[Discuss-gnuradio] Receiver gain issues with USRP2 and XCVR2450

[Srinivas]

Hello Everyone,

My questions are related to the Gain (set by 'set_gain()' method) set in the receive chain particularly in 'digital' folder in gnuradio-3.2. I am using USRP2 with XCVR2450.

The receiver gain is set to default (46 dB) in most of the scripts (digital, ofdm, usrp_fft etc.). My guess is that high gain is perhaps pushing the input signal out of the dynamic range of ADC and therefore saturating the ADC output. The SNR (as seen in usrp_fft.py) is low (20-25 dB) in this case. Lowering the gain (< 30) solves the problem (SNR 60-70 dB) and packets are successfully received in benchmark_rx.py. This problem was never found in USRP1. What is the cause of this problem to occur only in USRP2 ?

1. Does set_gain() set the analog gain ? (i.e. gain of a programmable gain amplifier before the ADC or on the daughterboard) or is it digital (scaling of digitized samples) ?

2. Shouldn't we have an AGC in all the 'receive_path.py' where gain is calculated as the mid-point of the gain range ?

I went through the previous posts on AGCs. They discuss the AGC settings in general which I understand, but my issue seems to be specific to USRP2's hardware. 

Any help in this regard would be greatly appreciated.

Thanks
-- 
Srinivas
WINLAB, Rutgers University
New Jersey