A SIMPLIFIED CONTINUOUS-PHASE MODULATOR TECHNIQUE

A low cost GMSK (Gaussian Minimum Shift Keying) modulation technique i s presented. GMSK is chosen as an example although the technique is su itable for any continuous phase constant envelope modulation with disc riminator, 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 out put. As a result of this approach, no in-phase and quadrature mixer or D/A converters are required. Furthermore, the desired signal can be g enerated directly at RF with no IF conversion stages. The look-up tabl e to generate the modulation is only one bit wide. DELTA-SIGMA techniq ues are used to obtain high accuracy through the long term average of a sequence of the single bits. The narrow band filtering of the transm it data is accomplished in two parts, a precise digital linear phase b and-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.