Ni daq boards can have a scan rate up to 10 MHz (using the S series).
Mseries boards can scan at a rate up to 1.25 MHz.
Obviously, you cannot sample a 20 GHz signal with these boards
You may use devices like Oscillosopes that can scan at high rates. You can then read the data collected using GPIB or whatever means is available to the computer
Is your signal a 20 GHz, or 20 MHz or simply 20 Hz ?
