[hpsdr] JANUS: Update on Alpha

[Robert McGwier]

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



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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



Ahti Aintila wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Lyle,
>
> I am very excited in hearing the news and testing results. AK5394A is
> my favorite choice.
>
> Ahti OH2RZ
>
>
> On 08/05/06, Lyle Johnson <kk7p at wavecable.com> wrote:
>   
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> New information on ADC performance has resulted in a possible change to
>> the design.
>>
>> The CS5381 as well as the two other chips tested so far are not good at
>> 192 kHz sampling.  The AK5394A appears to be good at this wider
>> bandwidth, pending testing.
>>
>> Stay tuned...
>>
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