The amazing yet economical SynthHD: 54MHz – 13.6GHz Dual Channel RF Signal Generator with Software GUI.
You can step the output frequency by 0.1Hz steps!
And the output level from -50dBm to +20dBm in 0.01dB steps!
Shift phase in 0.01 degree steps!
You can set each channel independently.
Modulation? How about Pulse, AM, and FM including FMCW radar chirp!
Complete Labview GUI source too!