Re: [Discuss-gnuradio] USRP and USRP2 FFT result comparison analysis help

[Matt Ettus]

ILKYOUNG KWOUN wrote:
> Hi, folks,
>
> I recently did some comparison test with my USRP and USRP2 boxes and
> got slightly different results.
>
> Test setup
> - Signal source : HP signal generator (forgot the name of the equipment)
> - Daughter cards for USRP and USRP2 : BasicRX
> - Test configuration
>  : I divided a 12.1MHz CW signal from the SigGen with a 3-dB divider
> and fed the outputs to both USRP and USRP2 boxes.
>
> Please find the two snapshots here.
> http://zoolu.co.kr/episodes/3
> Both have 4MHz span and centered at 12.3MHz with fft size of 8196.
>
> Questions
>
>
>  1. It seems that the noise floor of USRP2 is about 10 to 12 dB lower
> than that of USRP while the USRP has better SNR. Which are  considered
> relatively more important in general between a lower noise level
> (probably be able to detect weaker signals) and a better SNR when you
> plan to build a signal detect solution?
>   

What you are seeing here is the effect of the PGA (programmable gain 
amplifier) on the USRP.  In your usrp_fft plot you can see it is set to 
10dB.  The USRP2 does not have an equivalent to this PGA, so to make a 
better comparison, you should set the slider to 0dB manually, or use "-g 
0" on the command line.  Bringing the gain down will also lower the 
noise floor of the USRP1, but not down to the level of the USRP2.

In general, the USRP2 will have a lower noise floor, and lower spurious 
due to the 14-bit ADC.

Also, I think that the two python programs you are using will have the 
same number of points in the FFT, but you should make sure if you are 
trying to compare them.  Remember, you can keep lowering the height of 
the apparent noise floor by using more points in your FFT.  The real 
amount of noise will be the same, but you will be dividing it into more 
and more bins.  1024 is a good number to use, since it is comparable to 
the resolution of your screen.

>  2. I can see that the USRP has a bit more flatten noise floor shape
> but USRP2 has less spurious signals. Are those because of different
> performances of different ADCs or actually the two signals fed to each
> boxes are different although I branched from a single source?
>   

The spurs you see are from a variety of causes.  You didn't say what 
signal level you are feeding in to the boards, but about 9dBm will clip 
in the USRP2 and in the USRP1 with gain set to 0.  Your gain is set to 
10dB, so -1dB would be the clipping point.  ADCs are typically spec'ed 
at "-1dBFS" which means a sinusoidal signal 1dB lower than full scale, 
so try 8dBm.  Signal generators may only be accurate to .2 to .5 dB, so 
you may have to adjust in 0.1dB increments.

Also, the non-flat noise floor in the USRP2 may be a real part of the 
signal you are looking at.  It is very easy to see the phase noise of 
the signal generator you are using.  I actually wasted a couple of days 
trying to figure out why I was seeing that much phase noise, until I 
realized it was the noise of the generator and not the USRP2.  I had 
assumed that a high quality signal generator from Rohde and Schwarz 
would have better phase noise than the USRP2, but that actually isn't 
the case at low frequencies like this.  Once I got a better signal 
generator with the "low phase noise option" I saw it go down quite a bit.


>  3. I can see a rather strong peak of 12.3MHz signal in the USRP. I
> guess this is out of the local oscilator signal from CORDIC in the
> FPGA. However, I don't see the signal in the USRP2. Is this a leakage
> from the FPGA to ADC for the USRP unit?
> If then, is there any good way to get rid of this?
>   


Another cause of your spurs is probably harmonics from the signal 
generator aliasing into the passband.  Remember, the BasicRX does not 
have any filtering, and the ADCs will respond to signals well past 400 
MHz.  Even a very good signal generator may have 2nd and 3rd harmonics 
which are only 50 dB down, and even higher order harmonics are 
measurable.  Use a lowpass filter to get a purer tone. 

In testing the USRP2, I use a very high quality signal generator 
followed by a lowpass filter with more than 35 dB of harmonic rejection, 
and even then, the harmonics from the generator are still at a roughly 
comparable level to the USRP2s own harmonic performance.


>  4. I know that the relative signal level can be changed according to
> the numbers of FFT sizes. How do you guys calibrate the level? What I
> can think instantly is to measure a reference signal of know level
> (ie, 0 dBm) and put some codes to calibrate the signal level. Just
> wonder if there is any better method used in general.
>   


We automatically scale so that the height of the peak for the fft of a 
sinusoid (or any of the discrete spurs) should stay the same no matter 
how many points in the FFT.  Anything which is wideband, like noise, 
will show up at reduced levels when you have more points.  To calibrate 
to an absolute signal level, it is easiest to just do as you said and 
put in a known amplitude.  You could also do your computations based on 
the gains of everything in the signal chain, but that is more work.


Thank you for performing these tests.  It is good to have many people 
measuring these things.

Matt