ADC overload myths debunked

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I've received some feedback that there is some confusion circulating on other ham radio reflectors regarding how analog to digital converters (ADCs) work in radio applications.  Specifically, some of the comments tend to say that direct sampling ADCs just won't work in strong signal environments so I'd like to explain why this is not factual for those who are interested. I have a few points to illustrate this.

As hams we tend to think of strong signals in terms of their total power, how many total Watts they are.  When you think of signals in this way, you can add their power in your head and think: two -10dBm signals add to -7dBm total power (3dB increase).  In fact, you can take multiple signals and add them together in a power meter and the power meter will show the total power of all signals.  But this is the average and not instantaneous power.

An ADC, on the other hand, is really a discrete signal device.  All of the signals get chopped into samples and so the real question is: how do the signals add together in the discrete time domain?  To answer this, we have to look at the signals and how they interact.  An RF carrier is like any AC signal -- it is a sine wave that varies from negative to positive voltage along the curve of a sine wave.  If we add two sine waves of exactly the same amplitude, frequency and phase, the peak voltage will be doubled (6 dB).

But two signals of the same amplitude and phase on the same frequency isn't reality.  Reality is signals all across the bands that are totally unrelated (uncorrelated) -- for example one at 14.100374 and another at 21.102392, etc.  The variance of the algebraic sum of these signals will decrease with the square root of the number of signals present.  As more signals are added, there is a decreasingly small probability that these signals will add (precise alignment of the highest voltage peak of the signals) and the algebraic sum of the signals will degenerate into a quasi-Gaussian distribution.  To get a fabled 6dB voltage rise, they would have to already be exactly the same voltage, frequency and phase (this is what is done in a power combiner in an amplifier and it’s hard to make that happen).  If one is stronger, the addition of a weaker signal will not add much to the total level.  

If we're talking about a large number of signals across a wide spectrum, it's the same situation.  They would virtually never all add at the same time so they will not combine at just the point where the peak of all signals occurs.  It just doesn't ever happen.  As a mathematician friend of mine pointed out, the two primary principles involved are the Law of Large Numbers (https://en.wikipedia.org/wiki/Law_of_large_numbers) and the Central Limit Theorem (https://en.wikipedia.org/wiki/Central_limit_theorem) which you can peruse for more insight.

As an intuitive analogy, we could look at our solar system.  Let's discuss the likelihood that the planets will cause the ocean to rise and cover up the state of Hawai'i. The planets all have their own period around the sun (frequency).  They are all different amplitudes as well (gravitational influence on the Earth if we're thinking about rising tides).  The questions are:

1) How often do all the planets align?
2) When they do align, will the ocean cover Hawai'i (overload)

There was a book published on this in the 70's called The Jupiter Effect (https://en.wikipedia.org/wiki/The_Jupiter_Effect) which proclaimed death and destruction when this was to occur.  The book was, of course, proved wrong but not before it became a bestseller.  First, the planets almost never come into alignment -- even in the book the planets were only going to be on the same side of the sun, within a 95-degree arc.  Second, when they do align, the amplitude from the outer planets is so low, it just doesn't matter.  My college physics professor was asked about this problem and worked the equations and showed that even if they were all in precise alignment, the ocean would rise by an additional 1/4" briefly... just not worth worrying about.  It is the same situation in ADCs.  The real truth is that more and stronger signals actually make an ADC work better through a process called linearization.  Everyone that has studied ADCs knows this -- the irony here is that lots of strong signals are a benefit, not a detractor like they are in old technology superheterodyne transceivers where IMD dynamic range degrades rapidly with signal strength.  Translation: Strong signals -- Bring it!

Another point to make is that all overloads are not created equal.  Overload sounds like an undesirable situation, but a momentary overload has no significant effect on a direct sampling radio.  Why is this so?  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.  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.  This effect is called "soft overload" because momentary overloads just don't have an impact on the radio.  It takes much more significant and sustained overloads to cause a real problem.  The overload that folks are talking about is a non-event.  Even if it did happen, it's not going to affect the radio's performance.

Finally, there's often confusion about dynamic range from wideband ADCs.  The confusion generally works like this -- someone will lookup a data converter that runs at 100MHz and see that it has a dynamic range of 70dB and assume that it could never beat a radio with an 85dB dynamic range.  The problem is that this is an apples and oranges comparison.  You cannot talk about instantaneous dynamic range without talking about detection bandwidth.  For ham radio, this is the width of the actual receiver.  We use a standard 500Hz bandwidth receiver for comparison purposes but it could be 2700Hz for sideband or 50Hz for CW, for example.  

