Pls help me: FFT data output convert to dB

[dathoang86]

Hi,
I had a FFT, data output is through is real and image. This real and image data is squared, add together and go to square root block.
After that, we have 16 bits unsigned fix point.
But in software, i don't know to convert data to dB.
Example, i transmit a -60dBm from the signal generator through a cable, i want to see this signal peak in my software is -60dBm, the maximum display in dB is 0 dBm, the minimum is -140 dBm.
pls help me what i need to do, or give me some documents.
Thanks,
Dat

 

[ashwini1]

if you are using simulink FFT display block of signal processing blockset to visualize the frequency components then it automatically computes FFT and plots the results in terms of frequency(Hz) vs. Magnitude (dB).

If not using the above mentioned block write in detail about your implementation.

-- Ashwini

 

[dathoang86]

Thanks,
My ADC is 14 bits, SNR = 74 dB. FFT 4096 points,scale, 16 bits .
I design in FPGA, use FFT Xilinx Coregen.
FFT data output is real and image. It is squared, added together and squared root (in FPGA). After that, all data is sent to my PC. I'm sure I receive correct data, but I don't know calculate it correctly.
Example, if I have a pure sine wave Asin(2*pi*f*t), I can calculate input signal power in db through equation Pin = 20*lg(A/1). Is it correct ?

If i use signal generator, i transmit -60 dBm signal, can i convert this signal to voltage us above equation ?

After FFT, I don't know to calculate signal power. does it equal above Pin ?


Pls explain me, i'm a newbier in DSP.
Thanks in advance,
Dat