DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=btinternet.com;
  h=DKIM-Signature:Received:X-Yahoo-SMTP:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:References:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE:X-Antivirus:X-Antivirus-Status;
  b=tASYVfIAovSZluuWEJhLKf8NgPiY8XdvBs43x27PtGo95MBTFfVrYbKx8syJoKXN7BP6ueeeptDMUFApwEF42JIIIlpjeE5C1+wHL3eMqOE1WuYo18lu4QbBsJ8Joc4blS2ywj85+b0vp/j0wzfRzDn0JI1wg6vCv6iH4T6GRrA=  ;
Message-ID: <004d01cb7883$961be310$4001a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4CCAED49.6030007@o2.ie><4CCC2277.9040801@o2.ie><045201cb7844$ca1df020$0201a8c0@Clemens04><006b01cb7857$f564e870$4001a8c0@lark><AANLkTikqi2FqYK5z4MVxeHZEquuC9AMp6-N-ihr26cgr@mail.gmail.com><00f001cb7865$ad833bc0$0201a8c0@Clemens04> <AANLkTin5pUVkfzO3Nh9D0AU1goGibD35iLJuqf=nXRC=@mail.gmail.com>
Date: Sat, 30 Oct 2010 23:36:46 +0100
MIME-Version: 1.0
DomainKey-Status: good (testing) 
Subject: Re: LF: RFspace SDR-IQ
Content-Type: text/plain;
	charset="iso-8859-1"
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Content-Transfer-Encoding: quoted-printable


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 -----=20
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
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 wh=
o
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 ne=
w
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 inp=
ut
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 som=
ething
like this and asked me to confirm with the more controlled measurement=
s

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 quanti=
sation
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 co=
ntain
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 spectr=
um and
those on the Lowest Significant Bits will probably remain pretty const=
ant
whatever the input amplitude,  provided it is above the minimum quanti=
sing
level.  When a third non related tone in intoduced, *whatever its leve=
l*,
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 ca=
n 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 resu=
lts
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 be=
st in
the presence of multiple signals with a spread of amplitudes.  That wi=
ll
ensure the spectrum of RF leakage from the digital outputs will be noi=
selike
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 optio=
nally
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 specific=
ations
and results.

I believe a formalised route to DD Receiver specification is being dev=
eloped
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 <cpaul@gmx.net> 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 isol=
ation
> is achievable.
> Also the 70dB resistive isolation between the two crystal oscillator=
s
seems
> to
> be a bit on the short side.
>
> 73
> Clemens
> DL4RAJ
>
>
>
> ---- Original Message -----
>
> *From:* Andy Talbot <andy.g4jnt@googlemail.com>
> *To:* rsgb_lf_group@blacksheep.org
> *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 <alan.melia@btinternet.com> wro=
te:
>
>> 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 dang=
er 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" <cpaul@gmx.net>
>> To: <rsgb_lf_group@blacksheep.org>
>>  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 spac=
e or
>> > Perseus.
>> >
>> > 73
>> > Clemens
>> > DL4RAJ
>> >
>> > ----- Original Message -----
>> > From: "Tony" <ei8jk@o2.ie>
>> > To: <rsgb_lf_group@blacksheep.org>
>> > 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.
>> > >>
>> > >>
>> > >>
>> > >
>> > >
>> >
>> >
>> >
>> -------------------------------------------------------------------=
------
-
>> ------
>> >
>> >
>> >
>> > Eingehende eMail ist virenfrei.
>> > Von AVG =FCberpr=FCft - www.avg.de
>> > Version: 9.0.864 / Virendatenbank: 271.1.1/3226 - Ausgabedatum:
>> > 10/29/10 20:34:00
>> >
>> >
>>
>>
>>
>  ------------------------------
>
>
> Eingehende eMail ist virenfrei.
> Von AVG =FCberpr=FCft - www.avg.de
> Version: 9.0.864 / Virendatenbank: 271.1.1/3227 - Ausgabedatum: 10/3=
0/10
> 08:34:00
>
>