[hpsdr] Mercury Design assistance required

[Robert McGwier]

Eric Blossom wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> On Fri, Jun 23, 2006 at 09:03:56AM +0800, pvharman at arach.net.au wrote:
>   
>> Hi Eric,
>>
>> Thanks for the suggestions, much appreciated.  The blocks you suggest are very 
>> close to what I am using already. The issue that I don't understand is that 
>> the AD6620  design notes say that to get 100db of aliasing rejection at 
>> 100MHz/100kHz then the maximum decimation you should use in the CIC is 2. AD 
>> say that if you increase the decimation level of the CIC then you can't meet 
>> the 100dB aliasing requirement. 
>>     
>
> OK.  Not sure why that would be.  Maybe they're not carrying enough
> internal precision for the additional decimation.  If you find out why,
> please let me know.
>
>   
>> I have a design working using a CIC with divide by 512 followed by the half 
>> band with your values and decimate by 2. It looks quite good but still needs 
>> to compensate for the sinx/x roll off of the CIC and also sharpen up the half 
>> band roll off - I guess that this can always be done in the PC.
>>     
>
> If you stack two halfbands, you'll sharpen the roll off and increase
> the alias rejection.
>   

That would be my suggestion.


>   
>> There is still some aliasing responses that are < 100dB down so not quite 
>> there yet. 
>>
>> Getting close, randomizing the output data to prevent it being picked up buy 
>> the LT2208 input has made a huge improvement to the number of spurs. Not a 
>> problem with the chip, just my breadboard lash up.
>>     
>
> Keep me posted!
>
> Eric
> _______________________________________________
>   


-- 
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
Laziness is the number one inspiration for ingenuity.  Guilty as charged!


 1151030948.0