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

[Yong J. Chang]

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
> address@hidden
> 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.