Direct frequency modulation of an ADPLL for bluetooth/GSM with injection pulling elimination

Abstract

We propose and demonstrate a Gaussian frequency-shift keying and Gaussian minimum-shift keying (GMSK) pulse-shape filtering for wireless RF transmitters with an arbitrary reference frequency. The filter is software controlled to work in a multistandard radio. Spurs, which are due to the frequency injection pulling, in cases when the reference harmonics are close enough to the oscillating frequency, are avoided by means of retiming the reference clock by the RF oscillator. Baseband clock for the pulse-shape filtering is derived through a simple fractional-N division of the reference frequency. This saves area and power since it is no longer required to create a low-jitter clock for baseband symbol generation and modulating data. It is especially advantageous when the available reference frequency is not an integer multiple of the symbol rate. The presented transmitter is realized without any explicit analog filtering and is part of a commercial single-chip fully compliant Bluetooth radio fabricated in a digital 130-nm CMOS process. We demonstrate the software programming capability through an experimental GMSK modulation for the global system for mobile communication.