What really happens is that we use a process called decimation ( https://en.wikipedia.org/wiki/Decimation_(signal_processing) ) which takes the data collected at an oversampled rate (100MHz for example) and then systematically reduce the sampling rate down to the bandwidth of interest.  In this process dynamic range is increased in what is called "processing gain" (http://www.dsprelated.com/freebooks/sasp/Processing_Gain.html).  In the FLEX-6500 and FLEX-6700, we operate the ADCs at 245.76 Msps so that the typical processing gain is on the order of 56dB.  When added to the 75.5dB quoted spec of the ADC, the calculated instantaneous dynamic range is on the order of 132dB.  This far exceeds the dynamic range of ALL superheterodyne receivers (Don’t believe what you read about blocking dynamic range as it is irrelevant if the radio falls apart due to phase noise before this level).

In reality, it is impossible for any receiver to have blocking dynamic range or IMD dynamic range greater than its phase noise dynamic range (PNDR) otherwise known as reciprocal mixing dynamic range (RMDR).  In all cases and no matter the architecture, if RMDR is less than BDR or IMD DR for a given tone spacing, the phase noise will cover the signal of interest before blocking or IMD will be a factor.  In fact there is not a single transceiver from any manufacturer on the market that would not have its blocking dynamic range limited by its internal phase noise much less first by the noise from the transmitted signal.  

Most of the old technology superheterodyne transceivers on the market have horrible RMDR numbers.  When a strong signal is heard by them, their oscillators spread the signal all around the band as noise covering up signals you are trying to hear.  Here's the simple test: Take two of your favorite legacy radios and transmit in one while listening in the other and watch what happens to the noise floor at 2, 10, 20, 50 and 100kHz from that signal.  You will see that these receivers show significant noise floor increases that prevent operation near each other.  This is the practical concern -- there's no reason to talk about a number of mythical strong signals of all the same power that might correlate to cause an overload in a new type of receiver... the real problem is the superheterodyne receiver that folds under a single strong signal in the vicinity of small signals you are trying to copy.  Most contesters have experienced this first hand when two radios are being used.  If you have to tell your operating buddy in the same band to stay so many kHz away from you, you know the problem well.  This is also a classic Field Day problem.

We have thousands of radios in the field and if any of these issues were real, we (and you) would have heard about it.  You should have confidence that you have the best transceiver on the market -- experienced and knowledgeable people have said so.  They have said so because it is proven out in test after test and it is simply mathematically true.  FlexRadio Systems makes the best amateur transceivers available.
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Steve - N5AC, VP Engineering / CTO

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Posted 3 years ago

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DrTeeth

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Thank you for another article clarifying an otherwise difficult topic to this (almost) OM.

It is a rare talent that you have. Keep 'em coming.
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Winston VK7WH

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Well said. I second that

Winston VK7Wh
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Lee, Elmer

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I have confidence as well as experience  :)

73  W9OY
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Bob G W1GLV

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You cannot beat an engineer that knows what he is talking about. Fine job Steve.
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Corey/ KC0YNS

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Thanks Steve,  Spoken like a true engineer, Over my head, But I'm learning....
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Bill -VA3WTB

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I just love it when he does that!!!
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N4HY, Elmer

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Steve it is awesome, just awesome to have watched the growth of your understanding and the never ending quest for knowledge that drives you every day of your life.  I want to know when you want to come and guest lecture my communications theory students.   Spock to Kirk: "You have been and always shall be my friend".
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N4HY, Elmer

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Boris doesn't understand the difference between overload of a power amplifier with the crest factor problem in a power amplifier in the sum of few hundred channels and the overload of a ADC on receive. It's a poor attempt at sarcasm that fails to know it is off the mark.
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N4HY, Elmer

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Like my sideways picture
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Tim - W4TME, Customer Experience Manager

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YES!  Quadrature Bob.
(Edited)
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Greg - K5GJ, Elmer

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Dr. Bob has ALWAYS followed his own vector!
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N4HY, Elmer

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What's your vector Victor?
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Greg Zenger [N2GZ]

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QED
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Barry N1EU

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Thanks Steve, great post.  My feeling all along is that the guys on the E reflector are just missing the boat.  The days are numbered for superheterodyne front-ended flagship hf xcvrs.
(Edited)
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Steve Conklin

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Thanks, Steve !

