On 01/23/2010 09:48 AM, Yong J. Chang wrote:
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
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
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
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