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r/i~ HEWLETT ~~ PACKARD TECHNIQUE FOR MEASURING PHASE ACCURACY AND AMPLITUDE PROFILE OF CONTINUOUS-PHASE-MODULATED SIGNALS Application to GMSK.3 Dr. Raymond A. Birgenheier, 8M IEEE www.HPARCHIVE.com ABSTRACT A method and apparatus for determining the phase and amplitude accuracy of continuous- phase-modulated signals is described. The modulated RF signal generated by the transmitter unit under test is down-converted to a relatively low intermediate frequency which is filtered and sampled by a high sampling rate analog-to-digital converter. A digital signal processor processes the digital signals to produce measured amplitude and phase functions of the modulated transmitter signal. From the measured values of amplitude and phase, ideal ampli- tude and phase functions of the transmitter signal are synthesized mathematically. The synthesized functions are compared to the measured functions to determine instantaneous amplitude and phase errors as a function of time. I. Introduction A number of manufacturers manufacture and market radios for use in communications, such as digital cellular radios and the like. Typically each manufacturer provides its own specifica- tions for its products. Traditionally the accuracy of these specifications has been measured using many separate, possibly indirect methods. Phase accuracy of the transmitted signal, for example, typically is indirectly determined by measuring spurious signals, phase-noise, modula- tion index, frequency settling time, carrier frequency and data clock frequency. Furthermore, amplitude measurements present special problems because the amplitude versus time profile must be synchronized to the data typically utilizing external equipment. It has been proposed that a standardized mobile digital radio system be implemented throughout Europe. Such a radio system would require that all components such as transmitter and receiver, for example, be manufactured to standard specifications measured by a common method. The Groupe Special Mobile (GSM) has proposed a measurement technique to measure the accuracy of the modulation of the transmitted signal. In the measurement technique described in this paper, a sampled measurement of the phase trajectory of the transmitted signal is obtained. This measurement is compared with the mathematically computed ideal phase trajectory to determine the phase difference between the transmitted signal and the ideal signal. The phase difference is fitted to a linear regression line. The slope of the regression line provides an estimate of the frequency error of the transmitter, and the regression line subtracted from the phase difference provides an estimate of the instantaneous phase error. The utilization of the standard method such as this would simplify the testing and manufacture of radios. An individual manufacturer would then only need to ensure that the