[hpsdr] JANUS: Update on Alpha

[Robert McGwier]

Robert McGwier wrote:
> For future reference,  typical Codec manufacturers in doing designs 
> for audio applications,  do the internal mixing, matching, filtering 
> tricks to push noise "out of band".   The CS5381,  as does the TI, the 
> Wolfson,  etc. all produce a noise floor that looks a cross section of 
> a hot tub in a deck
>
>
>
> *****            ******
>           *        *
>             ****
>
> where the bottom is -50,50 kHz.  The reason to go to 192 for us is to 
> allow nearly 200 kHz in bandwidth through to the software.
>
>
> The Lynx L22,  and the Emu Creative cards all are as flat as west 
> texas at 192 kHz from -95 to 95 and then the noise floor GOES DOWN, 
> not up because the filtering kicks in.  Both of these cards (as you 
> know already) use the AK5394A.
>
> In the case of the CS5381,  these "shelves"  are 20+ dB above the 
> floor.  Almost surely we would have seen reciprocal mixing as a result 
> with louder signals.    We BARELY caught this before hardware got 
> built.   I had believed that the Lynx and Creative Emu had done 
> "something special" and indeed they have.  They have built very fancy 
> mixers and or fancy DMA engines and hardware based mixers/resamplers 
> as well as dithering (FPGA/ASIC)  and the ability to generate and 
> distribute all sorts of sync mechanisms but the inherent noise floor 
> and performance is determined by the  ADC.
>
> In the case of the AK5394A, the  performance gets just a little worse 
> at 192  kHz  BUT  as a system,  given  downsamples and filtering,  we 
> will get  processing gain  that might actually allow it to EXCEED the 
> performance of the part at (say) 48 kHz.   It is just not good to 
> isolate these pieces.  One must ALWAYS consider them as part of the 
> whole.
>
> In this case,  for example,  coupled with the SDR-1000  the Janus 
> board with the Akashi the SDR-1000
>

The instantaneous dynamic range of the SDR-1000 should IMPROVE because 
the noise floor will fall.  We already know that the noise floor of the 
system is already set by the preamp (SGA part) and the Delta 44 
recommended.

Bob
N4HY



-- 
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
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