[hpsdr] Selectivity design/measurement

[Robert McGwier]

Tayloe Dan-P26412 wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
>   
>> And the fancy audio generator results:
>>
>>     
>>> Date: Fri, 27 Oct 2006 14:57:23 -0400
>>> From: Robert McGwier <rwmcgwier at comcast.net
>>>       
> <http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org> >
>   
>>> With a nice audio generator making 11 kHz at -20 dBm and the PowerSDR
>>>       
>
>   
>>> calibrated to -20 dBm,  we then removed the signal generator and 
>>> terminated both inputs with 600 Ohms resistors.
>>> The MDS was -135 dBm in 500 Hz with the attenuators jumpered.
>>>
>>> Bob
>>>
>>>       
>
> If the MDS is -135 in a 500 Hz bandwidth with just a signal generator,
> what was the largest signal that be injected without harmonic artifacts
> showing up?  
>
> >From the specifications of the AKM5394A, it appears as if the difference
> between the differential inputs is 2.25v max. If I think of this as the
> direct detector outputs (no post detector audio pre-amp at all), then
> the largest input signal will be half of this or 1.12v pk, or 2.25v
> pk-pk, since the A/D converter will have at is balanced input signals
> with the polarity of +S and -S at the same time.  2.25 volts pk-pk makes
> sense, since the A/D converter will differently sum these two
> differential inputs into a new signal of 4.5v pk-pk, a reasonable limit
> given the 5v supply voltage used.
>
> Since ~ 610 mV pk-pk corresponds to a 0 dbm RF input on the detector,
> 2.25v pk-pk is a maximum input of +11.5 dbm before blocking.  Thus the
> blocking dynamic range should be +11.5 - (-135) or 145 db of blocking
> dynamic range.  That would indeed be excellent performance.  
>
> When I designed the NC2030 (a narrow band analog DC phasing receiver)
> using this type detector, I got 142 to 145 db blocking dynamic range 10
> KHz away from the desired frequency.  At that point, the limiting factor
> that I ran into was LO phase noise and the LO that I used was very
> clean.  This implies a strong emphasis on LO phase noise in order to
> take advantage of the outstanding performance of this new A/D converter
> board.
>
> No post detector amplifier needed!  145 db of potential blocking dynamic
> range!  Wow! HPSDR indeed.
>
> Great job guys!
>   
QUITE.   You have it exactly right.  The SDR-1000 and others that use 
the Janus will see an immediate improvement in BDR if the post mixer 
gain is removed.  NOTICE in the SDR-1000 and others that use the INA or 
something similiar, this cannot be accomplished by setting gains equal 
to zero in most cases.  Op Amps, etc.  like these HATE zero gain and 
raise the noise floor considerably.

Janus definitely improves the performance of the unmodified SDR-1000 but 
i suspect it will be an even bigger improvement on the differential 
output hook up which bypasses the INA's.

I am looking forward to making the measurements.


> Was there any testing on the IP3 of the new sound card?
>
> With no post detector amplification needed, it implies that the detector
> should have very, very short audio connections to the A/D converter.
>   
Yes but I have basically  been away from home for 3 weeks and have not 
dredged this up.  Almost all A/D with any kind of decency have excellent 
IM characteristics until you overload them and then they die.   The 
Janus board has above excellent IP3 because of the AKM5394A and a good 
design and layout.  The Phil's and Bill have done a super job here.  I 
am also looking forward to getting my hands on the Mercury to write some 
"scan all bands on HF"  apps.  HPSDR is looking good as is DttSP V2.0 
and sdr_linux.
> - Dan, N7VE
>
>   

Bob
N4HY

-- 
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"Taking fun as simply fun and earnestness in earnest shows
how thoroughly thou none of the two discernest." - Piet Hine


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