Is there any daq board available for getting analog input at the rate of 20 Mhz and converting it into a digital output and pass the values to the PC using USB?
How many channels? What voltage range is your signal? What form factor do you want? How many bits of resolution? NI has <a href="http://www.ni.com/digitizers/" target="_blank">digitizers</a> that go up to that rate but you really need to provide a lot more information. Have you tried to contact your local sales engineer?