transfer function using impulse

Hi,

I am trying to get the transfer function of a system by applying an impulse input. I generated an impulse using the Impulse Pattern.vi in LabVIEW and applied it to a simple low pass filter circuit on the NI ELVIS II prototyping board. I tried to plot the magnitude spectrum of the acquired signal. But nothing is being plotted. Following are the details:

OS: Windows 7

LabVIEW Version: LabVIEW 2012

I gave '10' samples with delay between samples '1' for the Impulse Pattern.vi. Kindly help.