[USRP-users] 12 bits I/Q samples

Marcus D. Leech patchvonbraun at gmail.com
Tue Jan 21 12:29:03 EST 2020


On 01/21/2020 05:06 AM, Arash Jafari via USRP-users wrote:
> Hello Everybody,
>
> according to FPGA implementation of my board which a USRP-b200, the 
> Least significant 4bits of I or Q samples are always 0 (0x???0), which 
> in turn means the recorded sample as short complex 16 bits (sc16) cpu 
> format should have four zero at LSB.
> But almost 10% of recorded samples do not obey this rule!!!!
> Any comment is highly appreciated!
>
> Kind regards,
>
>
This is because the FPGA implementation always scales the ADC values to 
whatever is appropriate for the wire format, which by default
   is 16-bits.

The idea is to make the sample formats somewhat independent from the ADC 
of whatever hardware you happen to be using.  This happens
   both implicitly, by virtue of the way a fixed-point DDC operates, and 
explicitly by scaling code in the FPGA.





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