[Discuss-gnuradio] Question about 16 bit signed integer samples

[Sergio Benco]

I'm interested in extracting samples from USRP to perform some spectrum
analysis on them. I use an USRP and a XCVR2450 to receive signals in the
ISM 2.4GHz band. My setup in GRC is a USRP (decim 64, gain 0dB, baseband
2.405GHz) connected to a file sink. This file is then processed by
read_complex_binary.m located in the directory .../gnuradio-core/utils/.
Its output, for a sampling rate of 1MS/s, gives (I printed out only a
small part):

     -18 -    19i
     -22 -    12i
      -2 -     8i
     -23 -    31i
      21 +    62i
     -62 -   231i
      76 +   510i
    -128 -  1023i
     240 +  1925i
    -571 -  3749i
    3643 + 10423i
   14675 - 14352i
   21379 +  3772i
   -1807 +  4089i
     851 -  2080i
    -483 +  1112i
     251 -   553i
    -135 +   256i
      54 -    96i
     -24 +    13i
      -3 -    12i
      -8 -     1i
      -8 -     1i

Now I would to know how the 12 bit samples from ADC are mapped in 16 bit
signed integers. Since now I know that the samples (12 bit, 4096 levels)
come into the USB after a mapping to USB's 16 bit signed integers with 4
bits zeroed. I would understand better the process of sampling in the
USRP from the analog signal, to 12 bit samples (signed? so 2^11 levels =
2048), to 16 bit signed integers (2^15 = 32768) as seems to be appened
from my printout. How could I correlate the signal on antenna with these
16 bit samples?

BR, 
Sergio Benco