Re: How to convert dB to dBm and dBFS in GNURadio with USRP X310 and UBX-160

[Marcus D. Leech]

On 24/06/2023 21:10, Steve Hubbard wrote:

Hi Dor,

I don't follow all your email but in general you can add the powers (magnitudes squared) of the FFT bins to give you the equivalent of power in the time domain (Parceval's theorem). Strictly speaking I recall there is a slight tweak to the DC bin value but I don't remember the detail. It goes without saying that baseband signals and FFT bin values are complex (IQ).

Keep in mind that when you feed a single frequency into an FFT, it may fall between 2 bins. In this case the peak will be lowered (scalloping loss). You might want to consider the use of a suitable window to compensate for this. The window will lower the total power, which you also need to allow for.

I think -15 dBm is the point beyond which saturation, distortion or worse might occur. The point at which you reach full scale on the ADC will depend on the front end gain setting. I don't know what this might be for your set up. At some gain settings compression might occur in the analogue front end before you reach full scale. Gain will also be frequency and temperature dependent to some degree. The UBX-160 datasheet lacks detail.

I have experience calibrating the X310 but with a different front end, using GNU radio to capture the baseband signal but Matlab to process it.

Regards,

Steve

On 24/6/23 21:45, Dor Ratz wrote:

Hello,

I wish to use USRPX310 with UBX-160 daughterboard as a calibrated spectrum analyzer to show the received signal power in units of [dBm] and [dBFS] for my setup.

I've read discussion and the GNURadio FAQ before so I know I must calibrate my setup because the values in GNURadio don't mean a thing.

So I wanted to ask if this procedure seems to make sense for you. I will  be glad to receive any suggestions.

I'm gonna connect my signal generator to spectrum analyzer to measure the cable loss.

After that, gonna connect it to the USRP to measure the power[dB] in order to understand what is the power[dBm] in GNURadio.

Is this table correct to calibrate and calculate the Power[dBm] on GNURadio in FFT?

Signal generator Power[dBm]	Power [dBm] on spectrum anlayzer	Power[dB] on GNURadio in FFT	Power[dBm] on GNURadio in FFT
-60	-61	-66	-61
-65	-66	-71	-66
-70	-71	-76	-71
-75	-76	-81	-76
-80	-81	-86	-81
-85	-86	-91	-86
-90	-91	-96	-91

Procedure for wideband signal (not CW)

Right now the power[dB] must be calculated with the occupied FFT bins, let's assume this is the signal and the NFFT=128, so we can see that the signal occupies only 2/10 grids in the FFT so:

Power_of_signal[dB] = Peak_power[dB]+ 10*log(FFT_bins_with_signal) = (-15dB) + 10*log10((2/10)*(128)) = (-15dB) + (14) = (-1dB)

Convert to dBFS for the USRP X310

How do you propose to do it? 

The ADC of X310 has 14 bits. so the MSB values are 2^13 = + - 8192 (signed values).

Power[dBFS] = 20*log10(voltage_value_measured/8192)

Some questions:

1. Does the gnuradio amplitude axis in QT GUI TIME DOMAIN are always float-point values in the range {-1.0,+1.0} for a received signal from USRP? 
2. If the answer to 1 is yes, then if I see amplitude = 0.5 , does it mean 20*log10(0.5)=(-6dBFS) ? is that correct?
3. I know that the maximum input power for the UBX is (-15 dBm), but what does it mean that (-15dBm) input power is 0[dBFS]? 
4. Has anyone had experience with RX power calibration of USRPX310 with UBX-160?
   If I understand correctly, it is supporting the power API:

Thanks

Dor

    Generally, a calibration exercise has to happen over a
    multi-dimensional set of variables:
    
    
      o   Center frequency
      o   Gain setting
      o   Bandwidth setting (if applicable)
      o   Sample rate
    
    Gain settings are mostly-linear, so if you have a good set of
    calibration coefficients at one gain setting, they can be
      extrapolated (coarsely) to others.  It depends on your desired
    accuracy.
    
    Because different filter topologies are used at different sample
    rates, you pretty much have to include that in your
      calibration tables.
    
    I've not used the CAL interface at all, but it relies on lab
    equipment that speaks the appropriate protocols to allow the
      calibration procedure to work.
    
    Using an SDR as a laboratory-grade spectrum analyzer requires,
    necessarily, that you undertake the same kind of
      calibration exercises as they do when sending laboratory-grade
    spectrum analyzers "out the door" (and that exercise
      has to be repeated on a egular basis for some lab-management
    doctrines).
    
    The main thing to recognize is that you cannot just "add up all the
    gains and losses in the analog chain" to produce
      a kind of empirical calibration model.  You have to actually
    measure, and you have to have known calibration sources.
      Often, one uses a noise source, rather than a CW signal generator
    to do this.