Thanks for your great answer Matt. I have another quick question. We know that the controllable range of rx-gain is 0 to 90dB in RFX-2400 d'board. But I only can find a programable gain amplifier in ADC chip whose range is 0 to 20dB. Where is the received signal actually amplified according to the 'rx_gain' value?
Thanks in advance again. Matt Ettus wrote: > > On 01/23/2010 09:48 AM, Yong J. Chang wrote: >> >> All, >> >> I'm trying to set all USRP and RFX2400 parameters comparable with Micaz >> in >> the context of receiver sensitivity. But I've observed a non-linear >> behavior >> of ADC. >> >> By using usrp_fft.py, we can see noise floor level. When I change a >> rx-gain >> in a range of 0~45dB, noise floor level does not change. However, in a >> range >> of 45~90dB, noise floor linearly increases according to rx-gain. Does >> this >> affect receiver sensitivity? My thought is that high-gain of ADC >> introduces >> additional noise figure. Please give me a clue. Thanks in advance. >> > > There is nothing nonlinear about this. This is how all receivers will > behave, USRPs, other SDRs, and all radio receivers in general. > > The first thing to understand is that noise figure is a function of the > gain setting. > > The second is the difference between the displayed noise floor and the > noise figure. Noise figure is a function of the difference between > signals and the displayed noise floor, not the absolute displayed noise > floor. > > It is more instructive if you put a weak signal in to the receiver when > you look at the fft display. > > As you increase gain from zero, the displayed noise floor does not rise, > but your desired signal does. This indicates that the noise figure is > improving by roughly 1 dB for every additional 1 dB of gain. > > Then there will be a range of gain settings for which 1 dB of additional > gain causes your desired signal to rise by 1 dB, but the displayed noise > floor will rise some amount less than 1 dB. In this range you are still > improving the noise figure, but by less than 1 dB per 1 dB of gain > improvement. > > Next there will be a range of gain settings for which a 1 dB increase in > gain will result in your signal going up 1 dB but the displayed noise > floor will also rise by 1 dB. This indicates that you have already > reached the minimum noise figure, and that increasing gain will no > longer improve the noise figure. > > Finally, if your signal is strong enough, there will eventually be a > gain range at the top for which increasing the gain by 1 dB will no > longer cause your signal to increase in amplitude by 1 dB. This is > known as gain compression, and it indicates that you have too much gain. > You will start to see strong intermodulation products here because of > the nonlinearity. > > Matt > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > -- View this message in context: http://old.nabble.com/Noise-floor-and-Rx-Gain-tp27288261p27290059.html Sent from the GnuRadio mailing list archive at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
