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

[Firas A.]

Hi,


1) Using high accuracy function generator and USRP with Basic-RX, I tried to
investigate the dynamic range of this board. The following behaver was
obtained (0 dB gain was used in all experiments with decimation of 8):

a) With Sinewave signal at frequency =250KHz and 0dBm input power to BasicRX
we get ,

Theoretical calculations :
When 0dBm signal injected into 50 Ohm load, then signal RMS = 0.225Volt and
Signal voltage P-P = 0.636396103Volt.

USRP readings :
The maximum counting obtained from USRP for this signal was 4845.

b) With Sinewave signal at frequency =250KHz and 5dBm input power to BasicRX
we get ,

Theoretical calculations :
When +5dBm signal injected into 50 Ohm load, then signal RMS = 0.4Volt and
Signal P-P = 1.13137085Volt.

USRP readings :
The maximum counting obtained from USRP for this signal was 8337

c) With Sinewave signal at frequency =250KHz and 9dBm input power to BasicRX
we get ,

Theoretical calculations :
When +9dBm signal injected into 50 Ohm load, then signal RMS = 0.64Volt and
Signal P-P = 1.81019336Volt.

USRP readings :
The maximum counting obtained from USRP for this signal was 13096

d) With Sinewave signal at frequency =250KHz and 10dBm input power to
BasicRX we get ,

Theoretical calculations :
When +10dBm signal injected into 50 Ohm load, then signal RMS = 0.71Volt and
Signal P-P = 2.008183259Volt.

USRP readings :
The maximum counting obtained from USRP for this signal was 13570
(Saturated).

Discussion :
>From the above results we see that the BASIC-RX is saturated when the input
signal power was +10dBm. This is logical, since the maximum input signal to
the USRP ADC AD9862 is 2Volt P-P (according to its data sheets).

But, what is not logical is the value obtained from USRP FPGA. Since the
incoming data is 16bit signed number (for I and Q), we should get +/- 32767
readings for the maximum input signal of 2V P-P. Instead, we have maximum
reading of 13570. I don't know how this number was generated, but it should
be scaled to make use from the full dynamic range offered by using 16 Bit
data transfer. 

2) I Think there is a problem in the 8 bit data transfer. As we know, if we
have 12 bit number (for example), and we would truncate it to 8 bit number,
we should take the most 8 significant bits from it and discards the rest
least 4 bits.

This is not happening in the 8 bit data transfer. When the input signal is
power is low, we get weird waveforms as shown in the following snapshots.

http://rapidshare.com/files/78343220/8-bit-samples-scope.tar.gz

As shown, unless the input signal power is high (9dBm was the best) we get a
very distorted signal when using 8 bit data transfer.

Note:
I'm using the latest trunk USRP rbf file (the 8 bit samples transferring was
restored).

3) FYI, the maximum measured SFDR for Basic-RX was about 55dB (when the
input signal was +4dBm and the gain was 0dB which gave this maximum SFDR)
 
4) FYI, the maximum measured SFDR for DBSRX was about 40dB (when the input
signal was -20dBm and the gain was 26 dB which gave this maximum SFDR).

I hope we use these experiments to enhance our magnificent USRP.

Best Regards,

Firas A.





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