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

[Tim Shoppa]

The opinion expressed in the Flexradio editorial about noise blankers being
completely harmless, is not an opinion, it is even false.

What Flexradio says: ""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".

What I have found: I have often had to resort to noise blankers for reasons
varying from the Russian Woodpecker to local 120Hz utility RFI. Noise
blankers can be incredibly effective when there are only very weak signals
on the band. But if there are other loud signals on the band (aka any
contest), mixing from simple blanking action starts mixing all the other
loud signals together and smooshing them everywhere across the band.

Looked at from a "reciprocal mixing" standpoint, think of blanking (either
because of noise blanker or ADC overload) as chopping your LO signal. This
causes wide wide noise sidebands in your effective LO. When there are only
weak signals on the band you probably do not notice the reciprocal mixing.
But the reciprocal mixing quickly becomes a limiting factor in contest
conditions.

I think some of the other Flexradio opinions are similar - when there is
only a single weak signal they may have some validity. But the instant you
start "removing samples" when there are loud signals on the band, you cause
mixing and intermod amongst all the loud signals.

There have some improvements in noise blanking in recent years. The best
DSP noise blankers - the K3 and Orion in particular comes to mind - seem to
make some effort to reduce reciprocal mixing by (I think) shaping the
blanking pulse to reduce the bandwidth of resulting LO sidebands and
limiting the range over which the reciprocal mixing action happens. Again,
this DSP noise blanking only works if your DSP has the dynamic range to
begin with.

Tim N3QE

On Tue, Oct 6, 2015 at 7:56 AM, James Rodenkirch <rodenkirch_llc at msn.com>
wrote:

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> From Stu, K6TU
>
> As with any technological change, there are many myths, past truths or
> part truths that get repeated endlessly and out of context by those
> frightened or challenged by the change.
>
>  Software defined radios are no exception to this in the world of Ham
> Radio.
>
>  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
>
>
>  Here is an extract of the first few paragraphs to whet your appetite -
> well worth a read as extending education on the world of Software Defined
> Radios…
>
>  Stu K6TU
>
>  ADC overload myths debunked
>
>  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)...
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>
>
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