Please make this available as a paper or pdf posted somewhere outside the community site, as I don't think that the people who could most benefit from this are members here.
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Peter K1PGV, Elmer

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Super interesting.

The ability to clearly describe complex technical concepts this well is rare. Well done, Steve.

Peter
K1PGV
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DrTeeth

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We need to have a "Steve's Corner" for all of his great posts. Then, when there are enough, they should be (self-publushed?) as a book. I'd be the first in the queue to buy it.
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Mike - W8MM

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Jupiter and Hawaii was a great analogy to vivify the explanation.  Great writing!!!!
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Peter Bentley

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 FlexRadio Systems makes the best amateur transceivers available.

Agreed..... Thank you Steve

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KD4HSO

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Great write-up.  I take it, from the 75.5 dBFS SNR, you guys are using the AD9467-250?  Can you share what clock synthesizer you are using?
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KD4HSO

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I thought jitter set the ENOB (effective number of bits) from the ADC, which I assume is around 12, which limits ADC dynamic range. So for closely spaced tones, which is main concern in this application, does the phase noise mask directly image onto the received tones, just like a standard superhet?
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Steve - N5AC, VP Engineering / CTO

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In a well designed synthesized superheterodyne transceiver, the low noise master clock will generally be the highest frequency where you can acquire good phase noise.  Then the this clock is divided down in either a DDS or with a PLL (the former has significantly better phase noise properties).  As you go lower in frequency, the division effect in the synthesizer produces better phase noise -- it is not generally consistent across the served frequency range.

In a direct sampling receiver, the phase noise at the sampling clock is turned into phase noise in the receiver at the sampling frequency.  This means that the phase noise present at the given offsets is not divided down for better effect.  This makes the phase noise requirement on a direct sampling clock much higher than that for a synthesizer.

Jitter is integrated or averaged phase noise.  It does not tell you, specifically, at what offsets you are most likely to observe the clock at any given time.  The type of source clock will have a phase noise pattern based on how the clock functions, if it locked to a source, the loop bandwidth on any PLL locking the clock, etc.  The detailed phase noise allows us to calculate noise floor rises at offsets from carriers that just knowing the jitter would not allow us to do.  It also provides information on the application that is useful.  

In HF radio applications, phase noise to about 100kHz is important because we have strong signals in that range, especially in FD and M/x applications.  In microwave (say 10.3681 GHz operation) though, we are significantly more concerned with close-in phase noise.  Because clocks are multiplied up, a small offset (10Hz) may end up being 1kHz after multiplication and phase noise here could interfere with a CW single close to you.  On HF, 10Hz phase noise isn't generally as important.
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Gerald - K5SDR, Employee

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Note that the lab measurement that will tell you performance in this area is Reciprocal Mixing Dynamic Range (RMDR).  This will usually be the limit of performance for single tone blocking dynamic range in most all modern ham receivers that I am aware of no matter the architecture.  If RMDR is less than BDR, noise will dominate to cover up weak signals.  You can directly compare these measurements in QST reviews at 2 kHz, 5 kHz and 20 kHz offsets from the carrier.  RMDR for the FLEX-6500 and FLEX-6700 is 116/117/124 dB respectively.  For the FLEX-6300 it is 116/117/121 dB respectively.
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KD4HSO

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Wow, thanks for the detailed responses guys; your are true engineer's engineers!  I have lost hair over TCXO for narrow band microwave comms for just what you stated.  Ended up characterizing several manufacturers SMT TCXO for phase noise below 10 Hz; they never publish that and they are not all equal, and some use digital vs analog temp comp (been burned by digital with phase hits over temperature).

I assume you'll be using ultra-low phase noise optical oscillators for the 7000 series ;-)

http://www.oewaves.com/technology
(Edited)
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N4HY, Elmer

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I know the OEWaves people. They are friends of mine. Lute Maleki and Alfred Pais have done a great job. To finish the work of applying the whispering gallery mode resonators to Flex, well, ithe might currently cost about 1/3 of the entire value of Flex just for NRE and no one could afford the radio if the NRE were donated!
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EA4GLI - 8P9EH - Salvador

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Hi Steve. Great article. I took the liberty to translate it into Spanish and publish it in the URE.es forum giving you credit as author. I hope you are ok with it. Otherwise I can promptly remove it.
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Tim - W4TME, Customer Experience Manager

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This technology description is also available as a link on our HelpDesk
https://helpdesk.flexradio.com/hc/en-us/articles/205527943-ADC-Overload-with-Direct-Sampling-SDRs-My...
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EA4GLI - 8P9EH - Salvador

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Do you prefer if I link to the helpdesk url instead of this forum post?

