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

[Robey, Frank]

>  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?
>

Both are important.  Overall dynamic range is also important for many 
applications where you have large signals and are looking for smaller ones 
(that is, in almost any over-the-air application).

>  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?

One source of the center band spur in the USRP is truncation in the FPGA signal 
processing chains.  A DC offset in the IQ data shows up as a spur at the center 
of the band.  Kyle Pearson showed that modifying the FPGA code to round rather 
than truncate makes most of this go away.

>
>  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.

Measuring a known level is best for accuracy but you should already have 
calculated the gain and should know within a few dB what the reference signal 
should measure.  You want to use the reference signal to remove the last bit of 
analog gain uncertainty.  In that way you know all through the signal 
processing chain (analog and digital) what the noise and signal levels are to 
maintain dynamic range.

Frank