Topband: SDR Mythbusters - ADC Overload myths debunked...

[Richard (Rick) Karlquist]

>   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