----- Original Message -----
Sent: Saturday, October 30, 2010 9:48
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.
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 -----
Sent: Saturday, October 30, 2010 7:47
PM
Subject: Re: LF: RFspace SDR-IQ
Showing some of the minor peculiarities of DS SDRs
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
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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