Re: [Discuss-gnuradio] Noise floor and Rx Gain

[Matt Ettus]

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