[hpsdr] An high dynamic QSD front-end

[Giancarlo Moda]

Hi Bob,

Marco, IK1ODO, has done a very interesting and
welcomed analysis of QSD in SDR. Also Ahti, OH2RZ, has
defined an interesting way to improve QSD.

I would like to report a few comments to your notes
below.
I believe discussions are very important as are way to
exchange ideas and improve designs in the ham or
business field.

Date: Sun, 14 Jan 2007 17:37:06 -0500
From: Robert McGwier <rwmcgwier at gmail.com>
Subject: Re: [hpsdr] An high dynamic QSD front-end
To: Ahti Aintila <oh2rz.sdr at gmail.com>
Cc: Marco IK1ODO <IK1ODO at spin-it.com>, hpsdr at hpsdr.org
Message-ID: <45AAB092.8020300 at gmail.com>
Content-Type: text/plain; charset=ISO-8859-1;
format=flowed

Congratulations.  Very nice analysis,  very
straightforward,  not 
requiring the machinery of Laplace transforms to
understand the reasons 
which the "intuition impoverished mathematician" had
to do to gain 
intuition.   If you study Leif ?sbrink's WSE hardware
you will see the 
great lengths he went to do his "transient fix".   He
was working very 
hard to supply the current from a suboptimal current
source to overcome 
the transients problems with the four phase  capacitor

pseudo-integrator.  He had a heuristic understanding
of the need some 
time ago but did not go fully virtual ground, current
mode with the 
necessary integrator to solve this in the utterly
simple ISD Ahti has 
shown us.

Thank you both for a serious elevation of this
discussion.  It really 
is 
time to build hardware.  WARNING:  We definitely want
to steer away 
from 
the FST switches.  FROM MEASUREMENT,  coupled with DDS
issues,  they 
are 
the source of the increasing noise figure.  Marco's
plots, as well as 
Phil C's and my measurements of various QSD's using
them show this 
inescapable fact.  

> It was my understanding that Marco did try to
demonstrate the opposite about QSD noise


I think various pieces of analysis have incorrectly 
attributed the measurements but the switches are a
major contributor.  

> SDR is an interesting field in radiocommunication
but I feel there is still a lot of work to be done as
we do have to learn on how to do measurements with
this kind of receivers contrary to the analog radio
equipment ...


They solved an early problem and we can thank Gerald
for finding this 
part and his very gracious publication in QEX and with
the SDR-1000.  

> I would like to underline that I am the ONE that had
the idea, early 1998, to use the Fast Bus Switches
like the FST3125 in mixers to replace diodes and MOS
in particular for the G3SBI H-Mode Mixer. The Idea of
FST use did improve the Tayloe QSD.
 
But it is time to leave them behind FOREVER.

> Sorry but I do not understand your "FOREVER" in the
abouve phrase. Leave the Forever for all radio project
or FOREVER for SDR ?

I believe we should never close any door to the past
or to the future. I am sure the HPSDR group will
rlease SDR project very advanced going totally with
digital solution ... but we should never forget we are
Amateur Radio and not Business, although there are
many involved in SDR that are also in the industry.

> There are other colleges experimented with recent
switches like the FSA322 ... that may give H-Mode
Mixer with IP3 around +50dBm .... what will be the
value of IP3 in HPSDR?

Thanks and best 73

Gian
I7SWX

73's
Bob
N4HY



 
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