Re: 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_medium=email_campaign=new_topic_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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Re: Topband: SDR Mythbusters - ADC Overload myths debunked...

[Tom W8JI]

The opinion expressed in the Flexradio editorial about noise blankers 
being

completely harmless, is not an opinion, it is even false.



It really isn't an accurate or factual technical article. It is a sales 
brochure meant to enforce the love each of us have for a particular system, 
in this case the magic of SDR. You might as well read an MFJ advertisement 
for technical accuracy, watch a year old VW diesel commercial, or listen to 
a politician of any party telling us how they will fix everything. It is all 
sales and stoking people's emotions or existing opinions, with just enough 
truth woven in to avoid prison.


Oddly, the K3 Elecraft is largely SDR. It is an SDR receiver with a standard 
analog front end as a frequency converter with roofing filter. IMO, the 
worse part of the K3 is the SDR part.


Try this test. Poll users who work very weak CW signals routinely in all 
sorts of station environments, just the general population of weak signal CW 
DXers.  Ask them if analog detection is better for pulling a weak signal 
below noise floor out of noise, or if a DSP detection system is better.  The 
results are always that DSP detection systems are viewed unfavorably over 
good analog systems by significant number of people.


The limits of this case are digging weak signals, that are at or below 
noise, out of the noise.


There is always a certain loss of dynamics to me, when I listen to any SDR. 
It has been this way for me with any SDR or DSP detection radio. Something 
my ear depends on to know the difference between a signal below noise and 
just noise without signal is lost in every single DSP detection system I 
listen to. It melts the signal into the noise with a distortion.


We did a blind A-B test (I have audio lines that bring every receiver to a 
jack ). It is easy to tune two receivers to the same signal, and have 
someone else switch the lines.


People who cannot copy below noise signals seem to not notice this effect.

73 Tom 


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

[JC]

Hi guys

I would like to add my 2 cents on this matter. If you not  good, or a very
good, RF engineer it is hard to understand what DDU/DUC radio do for you.
The most important thing, DDC/DUC  radios are not the same just because of
the DDC. Number of bits, jitter on the clock works like phase noise from LO.
What's in front of the ADC sets the performance as well the software and the
ADC

ADC overload only can be the limit when everything before the DDC can handle
more power than the ADC itself.  For the band you are listen. Most SDR
radios are using preamps to cover  1 to 50 MHz and guess what? Poor
performance on 160m. Example a common used T50-2 toroid barely can handle
20db IP3!!! 

Most of DDC uses the Linear Technologies LTC2208 16 bit.  Or similar DDC
with 16 bit.

http://cds.linear.com/docs/en/datasheet/2208fc.pdf

These ADC devices overload near +10dBm , that's very high, its over +50db
IP3. However everything before that should be +50db better, and that is not
the case. I like the QS1R because does not have preamp and the LPF/BALUN is
quite good to handle strong signals. 

Another issue with DDC radios available nowadays is the lack of shield, they
looks like a computer board and that's it, performs like a computer board.
No good RF practice to protect the noise floor or common mode noise. Really
not ready for field use.

With all the new software development, construction and good RF design is
way behind. Lab tests on lab environment also does not tell the true value
of a radio. It gives you specific results for specific test conditions.

I believe DDC is the future, but we are not there yet.

Regards
JC
N4IS




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Re: 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 
wrote:

>
>
>
>
>
> 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_medium=email_campaign=new_topic_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)...
>  ___
> > CQ-Contest mailing list
> > cq-cont...@contesting.com
> > http://lists.contesting.com/mailman/listinfo/cq-contest
>
>
>
> _
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>
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Re: Topband: SDR Mythbusters - ADC Overload myths debunked...

[James Rodenkirch]

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 TheoremI shoulda put on my "Huh?" hat so I'm almost 
embarrassedhihi
 
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_medium=email_campaign=new_topic_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

Re: Topband: SDR Mythbusters - ADC Overload myths debunked

[Jim Brown]

On Sat,10/10/2015 2:15 AM, Steve Ireland wrote:
Of course, I was talking about using a DUC/DDC in a single 
transmitter/receiver urban/semi-rural station setting on a quarter to 
half an acre block, which is the setting in which the vast majority of 
people who subscribe to this reflector would be using a transceiver.


Your use is not nearly so much a majority as you might suspect. A fair 
percentage of contesters operate SO2R. MANY hams live in proximity to 
high power broadcast stations. Hams with good antennas in locations 
exposed to many in-band and out-of-band signals are likely to encounter 
far higher voltages at the input to the radio than you do. VK, for 
example, is exposed to far fewer in-band signals than EU or the eastern 
half of NA.


