Low Harmonic Distortion
All RF signal generators have some amount of harmonic distortion. Small, low-cost RF signal generators can have more harmonics than expensive benchtop-style RF signal generators. Harmonic distortion is easy to see in the frequency domain but shows up as a non-sinusoidal waveform in the time domain. To combat this problem it is possible to use screw-on external low cost, low pass filters.
Windfreak also sells the SynthNV Pro, which has a multi-band switching RF lowpass filter on its output.