A digital compensation method is described that allows fractional-N frequency synthesizers to be directly modulated at high data rates while simultaneously achieving good noise performance. The technique allows digital phase/frequency modulation to be achieved at high data rates (> 1 Mbit/s) without mixers or D/A converters in the modulation path. The resulting transmitter design is primarily digital in nature and reduced to its fundamental components–a frequency synthesizer that accurately sets the output frequency, and a digital transmit filter that provides good spectral efficiency.

The synthesizer is implemented as a phase locked loop (PLL). To achieve good noise performance with a simple design, the PLL bandwidth is set to a low value relative to the data bandwidth. A digital compensation fiiter is then used to undo the attenuation of the PLL, transfer function seen by the data. This filter adds little complexity to the transmitter architecture since it can be combined with the digital transmit filter; the overall filter is efficiently implemented by using a ROM to perform the required convolution with input data.