That said, there's one element of the RX system that is often overlooked 
in this discussion -- the bandpass filters that are a part of virtually 
any competent multi-transmitter station. So, while the radio itself 
lacks a pre-selector, the outboard bandpass filter can provide the 
pre-selector function.


So far, the only SDRs I've used have been K3s and KX3s. I like them for 
their combination of features, RF performance, and user interface. What 
I've seen of the UI for other SDRs makes them OK for casual operation 
but useless in my station. That's something that, IMO, must also be 
greatly improved before I'd consider using one of them.


73, Jim K9YC
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Re: Topband: SDR Mythbusters - ADC Overload myths debunked

[Tom W8JI]

On Sat,10/10/2015 2:15 AM, Steve Ireland wrote:
Of course, I was talking about using a DUC/DDC in a single 
transmitter/receiver urban/semi-rural station setting on a quarter to 
half an acre block, which is the setting in which the vast majority of 
people who subscribe to this reflector would be using a transceiver.


Your use is not nearly so much a majority as you might suspect. A fair 
percentage of contesters operate SO2R. MANY hams live in proximity to high 
power broadcast stations. Hams with good antennas in locations exposed to 
many in-band and out-of-band signals are likely to encounter far higher 
voltages at the input to the radio than you do.


I thought they were talking about overload from local transmitters.

If you look at Sherwood Engineering's tests, they show the problem we had. 
The wide spaced dynamic range of SDR's is only 96-99 dB.


The wide spaced dynamic range of the K3 is up around 105 dB. That 5 or 10 dB 
is helpful with local transmitters, but the real difference we noticed was 
the SDR just totally goes goofy when it overloads, losing everything, while 
the regular receivers just "noise up" or de-sense.


Overload characteristics for strong signals have nothing to do with how a 
receiver works without strong signals to overload it.:)


73 Tom 


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Re: Topband: SDR Mythbusters - ADC Overload myths debunked

[Tom W8JI]

None of the below has anything to do with overload from local transmitters. 
:)


Or did I miss that part?

- Original Message - 
From: "Steve Ireland" <vk...@arach.net.au>

To: "Topband reflector" <topband@contesting.com>
Sent: Tuesday, October 06, 2015 9:51 PM
Subject: Re: Topband: SDR Mythbusters - ADC Overload myths debunked



G’day all

As someone who has extensively used a digital down conversion/digital up 
conversion transceiver (original HPSDR) on the Topband, it is good to see 
this myth being debunked publicly as much as possible. Another station who 
has used an HPSDR even more extensively than me on Topband is Greg ZL3IX, 
who may be interested in commenting.


For about 18 months I used the HPSDR transceiver alongside a ‘benchmark’ 
Elecraft K3 in contests and for DXing, alongside keen contester/DXers 
VK6LW (about 3km away) and VK6DXI SK (about 30km away) running full power 
on Topband and the other bands.  I could work equally close (within 500Hz) 
to Kevin and Mirek on the HPSDR as I could on the K3.


The brick-wall sided constantly variable filtering provided by the fully 
digital HPSDR and its associated OpenHPSDR software and the way this 
filtering handled noise in comparison to the K3’s digital filtering made 
the HPSDR a superior radio for weak signal DXing – simply put, I could 
read S2/S3 signals on the HPSDR in a 50Hz bandwidth that were unreadable 
on the K3.


The only problem for me is that using computers day in, day out at work, I 
loathe using them for radio and hate spending my evenings fiddling around 
with software, so the HPSDR has finally gone (mind you, so has the K3 – 
loved the radio but hated the audio and went and bought an old Orion 2 
which sounds waaay better to my ears).


DDC/DUC radios handle strong signals as well as the best of analogue 
radios – the only question for topbanders is whether you like their 
ergonomics and the whole idea of having a digital radio.


Vy 73

Steve, VK6VZ


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

[W5JR Mike]

Mike, what version of SSDR is he running? I believe the current 1.5 release has 
a reported issue on transmit with some (all?) 6300's. I understand a point 
release is being readied to address that. I suppose he could go back to 1.4.6 
in the meantime. 

I rarely ever plug a mic into mine. 

