How to show dB instead of dBFS in the FFT-Plot Sink


#1

Hi All,

and thank you All for your Answers in my last thread.

Now i want to know, how i can convert the showing
values in the Y-Axis of the FFT-Plot Sink to dB, so that
i can interpret this correctly.

Do i have to define the max and min input value first ?
Results an input voltage 3.3V in the max Value(0dBFS)
i can watch in the FFT-Plot sink ?
Results an input voltage 0V in the min Value i can
watch in the FFT-Plot sink ?

As Stefan Brüns said in my last Thread:
"The reference point is 0dbFS - FS meaning Full Scale,
-32768…32767. The signal power you need to get this
readings from the ADC, or more exactly from the FPGA
depends on the daughterboard, PGA setting on the daughterboard,
decimation …

You have to calibrate your USRP to get from the sampling
values to input power in Watts."

So if i read for example the value +7.079 from my FFT-Plot
Sink, what does this mean ?
Is this higher than the reference(0dBFS) Point ?
I thouht the reference is the highest point.

How can i calibrate my USRP to get from the sampling values
to input power in dBW ?

I experimented with an example(See Attachment), but
this example use a virtual signal(software generated signal)
Amplitude of Signal: Value of FFT-Plot:
10e-3 -12.921
100e-3 7.079
1000e-3 27.079

So inreasing the Value with the multiply of 10 let the
y-Axis of the FFT-Plot increase by 20dB.
This behaves like dBW not dBV ?

Regards Markus


#2

Markus F. <feldmann_markus gmx.de> writes:

Hi All,

and thank you All for your Answers in my last thread.

Now i want to know, how i can convert the showing
values in the Y-Axis of the FFT-Plot Sink to dB, so that
i can interpret this correctly.

Maybe i can do an offset to the usrp_source ?

I found the following function in <gnuradio.usrp> :
set_adc_offset(self, int which_adc, int offset)

But it doesn’t change anything in my program.
How to use this function correctly?