[hpsdr] Mercury – Design assistance required

[pvharman at arach.net.au]

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. 

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.

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.

73's Phil...VK6APH 


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