[USRP-users] N210 DDC gain

Andrew Senior a.senior at lancaster.ac.uk
Wed Sep 12 10:25:48 EDT 2012

On 12/09/12 10:04, Josh Blum wrote:
> On 09/12/2012 01:32 AM, Andrew Senior wrote:
>> 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.
> Hey Andrew,
> It looks like I found the bug. The scale factor is 18 bits, but the
> fixed point adjustment assumed 17. Whats odd is that the tx chain code
> (which is a near mirror of rx) was correct. I pushed the following patch
> to the maint and master branches of UHD:
> http://code.ettus.com/redmine/ettus/projects/uhd/repository/revisions/f969650555e1fbf87548d861e6b5126e8a641b3e/diff/host/lib/usrp/cores/rx_dsp_core_200.cpp
> -Josh

Great, thanks very much! I can confirm that, with the patch, I now see a 
sensitivity of 54500 counts/V with 6 dB gain enabled. As expected, this 
is twice what I saw before +/- measurement error.

It still seems to be ~1.6 dB short of what I'd expect though. I guess 
maybe losses in the analogue circuitry in the Basic RX board might 
account for ~0.5 dB, but I'm not sure about the rest. I wonder if this 
sort of result is typical of real-world measurements?


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