[Discuss-gnuradio] Digital amplitude in benchmark_tx.py

[neha kochar]

Hi,

I was trying out some experiments using USRP1 and the files benchmark_tx.py and benchmark_rx.py in the digital folder. I wanted to know few things.

(i)  I saw the option of digital amplitude in benchmark_tx.py. Could someone tell me the gain of which component on the USRP board is controlled by this digital amplitude.

(ii)  Also I tried to connect two USRP1 in a wired fashion and put one to transmit using benchamrk_tx.py and the other to receive using benchmark_rx.py with the default gain of 45. I noticed that at digital amplitudes (at the transmitter) of 0.64 to 1, the BER suddenly increases inspite of wired connection. At values 0.63 and below, the bit errors are zero as expected. I was wondering if someone could explain this behaviour.

(iii)  In addition, I measured power levels at each digital amplitude in increments of 0.1 (i.e. 0.1, 0.2, 0.3, ....., 1) and I observed the following values using a spectrum analyzer:

Digital Amp   Power(dBm)

0.1	-8 to -4
0.2	-2 to 2
0.3	1 to 5
0.4	4 to 7
0.5	5 to 8
0.6	5 to 9
0.7	6 to 10
0.8	7 to 9
0.9	8 to 11
1	8 to 11
As can be seen, at larger digital amplitudes, there is not much difference in power levels between adjacent values of digital amplitudes. It seems like something at the transmitter is saturating, not allowing more than a particular amount of power to be transmitted. Could someone please elaborate on this behaviour and its reasons? Thanks in advance. Regards, Neha.