Thanks Gerald.
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Tim - W4TME, Customer Experience Manager

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Salvador - either way will be fine. 
Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

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OK Thanks Tim. FYI here is the post on ure.es http://www.ure.es/foro/6-tecnico/223735-todo-flex-y-smartsdr.html?start=60#291760

I try to keep the spanish hams abreast of Flex updates on that forum. URE is Spain ́s ARRL. Not all of the EA understand English and Google translate butchers technical texts. For example, it translates Hams as the cured meat and not as amateur radio operators. :)
(Edited)
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Tim - W4TME, Customer Experience Manager

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Muy bien mi amigo. Hams no son barbacoa!
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Ken - NM9P, Elmer

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I have really enjoyed this thread.  It has been very informative, and it adds to my appreciation of my 6500.  I really appreciate that several of the staff at FRS, including Lead Engineers, Customer Experience Managers, and even the President of the company are willing to take the time to converse with their end-users.  How cool is that?  Awesome! 
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Peter K1PGV, Elmer

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What Ken said!

I don't even *understand* the details of Steve and Gerald's posts, and I *still* enjoyed reading them.  As Ken so aptly said "it adds to my appreciation"... and I love hearing about the factors that are germane to areas of engineering other than my own (OS software). 

Thanks guys!

Peter
K1PGV
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Bill -VA3WTB

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Oh, don't all the other company's talk to their customers directly? LOL
(Edited)
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EA4GLI - 8P9EH - Salvador

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Well... maybe the others do, but in Japanese and we just don't know about it....
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Walt - KZ1F

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To be sure, more Japanese can speak fluent English, or American in Sarah Palin's world, than there are Americans that can speak Japanese.

However, if the goal is to have no one besmirch FRS or their products, people here shouldn't be casting aspersions on other vendors or their products.
(Edited)
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EA4GLI - 8P9EH - Salvador

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Walt, 
Was you reply directed at my statement? Because if that is the case I think you misunderstood my point.

I meant to literally say what I wrote, that maybe there are forums in Japanese from Yaesu, Icom and Kenwood where the owners "talk" to hams and we just don't know about it because of the language barrier. I know for a fact that a lot of people in Spain are not very aware of the ins and outs of SDR because there is very little information in Spanish. many people can understand basic English, but when it gets technical it is difficult to follow if it is not your mother tongue.
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Tim - W4TME, Customer Experience Manager

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Good point.  Those of us who are monolingual native English speakers sometimes miss this subtle, but important point.
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Walt - KZ1F

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Sorry Sal. No, that was not directed at you. I just now saw your question to me. My comment was more an observation that, I believe this country is unique on the planet in the percentage of its population unable to converse in any native tongue other than 'American', to once again jab at Sarah.

I try to but I don't get to converse much to German folk. I do try to use Italian when conversing with Enzo though.
(Edited)
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Bob - W7KWS -

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Steve,

Your comments on ADC overload are helpful & I've watched the video of your Chicago TAPPR presentation. The simulated signal traces that Gerald did added a lot of clarity. However, decimation remains a vague mathimatical concept for many of us. Could you please give us one of your very good, non-math explainations on how decimation equates to more ADC sample bits? This would go a long way in closing the loop for those I'm discussing this topic with.

Thanks!
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Steve - N5AC, VP Engineering / CTO

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Bob, ok here goes:

Suppose that you have a device that samples at 80sps (samples a second) and is an 8-bit device.  Now let's say that we want to decimate down to 40sps.  We take samples 1 and 2 and turn them into a new sample 1A.  Let's say the samples are each equal to 1 (binary 00000001).  In this case, our sample 1A could be set to 1 as well.  Now we look at samples 3 and 4 and they are equal to 1 (binary 00000001) and 2 (binary 00000010).  In this case, we have to pick either 1 or 2 or something in between.  So we decide that we will add one bit BELOW the decimal point and set this bit to a 1.  The resulting binary representation would be 00000001.1.  The bit just to the left of the decimal point is the "1s" bit and the bit just to the right is the "1/2s bit" so the value of this is 1.5.

