[hpsdr] Not Hpsdr - A Front-End with an IIP3 > +50dBm ?
Giancarlo Moda
i7swx at yahoo.com
Sat Aug 25 08:55:39 PDT 2007
Hi Ulrich and Bob (and All),
Thank you very much for your comments following my
report as on Subj following the work of Martein,
PA3AKE.
I believe that sharing information and experiences,
plus comments from people that have experiences like
not too many of us, is a way to help the progress of
and also a way of learning for all radio amateurs,
including the so called "button pushers".
With a technology bringing new components and moving
the amateur radio hobby into the future towards the
complete Digital transceiver, it may sound or look
"strange" if people are still fiddling the the analog
critical stages of our equipments and to improve them.
As hams we are not doing business, except some with
good ideas, we are doing something to better our
knowledge and when possible to share it with otthers,
but first we work and swet for our own pleasure ...
After the H-Mode Mixer (G3SBI) removed the mixer from
the list of critical stages in a receiver (also OK in
TX) we "discovered" our equipments IMD problems were
now produced by the passive components, like coils and
transformers, and, a very important one... we really
understood the "heard" problem due to the phase noise
of oscillators.
I do remember the articles in HamRadio, QST and QEX
were our friend Ulrich, at that time DL2LR, was
spending a lot of words to show us the critical points
of each rx stage, including the oscillators and all
his suggestions.
I believe a lot of water is gone under the bridges and
phase noise is getting lower with the improvements of
DDS, see the changes from the AD9851 to the AD9951 and
now the AD9910 and AD9912 (yes we have a new problem
... the spurs !).
Till the ADC will really perform as we hope, I do not
see why not to invest to improve today or yesterday
circuits.... maybe in a few years time we will have
oscillators, DDS and the like with so low phase noise
that we will have to spend our time to invent
problems.... So, why not to search for the best
improvement as possible in the front end and "live
temporary" with "phase noise" problems ?
This point just remind me of what happen when Enzo
Ferrari, the founder of the well known racing car
company, decided to stop to be a racing driver and
wanted to build his "dream racing car", after the
second world war. It seems one of the people with
which he discussed his project told him ..."why do you
want to design a car with a so advanced engine and
body when our fuel is so poor in octanes (it was
necessary to add castroil to the fuel at that time)
and the roads are not and will never be good for high
speed?" ... Well, if he had listen to this person we
would not had the fastest cars in Italy and no Ferrari
or Lamborhini... etc. Today we have super octanes
fuels and motorways where one can drive up to 300kmh
(nearly 200 miles/h) ..not possible due to law speed
limits 130kmh)...(our road phase noise)... hi
I believe many of us know how difficult it is to
perform measurement and particularly when equipment
stages have high performances.
Bob mentioned the ARRL tests on equipment and then go
and check the ones done by Sherwood (different). I am
a little puzzled with this. In Europe we are most used
to the measurements reported by Peter Hart, G3SJX, in
the RSGB RadCom magazine. His review reports figures
are often different from the ones published by ARRL
for the same equipment and he is and has always
reported the note "limited by oscillator phase noise"
and other interesting IMD, DR and IP3.
Regarding the subject, I have contacted Martein,
PA3AKE, and he kindly sent me a few notes I am sure
readers will find interesting. Please see below.
73
Gian
I7SWX
>From Martein, PA3AKE
"Phase noise of the local oscillator is indeed very
important at these
intercept point levels. This is the phase noise I
measured with the recently already out-dated AD9951
DDS clocked at 500MHz with a $6 digikey ECS 100MHz
quartz oscillator helically multiplied by 5 ala I0CG:
2KHz -137dBc/Hz
3KHz -140dBc/Hz
5KHz -145dBc/Hz
10KHz -147dBc/Hz
20KHz -148dBc/Hz
50KHz -150dBc/Hz
100KHz -150dBc/Hz
These measurements are done with the DDS at 12.6MHz
(80M LO) and measuring reciprocal mixing with a 500Hz
wide quartz filter filterering the pilot signal in the
80M band. The signal to noise ratio of the pilot
signal(3dB above MDS) was observed with SpectrumLab PC
software.
These results are not bad at all! This is partially
because we are using a DOWN-CONVERSION frontend
resulting in a relatively low LO frequency and hence
better phase noise from todays DDS chips. This is why
I choose for a down conversion HAM band only frontend.
