Re: [Discuss-gnuradio] USRP Dynamic Range and 8 Bit Problem

[Firas Abbas]

Hi Matt, Don

I did the tests. We definitely have improvement in USRP SFDR by more than 3 dB. I did the tests as follows :

1) Test Setup:

a) Using USRP Rev 4.5 with Basic-RX  board.
b)  Using high accuracy function generator.
c) Using decimation rate =8, and gain =0.
d) Using single tone (The SFDR is usually tested using single tone).
e) Using two frequencies 250KHz and 5250KHz. This is because I noticed  a large difference in USRP SFDR between DDC frequency =0 and DDC frequency  = 5MHz (for example).
f) Using tone power levels of (+4dBm) which gives about 1Volt P-P into ADC (according to AD9862 data sheets, this analog input signal gives best THD performance) and (+8 dBm) slightly below the saturating level which produces about 2Volt P-P into the ADC  (according to AD9862 data sheets, this analog input signal gives best Noise performance).
g) Using Intel Core2D PC with Ubuntu 7.10.
h) Using gnuradio 3.1.1.
i)
 Using usrp_fft.py.


2) Prepared USRP FPGA Work :

a) The first  FPGA rbf file (usrp_std_0.rbf) was generated by modifying only the cordic.v file.

b) The second FPGA rbf file (usrp_std_1.rbf) was generated by modifying the cordic.v and the cic_dec_shifter.v files.

 c) The third FPGA rbf file (usrp_std_2.rbf) was generated by modifying the adc_interface.v and the cic_dec_shifter.v files.

See the files at:
http://rapidshare.com/files/79109257/Files_Differences.tar.gz
http://rapidshare.com/files/79109346/all_fpga_rbf.tar.gz
http://rapidshare.com/files/79109391/Worked_files.tar.gz


3) Test results:

Note 1:
I used in my tests the original rbf file std_2rxhb_2tx.rbf , usrp_std_1.rbf and usrp_std_2.rbf  FPGA files (usrp_std_0.rbf was not used).

Note 2:
Let us assume that I have eye reading error by about (1dB - 2dB)!!!!.

Note 3:
See test results at
 :
http://rapidshare.com/files/79109205/Tests.tar.gz

a)  The generated rbf file by modifying the cordic.v and the cic_dec_shifter.v files (usrp_std_1.rbf) gave us more SFDR than the original FPGA file by about 7 dB in case of input signal 5250KHz and level=+8dBm as shown in graphs. I tested this rbf file for all input signal levels (from -90dBm to +13 dBm) and for all decimations (8 to 256). It is working fine and great and should  be used all the time instead of the original rbf file.

b) The generated rbf file by modifying the adc_interface.v and the cic_dec_shifter.v files (usrp_std_2.rbf) was not good as expected. 

       Although it gave us more SFDR than the original FPGA file by about 5 dB in case of input signal 5250KHz and level 4dBm as shown in the graphs, but the FPGA got crazy when the input signal level was 8dBm as shown n the graphs (I think the cordic was overflowed).
     
 When the FPGA  went crazy, I reduced the input signal gradually by 1dB steps until I reached  input signal =+4dBm then it worked back normal. Thus, the file is working good only and only if the input signal is equal or below 4dBm.


I think we should send this work as a patch to gnuradio to enhance our fantastic USRP device.

Best Regards,

Firas A.