Disclaimer - I don't work for Flex Radio but I've been known to alpha test SSDR 
releases. 

tnx
Mike / W5JR
Alpharetta GA

> On Oct 6, 2015, at 7:30 PM, Mike Waters  wrote:
> 
> I bought a partially-assembled SDR kit a few years ago. It's an older
> Softrock Xtall Lite 9.0 with the optional electronically-switched BPF,
> designed by Tony Parks. Well under $100 for the kit. I just "knew" that it
> had to be a cheap, compromise SDR receiver prone to overload, birdies, and
> anything else you can think of. The expensive ones like the Flex must run
> circles around it; you get what you pay for, right? ...
> 
> Well, in the process of learning what I need to know to finish this little
> tiny SDR and get it working, I'm hearing that it beats the receivers in the
> Flex 5000(A?) and the Flex 1500. (Owners of expensive Icom transceivers
> with SDR RX are using it for receive because the Softrock is superior.) The
> sources of this info appear to be reliable, but of course time will tell.
> (Newer versions of the Softrock are basically the same as what I have, but
> with attenuators in series with the 160 and 80 BP filters. And some
> transceive.)
> 
> As for the Flex 6300, a friend of mine has one. Maybe it has a better
> receiver than the earlier Flex SDRs, but its SSB signal is absolutely awful
> to my ears, no matter what mic he uses or how he adjusts the audio EQ
> settings. His is not the only one. You can read all about it at
> www.eham.net/ehamforum/smf/index.php?topic=104013.0
> 
> 73, Mike
> www.w0btu.com
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> Topband Reflector Archives - http://www.contesting.com/_topband
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Re: Topband: SDR Mythbusters - ADC Overload myths debunked

[Steve Ireland]

G’day all

As someone who has extensively used a digital down conversion/digital up 
conversion transceiver (original HPSDR) on the Topband, it is good to see this 
myth being debunked publicly as much as possible. Another station who has used 
an HPSDR even more extensively than me on Topband is Greg ZL3IX, who may be 
interested in commenting.

For about 18 months I used the HPSDR transceiver alongside a ‘benchmark’ 
Elecraft K3 in contests and for DXing, alongside keen contester/DXers VK6LW 
(about 3km away) and VK6DXI SK (about 30km away) running full power on Topband 
and the other bands.  I could work equally close (within 500Hz) to Kevin and 
Mirek on the HPSDR as I could on the K3.  

The brick-wall sided constantly variable filtering provided by the fully 
digital HPSDR and its associated OpenHPSDR software and the way this filtering 
handled noise in comparison to the K3’s digital filtering made the HPSDR a 
superior radio for weak signal DXing – simply put, I could read S2/S3 signals 
on the HPSDR in a 50Hz bandwidth that were unreadable on the K3.

The only problem for me is that using computers day in, day out at work, I 
loathe using them for radio and hate spending my evenings fiddling around with 
software, so the HPSDR has finally gone (mind you, so has the K3 – loved the 
radio but hated the audio and went and bought an old Orion 2 which sounds waaay 
better to my ears).  

DDC/DUC radios handle strong signals as well as the best of analogue radios – 
the only question for topbanders is whether you like their ergonomics and the 
whole idea of having a digital radio.

Vy 73

Steve, VK6VZ


---



>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_medium=email_campaign=new_topic_content=topic_link
 
 
 

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
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Re: Topband: SDR Mythbusters - ADC Overload myths debunked...

[James Rodenkirch]

His "explanation," if I read it correctly, seems to be a tad "at odds" with 
what Sherwood Engineering's measurements showor am I missing it
 
> From: w...@w8ji.com
> To: rodenkirch_...@msn.com; topband@contesting.com
> Date: Tue, 6 Oct 2015 18:29:09 -0400
> Subject: Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
> 
> 
> 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:>>>>>>
> 
> Regardless of the fact they cannot possibly overload, they overload here.
> 
> I don't understand his explanation at all.
> 
> We had ONE transmitter on while trying to receive. The ADC could not handle 
> the level of one transmitter.  Because there was no in-band front end 
> selectivity, the spacing did not matter.  We could not get the strong local 
> signal outside a roofing filter.
> 
> With two transmitters on different non-synchronized frequencies, there will 
> be periods of time when the voltages add. There will be periods when the 
> voltages subtract. Anything following the summing point has to handle the 
> random peaks, even an ADC.
> 
> The Flex we tried here, using it to listen while ONE transmitter was 
> running, just went nuts. It was equally bad no matter what the spacing. At 
> least with a radio with roofing filter, we could move the radio up or down 
> band and use it.
> 
> 73 Tom 
> 
> _
> Topband Reflector Archives - http://www.contesting.com/_topband
  
_
Topband Reflector Archives - http://www.contesting.com/_topband

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

[W5JR Mike]

Interesting. Here at my place - wires in the trees, I'm able to transmit on the 
K3 (100w, old SYN) within about 20 kHz of the frequency where the Flex 6300 
(zero band filtering radio) is listening without any detriment to the Flex's 
ability to hear signals on the frequency it's listening to.

I admit I was surprised at the results when I did this test a year ago. 

I've done this on several bands, 160 80 and 40. 