The LO, the mixer and the roofing filter are much
easier to build with the quality that is needed. I
have no ambition nor the means to even consider a DC
to visible light reciever with a +50dBm intercept
point.
AD9951 phase noise is excelent when giving it a clean
clock, but about 2 or 3 months ago the AD9910 DDS is
released that potentially has much improved phase
noise performance. If one takes the time to compare
the residual-phase-noise plots of the AD9951 and the
AD9910 it is obvious that the AD9910 adds much less
phase noise then the older AD9951. The AD9910 in
combination with a good (ultra) low noise clock will
outperform the AD9951 hands down.
Let us put this in perspective a little bit. The phase
noise of the LO of the KW7CD Star-10 tranceiver,
mentioned in this thread is shown in the
QEX-2001 beyond fractional article part 2. If I read
the graph correctly I get these figures:
2KHz -110dBc/Hz
10KHz -120dBc/Hz
Further out the noise and the spurs seems to get worse
again. This is a PLL based solution, although probably
state of the art at that time, it cannot
compete with todays mainstream technology.
The AD9910 and possibly AD9912 (no residual phase
noise plots are available at this time of the AD9912)
are the so called SUPER DDS's predicted by KW7CD in
the QEX series and they are available today and they
make the PLL based LO a noisy relic of the past.
AD9910 in combination with a Wenzel or similar quality
low phase noise VHF OCXO multiplied to 1GHz will give
the DOWN-CONVERSION front-end the LO that
complements the frontends +50dBm intercept point
possible today with off-the shelf affordeable
components.
Martein
pa3ake
From: KA2WEU at aol.com View Contact Details Add
Mobile Alert
Date: Thu, 23 Aug 2007 11:34:28 EDT
Subject: Re: [hpsdr] Not Hpsdr - A Front-End with an
IIP3 > +50dBm ?
To: i7swx at yahoo.com
CC: hpsdr at hpsdr.org
Given the current SSB Phase noise of oscillators,
this IP3 is useless.
73 de Ulrich
> >
> >
>
--- Bob McGwier <n4hy at idaccr.org> wrote:
> Indeed, especially if you compute IP3 the way the
> ARRL labs does, by
> raising the level of the two tones injected into the
> DUT until they 3rd
> order product gets to be S5. That would be really
> really difficult with
> the test equipment they have and a front end with a
> "50 dBm IP3" and for
> most radios to take that much signal, but let's
> suppose they did it.
>
> It appears from this test that the IP3 and the
> IMD-DR dynamic range are
> large. However, if dynamic range, which is the
> more important thing
> takes into account the rise in the noise floor
> induced by the two tones
> or a single tone, the dynamic range is more limited
> than would be
> indicated by the ARRL test method.
>
> I refer interested readers to the Sherwood test
> table:
>
> http://www.sherweng.com/table.html
>
> I then suggest that ARRL members compare the numbers
> Sherwood has
> measured to those measured by the ARRL labs for the
> receivers/transceivers that are compared. You will
> see that inevitably
> the dynamic range numbers are lower than those
> measured in the ARRL labs
> because Sherwood takes LO phase noise into
> consideration. You will see
> MANY entries in his table where they are footnoted
> with "phase noise and
> not IMD limited".
>
> I think his Ip3 numbers have been computed from the
> DR measurement when
> the more accurate way to compute IP3 is the S5 test
> but I would rather
> he compute USABLE IP3 and the imputed dynamic range
> by taking LO phase
> noise into account than the other way around.
>
> As usual, Ulrich is right on the money here.
>
> For the superior transceiver being built by Cornell
> Dentra and being
> written up in QEX (and which many know about from
> his writing and web
> page), he does get there with oscillators that have
> sufficiently low
> phase noise to use it. You cannot even begin to
> figure out how much it
> would cost to build such a thing. He uses parts
> that were used in
> serious projects and then surplussed and for which
> the government or
> commercial entity paid a boat load of bucks. You
> might be able to
> duplicate his work if you had the resources of Bill
> Gates or Warren
> Buffet to throw around. It is not reproducible
> (though it is brilliant).
>
> Bob
> N4HY
>
>
> > _______________________________________________
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>
>
> --
> Robert W. McGwier, Ph.D.
> Center for Communications Research
> 805 Bunn Drive
> Princeton, NJ 08540
> (609)-924-XXX-4600
> (sig required by employer, remove X's for phone #)
>
>
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