This continues and the net effect is that we pickup a 1/2-bit of resolution each time we decimate, but the sampling rate is lower by a factor of two.  After this first decimation, we now need to carry 9-bits instead of 8 and we've added 3dB to our dynamic range because we can now instantaneously represent 8.5-bits of information instead of 8.  In the FLEX-6000, we pickup about 56dB of dynamic range going from 245.76Msps to 24ksps.

Does that help?
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W1RE

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Excellent explanation Steve.

Thanks!

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Bob - W7KWS -

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Thanks Steve.

I'll probably look to you later as I try to determine the impact, if any, of the samples being taken at different instants & then combined.

You have given us plenty to chew on for the time being. Hopefully more of this will sink in before I see you in Dayton.

Best regards!
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Barry N1EU

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Barry N1EU

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yup  ;-)
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EA4GLI - 8P9EH - Salvador

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Looks like a fake to me.... let me know how it pans out.
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Barry N1EU

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Sal, we're just having fun
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EA4GLI - 8P9EH - Salvador

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:)
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Sergey, R5AU

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LOL
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Dan -- KC4GO

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W4TV isn't more that 25 miles from me and the only band he might get -130 signals is 10 or 6 meters 
 "Try copying a -130 dBm signal among multiple -30 to -50 dBm (S9+20 to S9+40 dB) amateur signals on 160 with a dozen AM signals also 
present at -30 dBm to 0 dBm at the receiver input.:" is in his dreams  This morning it's -100 with -10dB att. Flex 6500 Non resonant antennas.
When the band is open (160 the noise it much higher)   
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N4HY, Elmer

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Joe is a well known ham (microHam).  Flex is a major threat to sales of lots of equipment by lots of different people.  It is a clear indication of how far behind we are in certain technology areas that things I was doing years ago, using commercial parts, is disruptive hardware technology.  Flex is a terrific software company and this is most of the disruptive technology.  The hardware is mostly just superbly engineered.

Joe will get over it soon enough since he has no choice.
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Dan -- KC4GO

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I got my long wire to tune to 1.850 and got a noise level of -67 S9+ (From 6500 S-Meter) in Casselberry Fl 12.64 miles from Geneva, Fl Location of W4TV
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Lee, Elmer

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W4TV I think owns Microham and makes SO2R "glue" so when he talks about "talking book" it is actually he who is "talking his book".  He lives about 15 miles from me and I've talked to the guy and he is definitely a legend in his own mind and rabid on the K3.  

As far as 160 noise goes, I get -103 dBm on my dipole and -108 dBm on my vertical with WBN turned on but I live in a area of very low noise and my neighbor to the east is the Atlantic ocean.  His QTH is pretty rural as well so he may also have a pretty good noise floor, but I'm not sure what he is running for antennas
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Craig K9CT

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I used a Flex 6700 in the ARRL 160 contest. I did very well...the only people ahead were multiop or east of me by at least 800 miles. I did something very unusual in that I did SO2R on one band and had the second receiver on the band for copying while I transmitted on the run frequency. Full duplex operation. The RX antenna was 1100 feet from the TX antenna. I had another RX antenna on the run slice and that antenna is only 300 feet away...and it is not disconnected while TXing. At no time did I have IMD or burn up my radio. I don't have a noisy location but the band has way more noise than necessary for using a preamp. I insert attenuation all the time to max the dynamic range.

Next week I am selling every one of my microHam MKIIs. I don't need them any more.  Poor Joe....

73, Craig K9CT

 
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Lee, Elmer

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Craig your comment on the contest is telling regarding the future of SO2R! 

I also use the built in attenuation in my Flex for 160  Worked P40L tonight barefoot and he's S9 here, and I can hear 3XY1T so the band is pretty good

73
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N4HY, Elmer

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I was not going to claim his obvious conflict of interest but I agree with you completely now that you have said it.  
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w8mqw

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N5AC writes:

The variance of the algebraic sum of these signals will decrease with the square root of the number of signals present.  As more signals are added, there is a decreasingly small probability that these signals will add (precise alignment of the highest voltage peak of the signals) and the algebraic sum of the signals will degenerate into a quasi-Gaussian distribution.

This is faulty mathematics. The variance of a sum of uncorrelated random variables (signals) is the sum of the variances. You are stating the rule for the average of the signals, not the simple sum. By your reasoning, noise in a channel will decrease as you widen the bandwidth.

