[USRP-users] N210 DDC gain

Andrew Senior a.senior at lancaster.ac.uk
Wed Sep 12 04:32:35 EDT 2012


On 11/09/12 20:10, Josh Blum wrote:
>> It puzzles me because I would have expected the gain would be set so
>> that a full-scale input to the ADC gives a full-scale 16-bit output,
>> i.e. +/-32767, but at present the gain seems to be less than half that.
>> Is this how it should be and if so, why?
>>
>
> Indeed, it looks as if a bit has been lost. This should just be a
> software fix away. This patch doubles the multiplier scalar before the
> bits are clipped to 16. http://pastebin.com/VUCK6yzq
>
> I need to look into ddc_chain.v more before merging this.

Thanks for looking into this, Josh. Can I suggest you take a look at 
cordic_z24.v? I apologise if I'm barking up the wrong tree having failed 
to understand what's really going on, but it seems to me that the code 
allows some extra width for the growth due to the CORDIC algorithm but 
then divides the result by 2 at the output. Coupled with the correction 
for the CORDIC gain in host/lib/usrp/cores/rx_dsp_core_200.cpp, I 
wondered if this has something to do with it.

Andrew.






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