Hi Andy, Clemens .....see a couple of articles by the Ed of QEX in an issue
in the last 12 months about this topic.

Alan

----- Original Message ----- 
From: "Andy Talbot" <[email protected]>
To: <[email protected]>
Sent: Saturday, October 30, 2010 8:48 PM
Subject: Re: LF: RFspace SDR-IQ


AH,  forgot that note only gave the results, and didn't include any
explanation of the findings...
Perhaps the contents of this email should be added to it

The complete unpredictability - far more than just the IMP-3 asymmetry - was
the whole point of making the measurements.   It was actually G3PLX who
asked me to do them as he didn't have the test equipment.   What they  show
is that the 'classic' third order linearity model is not applicable to
direct sampling SDRs and high speed A/D converters and a completely new
approach is needed.

The fact shows up dramtically when you observe that the level of the third
order products remains *reasonably* constant with varying two-tone input
amplitude whereas conventionally you should see a 3dB/dB variation.

But then it changes dramatically when a third tone is introduced that
*cannot
itself contribute to the IP3 tone being measured*.   Peter spotted something
like this and asked me to confirm with the more controlled measurements

One explanation we can think of is that there is no "real" third order
product being generated at all, certainly not one above the A/D quantisation
noise, but there is leakage from the digital lines.      A single tone into
the A/D will give signal components on the digital A/D ouptuts that contain
components at the input frequency and  its harmonics, which can leak into
the RF path.

Two tone signals will include I/M sidebands as well within this spectrum and
those on the Lowest Significant Bits will probably remain pretty constant
whatever the input amplitude,  provided it is above the minimum quantising
level.  When a third non related tone in intoduced, *whatever its level*,
the LSBs will be jittered around a lot more, so reducing the level and
changing the spectrum of teh leakage.   This effect is observed and can be
seen in the measurements

All a bit empirical but if you web-search on "SDR Linearity"  you'll find a
large number of papers and observations now; many showing similar results
and offering similar conclusions.

So, in conclusion :-
We cannot use conventional analogue-receiver linearity definitions or
measurement techniques on direct samplibg SDRs.

What we can safely say, is that direct sampling receivers will work best in
the presence of multiple signals with a spread of amplitudes.  That will
ensure the spectrum of RF leakage from the digital outputs will be noiselike
with no discrete components and hence allow higher dynamic range.  In other
words, just what you see by connecting to an antenna.

In fact the very highest specification top end A/D converters do optionally
deliberately jitter the clock to spread out the leakage spectrum.  The
jitter is taken out digitally by DSP within the A/D chip itself so the user
sees a tranparant conversion, or this can be done subsequently by the user
if preferred.   Probably a perusal of the Analog Devices web site
www.analog.com  will reveal a plethora of papers on linearity specifications
and results.

I believe a formalised route to DD Receiver specification is being developed
and prbably has been by now, , but I have had no inclination to follow the
story these days - that's all a bit too much like the work I used to do and
couldn't wait to retire from.

Andy
www.g4jnt.com


On 30 October 2010 20:07, Clemens Paul <[email protected]> wrote:

>  Hello Andy,
>
> your test results show quite strong differencies between the
> upper and lower IM3 product,up to 18dB.
> Unsymmetrical IM3 products *always* imply  that there is more than one
> IM3 producing source.
> Maybe it's a good idea to check the inherent IM3 behaviour of your test
> setup itself.
> 3dB-combiners have only 30-35dB port isolation,true hybrid combiners (6dB)
> are better by at least 20dB,if they are made tunable up to 80dB isolation
> is achievable.
> Also the 70dB resistive isolation between the two crystal oscillators
seems
> to
> be a bit on the short side.
>
> 73
> Clemens
> DL4RAJ
>
>
>
> ---- Original Message -----
>
> *From:* Andy Talbot <[email protected]>
> *To:* [email protected]
> *Sent:* Saturday, October 30, 2010 7:47 PM
> *Subject:* Re: LF: RFspace SDR-IQ
>
> http://www.g4jnt.com/SDRIQ_Linearity.pdf
>
> Showing some of the minor peculiarities of DS SDRs
>
> Andy
> www.g4jnt.com
>
>
> On 30 October 2010 18:04, Alan Melia <[email protected]> wrote:
>
>> Hi Clemens, you seem to impute that there are no shortcommings in direct
>> sampling!
>>  :-))    I suspect there are, but they are just "different".  You may be
>> right about the units mentioned, but it is just as easy to make a poorly
>> preforming DSP radio as to make a poor analogue one. We are in danger of
>> being conditioned to accept that "digital" is the magic dust that solves
>> all
>> problems......it is definitely not so!
>>
>> Having said that, the performance of some units is quite remarkable and I
>> do
>> own a couple very good conventional receivers.
>>
>> Alan G3NYK
>>
>> ----- Original Message -----
>> From: "Clemens Paul" <[email protected]>
>> To: <[email protected]>
>>  Sent: Saturday, October 30, 2010 4:11 PM
>> Subject: Re: LF: RFspace SDR-IQ
>>
>>
>> > Tony,
>> >
>> > the most important difference between SDR-IQ/-IP(from RF Space)
>> > or Perseus and the $200 SDR on
>> > http://www.lazydogengineering.com/LD-1A_SDR.html
>> > is that  the latter is no direct sampling receiver.
>> > It uses a downconverting technique with a number of shortcomings,e.g.
>> > phase noise of the LO and others.
>> > I would go for a direct sampling SDR like those e.g. from RF space or
>> > Perseus.
>> >
>> > 73
>> > Clemens
>> > DL4RAJ
>> >
>> > ----- Original Message -----
>> > From: "Tony" <[email protected]>
>> > To: <[email protected]>
>> > Sent: Saturday, October 30, 2010 3:49 PM
>> > Subject: Re: LF: RFspace SDR-IQ
>> >
>> >
>> > > Thanks guys.
>> > >
>> > > You have just tipped the balance for me and I shall be placing an
>> > > order
>> > > for one on Monday morning.
>> > > It's about time I joined the 21st century, but it will be
>> > > interesting to
>> > > compare it to my beloved FT 102 rx.
>> > >
>> > > Tony, EI8JK.
>> > >
>> > >
>> > > On 29/10/2010 16:50, Tony wrote:
>> > >> Hello group.
>> > >>
>> > >> Has anyone any experience with this SDR ?
>> > >> It looks interesting and I'm a bit tempted, especially as it works
>> > >> full spec down to 500 Hz and "usable" to 100 Hz.
>> > >> But as I have never used an SDR, I haven't got a clue if it's any
>> > >> good
>> > >> or not.
>> > >>
>> > >>
>> > >>
>> > >
>> > >
>> >
>> >
>> >
>> -------------------------------------------------------------------------
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>> >
>> >
>> >
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>> >
>> >
>>
>>
>>
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