(Also the Central Limit Theorem involves dividing the sum of normalized random variables (signals) by sqrt(n) rather than n so as to maintain the identical standard deviations as this quotient approaches Gaussian.)

  This averaging idea is used in astrophotography, where multiple images (n > 50) are taken of the same object, then superimposed (averaged). In this case indeed the standard deviation (visual noise) decreases as 1/sqrt(n).

What you have to worry about with a ADC is the random variable

                         Y = max(X1 + X2 + X3 + ... + Xn),

whose s.d. increases with n.
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Steve - N5AC, VP Engineering / CTO

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@NO5J - I don't know -- I have a pretty positive view of the ham community.  I watch what happens when a new antenna design hits the market and I am amazed: there will be folks that immediately go buy the antenna, put it on the air, A/B it with existing designs and report what they've found.  Another group will go simulate the antenna in NEC and report on why the design does/does not work.  Still others will put it in an anechoic chamber and measure the antenna patterns.  All that info will be assimilated over time and a consensus about the antenna will form and it will take its rightful place in the ranks of antennas.  The best designs will be adopted and often achieve commercial success from performance attributes.  

It really is no different with radios, but radios seem to be more of a personal decision than antennas are.   I'm just expressing that I hope that in time, hams will find time to read up and understand direct sampling since it is and will continue to sweep across all aspects the hobby.
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w8mqw

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Steve:

  I agree with your intuition. There may be other factors as well---the flash sampling itself is an averaging process, etc. I've got to think about it. 

  Is it true that the lower the frequency of interference, the more destructive to ADC headroom?

Chuck
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Lee, Elmer

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Actually I think hams have tremendous potential to embrace the underlying concepts of direct sampling.  The key to understanding is education provided in the proper channels.  Back in the 60's hams didn't have the first clue about blocking dynamic range and 3rd order intercept, RMDR and all of the concepts that have been developed to describe a theoretical basis for thinking about receivers and comparing receivers in overload and near overload situations.  

In the 70's QST published some articles describing a standardized criteria and methodology by which to characterize receivers.  Many hams understood the topic, and many of those hams brought their understanding to their local ham clubs and helped others to understand the concepts.  So this narrow analogue understanding (prejudice) is the basis by which direct sampling is judged.  In addition there was an explosion of "direct conversion" receivers, mostly mediocre QRP rigs that occurred in the 70's.   The SDR-1000 was in some essence a direct conversion radio, and this I think has an effect as well.  Non-SDR Hams sometimes follow SDR development from a kind of arm chair perspective.  For example they may have read Gerald's QEX articles and understood SDR somehow was related to a direct conversion radio plus some kind of software/soundcard back end and promptly stopped learning at that point feeling they mastered SDR concept and it was somehow connected to those silly direct conversion radios.  The SDR-1000 in many respects is an analogue radio, so it would not be unreasonable to assume SDR's of that kind and therefore SDR's in general have the same front end vulnerabilities as a Kenwood or an Icom.  Even the KX3 (a so called SDR) is basically a digitized K1 with some kind of DSP soundcard like dongle hanging off the output of the receiver, and even the KX3 has a roofing filter.  So you can well expect the K3 crowd is going to assume the KX3 represents present day SDR thinking.  

If you want Hams to understand the power of direct sampling in today's SDR apart from the "direct conversion" of yesteryear, then you really should publish a primer, as in a QST article that introduces the average ham to direct sampling definitions and concepts and how they are different from the 1970's concepts of what constituted a "good" receiver.  

Demonstrated above in this thread are a theoretical basis/model to understand direct sampling from Steve further refined by W8MQW, an empirical model demonstrated by Gerald, and a successful experiential basis in real life situations by KY6LA, N6WM, K6TU, K9CT and others in environments which are challenging if not deadly to receiver performance.  N1DG wrote an interesting article in the SDR forum of eham describing how he characterizes the differences and KY6LA also developed a way to hierarchically understand SDR development, the problem is this stuff is not exposed to the ham community at large as would be the case in a QST article.  What you want to do is capture the interest of the ham who understands and then can bring that understanding to the local ham club.  

SDR is the future and that is clear.  Now is the time to begin to fix a correct understanding in the mind of the average ham.  If we don't do that someone else like Icom or Elecraft will.

73 W9OY
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Mike - W8MM

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Excellent observation and idea for action!!!!
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Lee, Elmer

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N1DG should be N9DG in the above