital filtering after a nonlinear front end cannot
> repair the damage caused by nonlinearity. Just
> like adding crystal filters to the IF in an analog
> receiver won't overcome front end overload caused
> by enabling the receiver's built in preamp.
>
> There is an assertion that the large amount of
> "noise" added by hundreds of signals results in
> "linearization", which I believe is referring to
> what is usually called "dithering". This is a
> complete misunderstanding of dithering, which uses
> small amounts of noise and does not involve clipping
> in the ADC. High quality ADC's have dithering
> and similar randomization processes built in and
> don't need help from external noise anyway.
>
> The paper then changes the subject to phase noise.
> This has nothing to do with ADC overload. I will
> note that digital radios are much more sensitive
> to clock jitter (IE phase noise) than analog radios.
> If anything, the phase noise issue is an argument
> against digital.
>
> There are various distractions such as the Central
> Limit Theorem and the Jupiter effect that don't
> add much to the discussion.
>
> The dubious argument is made that the
> existence of 1000's of receivers in the field
> without complaints from their owners "proves" that
> overload problems do not exist. Until last
> month, we could make a similar statement about
> the millions of satisfied Diesel Volkswagen owners.
>
> The concluding statement is quite a stretch:
>
> " it is simply mathematically true. FlexRadio Systems
> makes the best amateur transceivers available."
>
> Mathematically true? Maybe it's that new Common Core
> math.
>
> Rick N6RK
>
>
>
>
>
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