Hello Vamsi,
Interesting question. Are you looking for an RF Signal Generator? Or are you looking for a driver for your existing RF Signal Generator?
Case I : The web link that you provided gives you information about thye drivers that you can use if you already have an RF Signal Generator from Rohde and Schwarz.
Case II : If you are looking to purchase RF Signal Generator, then from which company you buy will depend on your specification, purchasing power etc. If you want to know more about NI RF Signal Generators, you can find it here :
http://www.ni.com/rf/
The left menu on the above link should guide you well in the domain on NI RF devices.
Please do let me know if you have any questions.
Regards,
Sastry
Sastry Vadlamani
RF Principal Software Engineer
Aerospace and Defence BU
National Instruments
Certified LabVIEW Architect