The Effects of Transmitter Source and Load Impedance on Harmonic Output Spectrum - A New Measurement Method

In the recent literature several papers have appeared which discuss the significance of considering the effects of both the transmitter internal impedance and the load impedance when determining the transmitter harmonic output spectrum. The considerable variation in the range of predicted harmonic emission levels which accrue from high source and load VSWR and which include the effect of intervening transmission line has been aptly presented.1,2,3 It is the purpose of this paper to present the results of recently conducted measurements and analysis aimed at more accurate prediction of these effects in terms of the complex reflection coefficients of both the source and the load impedances which exist at the harmonic frequencies. An analysis was made of the constant absorbed-power contours which can be plotted in the complex load reflection coefficient plane for specific complex source reflection coefficients. These constant power contours are circles in the load plane, and if superimposed on Smith chart coordinates permit ready determination of the power delivered to any given load impedance at the harmonic frequencies. This pictorial representation thus provides particular insight into the effects of source and load reflections on the harmonic power output. Measurements were made of the transmitter power delivered to specially designed loads comprising transmission-line sections, tunable stubs, and a linear dummy load. These special loads were novel in that they could be tuned to produce a perfect match at the fundamental frequency, while known non-zero reflections were obtained at the several harmonics. Thus, in principle, load impedances which are conjugates of the transmitter source impedance are those which absorb maximum power. The transmitter load power measurements using these loads were in good agreement with the theoretical load power contours and permitted an indirect determination of transmitter internal impedance at harmonic frequencies. An analysis of the transmitter tank circuit was made for both the active and passive conditions of the final amplifier stage. These results which will be discussed show that the transmitter internal impedance at harmonic frequencies is the same for both conditions.