Re: [Discuss-gnuradio] USRP2 Noise figure and Phase noise measurement methods

[Per Zetterberg]

On Sun, 2010-10-03 at 15:30 +0200, Jorge Miguel wrote:
> Hi all!,
> 
> I am going to measure both parameters, noise figure and phase noise of
> my USRP2+XCVR2450 transceiver.
> After some reading about different methods of measuring them I have
> some questions.
> 
> Phase noise. 
> I read that in the lack of a Signal Source Analyzer (SSA) the most
> spread method of measuring it is with the delay line technique. Is it
> the right way to measure the phase noise in the USRP? Has anybody of
> you done it before? 
> 

I just measure a CW and do an fft. Results: See the following post:

http://lists.gnu.org/archive/html/discuss-gnuradio/2010-03/msg00299.html

> Noise figure.
> The standard way of measuring it seems to be the Y-Method using a
> noise source connected to the DUT and then measuring the output power
> of the DUT when the noise source is on and off. However as I
> understand it, in this case the USRP2 is not a two port device. It is
> just a receiver. Is it valid to measure the USRP2 power with software?
> (usrp2_fft.py for example, as I have read in another mailing list
> posts)
> 

What I do is to insert a CW with known power and then estimate the SNR.
The NF can then be calculated from

SNR = -174 + log10(BW) + Pin + NF,

where SNR and NF are in dB, Pin in dBm and BW in Hz. The BW is the noise
bandwidth. Roughly it is sample_freq/2. 


> What are the range of values I should expect to understand I have made
> a good measurements?

Look at the data-sheet of the MAX2829. Note that at high Pin your SNR
will be dominated by the distortion and the NF will seem larger than in
actually is.

BR/
Per