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_...@msn.com; topband@contesting.com > From: rich...@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 > > > > > _________________ Topband Reflector Archives - http://www.contesting.com/_topband