[Discuss-gnuradio] get time before of fft block

[Carlo D'Elia]

Hi,
I'm a new user of gnuradio.
 I want to use this software to analyze  a traffic on wifi channels.I have a usrp2 hardware and I use the program file usrp2_fft.py to capture and then save into a file the result frequency and amplitude. The problem is : how can I get the time data sampling (in the simpler I think) that occurs prior to FFT ? 
PS: sorry for my english and thanks.