Investigation of a Closed Loop Modulator for Bluetooth Radios

Abstract: This rapport investigates the transmitter of the Ericsson Bluetooth radio. The open loop modulation scheme of the existing design is theoretically very simple but difficult to implement with available components. Frequency drift of the carrier is a common result of open loop modulation. This work therefore investigates an idea of frequency regulation where the regulation is active during modulation in order to reduce the frequency drift. Instead of completely opening up the phase locked loop, the rate of activity will be reduced to a level enough to compensate the frequency drift. The frequency regulator is a dynamic system with high complexity. Therefore the investigation has been done with simulators built with Matlab and Simulink. The simulations show that the new idea puts hard requirements upon the phase error and the asymmetry of the transmitted data when reducing the activity of the frequency regulator. The problem with the asymmetrical data is solved with an additional filter that the transmitted data goes trough. But in the case of large phase error, ringing will occur causing drift like problems. No satisfactory solution was found to overcome this problem. However, the work indicates that with careful optimisation of the loop filter and timing as well as with an increased reference frequency to 1 MHz the suggested idea is likely to work if some ringing of the frequency during modulation is allowed.