I routinely run these radios in SO2R mode (and with 1kw+ amps) on different 
bands without the aid of any bandpass filtering on the Flex, although I now 
have some Dunstars and plan to install them anyway on both transceivers. 

Same for the reverse, transmitting on the Flex (100w) and listening on the K3 
in the same band 20 kHz away. 

The antennas are a multiband OFC @ 50', a 160m Inv L and an 80/40 vertical, all 
within 100' of each other. Maybe they are all inadvertently placed in a way to 
provide a good bit of isolation between them, but it wasn't intentional. 

tnx
Mike / W5JR
Alpharetta GA

> On Oct 6, 2015, at 6:29 PM, Tom W8JI  wrote:
> 
> 
> 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:>>
> 
> Regardless of the fact they cannot possibly overload, they overload here.
> 
> I don't understand his explanation at all.
> 
> We had ONE transmitter on while trying to receive. The ADC could not handle 
> the level of one transmitter.  Because there was no in-band front end 
> selectivity, the spacing did not matter.  We could not get the strong local 
> signal outside a roofing filter.
> 
> With two transmitters on different non-synchronized frequencies, there will 
> be periods of time when the voltages add. There will be periods when the 
> voltages subtract. Anything following the summing point has to handle the 
> random peaks, even an ADC.
> 
> The Flex we tried here, using it to listen while ONE transmitter was running, 
> just went nuts. It was equally bad no matter what the spacing. At least with 
> a radio with roofing filter, we could move the radio up or down band and use 
> it.
> 
> 73 Tom 
> _
> Topband Reflector Archives - http://www.contesting.com/_topband
_
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Re: Topband: SDR Mythbusters - ADC Overload myths debunked...

[Mike Waters]

I bought a partially-assembled SDR kit a few years ago. It's an older
Softrock Xtall Lite 9.0 with the optional electronically-switched BPF,
designed by Tony Parks. Well under $100 for the kit. I just "knew" that it
had to be a cheap, compromise SDR receiver prone to overload, birdies, and
anything else you can think of. The expensive ones like the Flex must run
circles around it; you get what you pay for, right? ...

Well, in the process of learning what I need to know to finish this little
tiny SDR and get it working, I'm hearing that it beats the receivers in the
Flex 5000(A?) and the Flex 1500. (Owners of expensive Icom transceivers
with SDR RX are using it for receive because the Softrock is superior.) The
sources of this info appear to be reliable, but of course time will tell.
(Newer versions of the Softrock are basically the same as what I have, but
with attenuators in series with the 160 and 80 BP filters. And some
transceive.)

As for the Flex 6300, a friend of mine has one. Maybe it has a better
receiver than the earlier Flex SDRs, but its SSB signal is absolutely awful
to my ears, no matter what mic he uses or how he adjusts the audio EQ
settings. His is not the only one. You can read all about it at
www.eham.net/ehamforum/smf/index.php?topic=104013.0

73, Mike
www.w0btu.com
_
Topband Reflector Archives - http://www.contesting.com/_topband

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

[MU 4CX250B]

Seems to me the devil is in the details. Obviously, if you pump enough
RF power into the input of any SDR (e.g., 100W), eventually it is
going to collapse. If the ADC can handle a few volts, that should make
it pretty immune to any but the most extreme environments. I've not
noticed any overload with my Flex-6300, but I've never used it in
multi-multi-contest situations. I definitely prefer it to my K3 or
FTDX-5K on top band, because signals just seem cleaner with a quieter
background. Haven't tried any quantitative measurements, but I don't
think that's my imagination. (The sales guy at Flex told me the 6000
series radios are significantly better than the earlier 5000 series in
this regard.)
73,
Jim W8ZR



Sent from my iPhone

> On Oct 6, 2015, at 4:29 PM, Tom W8JI  wrote:
>
>
> 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:>>
>
> Regardless of the fact they cannot possibly overload, they overload here.
>
> I don't understand his explanation at all.
>
> We had ONE transmitter on while trying to receive. The ADC could not handle 
> the level of one transmitter.  Because there was no in-band front end 
> selectivity, the spacing did not matter.  We could not get the strong local 
> signal outside a roofing filter.
>
> With two transmitters on different non-synchronized frequencies, there will 
> be periods of time when the voltages add. There will be periods when the 
> voltages subtract. Anything following the summing point has to handle the 
> random peaks, even an ADC.
>
> The Flex we tried here, using it to listen while ONE transmitter was running, 
> just went nuts. It was equally bad no matter what the spacing. At least with 
> a radio with roofing filter, we could move the radio up or down band and use 
> it.
>
> 73 Tom
> _
> Topband Reflector Archives - http://www.contesting.com/_topband
_
Topband Reflector Archives - http://www.contesting.com/_topband