As a result of congestion in the traditional microwave portions of the ra- dio spectrum, new and emerging high rate wireless communication systems are migrating to the upper microwave and millimeter-wave frequency bands, where transceiver hardware architectures are less mature. For this reason, research into effective transmitter realizations at these frequencies is very timely. One way to realize an effective transmitter solution is to exploit the advantages of directly modulating the phase of a microwave carrier signal. Modulating the carrier phase directly, as opposed to the more traditional method of modulation at an IF frequency and use of multiple stages of up- conversion to reach the desired transmit frequency, results in a simple and elegant hardware solution for a microwave transmitter.

A modulation method requiring continuous phase control of the carrier signal over the full 360 degree range is Gaussian Minimum Shift Keying (GMSK). GMSK is a constant envelope continuous phase modulation method that has become popular recently, especially for mobile radio applications. 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 or millimeter-wave 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 which shifts the frequency by xN. 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 millimeter-wave region.

A prototype circuit is designed to verify this proposed method of carrier phase control 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 oscilla- tor 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 syn- chronization of antenna and oscillator arrays, phased array antenna beam steering, indirect frequency modulation, and ultra-small carrier frequency translation.

Performance results are presented for GMSK modulation of a carrier sig- nal at 18 GHz. Excellent performance in realizing GMSK at 18 GHz is obtained by employing Gaussian prefiltered phase control signals. The full GMSK phase control range of 360 degrees was exercized with less than 5 degrees of phase distortion in the modulated signal while maintaining the near constant envelope property desired for GMSK. The result of this per- formance is exceptional GMSK modulated signal characteristics at moderate data rates. The modulator provided an output level of -10 dBm at 18 GHz and good frequency selectivity as all undesired harmonics were maintained at < -30 dBc. The modulator was shown to be suitable for high frequency modulating signals and performed favourably at modulation frequencies as high as 300 MHz.

The results of this research are very encouraging and present an effective method of direct modulation that is very attractive for many applications emerging at upper microwave and millimeter-wave frequencies.