A simplified continuous phase modulator technique

A low cost GMSK (Gaussian minimum shift keying) modulation technique is presented. GMSK is chosen as an example although the technique is suitable for any continuous phase constant envelope modulation with discriminator, differential or coherent detection in the targeted receiver. In this technique, a fractional-N synthesizer is used to control the instantaneous frequency and phase of the phase locked synthesizer output. As a result of this approach, no in-phase and quadrature mixer or D/A converters are required. Furthermore, the desired signal can be generated directly at RF with no IF conversion stages. The look-up table to generate the modulation is only one bit wide. Δ-Σ techniques are used to obtain high accuracy through the long term average of a sequence of the single bits. The narrow band filtering of the transmit data is accomplished in two parts, a precise digital linear phase band-reject part using the one bit stored ROM look-up table and a less precise low pass analog part inherent to the PLL of the synthesizer