[hpsdr] An high dynamic QSD front-end

Mon Jan 15 14:27:54 PST 2007

Hello Gian:

Thank you for your many contributions in the mixer area, to the N2PK VNA 
group (both of mine use your circuit boards), and more.   Here is what I 
know from my own experience,  measurements done by Phil Covington,  Phil 
Harman, and others.  From several QSD's I have measured and have seen 
measured the rise in noise figure from (say) 14 to 54 MHz.   I see that 
Marco has done the same with his very nice work.   In each case,  the 
results are similar, if not identical to those I have measured or others 
have measured.   In those cases where I have seen these kinds of mixers 
done without the FST switches, but with different parts,  there is <<no 
such rise in noise figure>>.  Marco's work has one thing different from 
all of the other ISD implementations I have seen.  It uses the FST 
switches you discovered for us.  ;-).   Marco's circuit has the rise in 
NF which is NOT present in those ISD's I have seen measured which do NOT 
have the FST's.  I am certain Marco is not sloppy and his work is 
excellent and I am taking his graphs as the absolute truth.  The logical 
implication from this is immediate.  The FST switches are the cause of 
the rise.   I would like to minimize the amount of gain needed to make 
the receiver usable at 10 and 6 meters.

I acknowledge your work and that of Marco.  I am giving my opinions on 
the best way to proceed, fully aware of all of this work.  I read this 
group every day and am grateful for all of the wonderful contributions 
and sharing.

73's
Bob

Giancarlo Moda wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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>   

-- 
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"If you board the wrong train, it is no use running along the
corridor in the other direction. " - Dietrich Bonhoeffer

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