Direct GMSK modulation at microwave frequencies
Klymyshyn, David Mathew
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Congestion in the radio spectrum is forcing emerging high rate wireless communication systems into upper microwave and millimeterwave frequency bands, where transceiver hardware architectures are less mature. One way to realize a simple and elegant hardware solution for a microwave transmitter is to exploit the advantages of directly modulating the phase of the carrier signal. A modulation method requiring continuous phase control of the carrier signal over the full 360 degree range is Gaussian Minimum Shift Keying (GMSK). Unfortunately, it is very difficult to design a microwave circuit to provide linear phase control of a carrier signal over the full 360 degree range using traditional methods. A novel method of obtaining continuous, linear phase modulation of a microwave carrier signal over the full 360 degree range is proposed. This method is based on controlling a phase shifter, at a subharmonic of the desired output carrier frequency, and then using a frequency multiplier to obtain the desired output frequency. The phase shifter is designed to be highly linear over a fraction of the full 360 range. The frequency multiplier is a nonlinear circuit that shifts the frequency by *'N'. The subtle part of this nonlinear operation is that the multiplier also multiplies the instantaneous phase of the phase shifter output signal by *'N', thus expanding the linear phase shift range to the required 360 degrees. Using this nonlinear frequency multiplication principle, the modulator can readily be extended into the millimeterwave region. A prototype circuit is designed and performance results are presented for this method of carrier phase modulation at 18 GHz. The prototype circuit is realized with very simple hardware, containing only a single microwave active device. An extension to the modulator involving phase locking or injection locking of a power oscillator is also suggested for obtaining higher power modulated output signals. In addition to direct continuous phase modulation, the proposed method is also suitable for a wide variety of transceiver applications, including phase synchronization of antenna and oscillator arrays, phased array antenna beam steering, indirect frequency modulation, and ultra-small carrier frequency translation.